DC cable selection guide

June 11, 2016

I have recently installed solar and dual bettery systems on two buses, refreshed my own installation and am starting to think like an electrical engineer. After some extensive research, I have produced some cable selection tables for my toolbox. These tables can be used for auto electrics, marine electrics and small solar power systems. Before presenting the tables, I discuss the data sources.

Ampacity

Ampacity (amps capacity) is the maximum continuous current an electrical cable can carry without the insulation melting. Some major factors that determine ampacity are:

  • Cable size (small conductors have high resistance and become hot at low currents).
  • Insulation temperature rating.
  • Ambient temperature (heat dissipation from hot cables is slow at high ambient temperatures).
  • AC or DC current.

Also beware that old cables with corroded conductors and weathered or perished insulation will not perform like new.

There can be large differences between ampacity data from different sources. The following plot compares five standards. These curves show that AC ratings tend to be lower than DC ratings (e.g. NEC versus ABYC). For DC applications, the JASO and ABYC ratings are similar and the ISO ratings are about 15% lower. I selected the ABYC data, which seems reliable and covers a broad range of cable sizes and conditions.

Ampacity versus conductor area for copper power cables with 90°C insulation, at 30°C ambient temperaute. The American Boat and Yacht Council E-11 data I found at Blue Sea (marine small craft, DC). The JASO D609 data are from the manufacturer Tycab (automotive, DC). The ISO 10133 data I found at Energy Solutions LINK (marine small craft, < 50 VDC). The NFPA 70 NEC 2014 data are from Wikipedia (AC, ≤ 3 conductors). The AS/NZS 3008 data are from the manufacturer Olex (single-phase AC, single conductor).

Ampacity versus conductor area for copper cables with 90°C insulation, at 30°C ambient temperature. The American Boat and Yacht Council E-11 data I found at Blue Sea Systems (marine small craft, DC). The JASO D609 data are from the manufacturer Tycab (automotive, DC). The ISO 10133 data I found at Energy Solutions (marine small craft, < 50 VDC). The NFPA 70 NEC 2014 data are from Wikipedia (AC, ≤ 3 conductors). The AS/NZS 3008 data are from the manufacturer Olex (single-phase AC, single conductor).

Resistance

Voltage losses usually decide power cable size rather than ampacity. Resistance is largely determined by copper cross-sectional area. The following plot compares DC resistance data from five sources and the differences are not substantial. I selected the ABYC data, which covers a broad range of cable sizes.

Resistance versus conductor area for copper power cables, for 0 to 35 mm2. The American Boat and Yacht Council E-11 data (30°C) data I found at Blue Sea. The Tycab (20°C) data are from the manufacturer. The IEC 60827 data (20°C) I found at myElectrical Engineering LINK. The NEC data (20°C, solid core) are from Wikipedia. The Olex data are from the manufacturer.

Resistance versus conductor area for copper cables. The ABYC E-11 (30°C) data I found at Blue Sea Systems. The Tycab (20°C) data are from the manufacturer. The IEC 60827 data (20°C) I found at myElectrical Engineering. The NEC data (20°C, solid core) are from Wikipedia. The Olex data are from the manufacturer.

Cable ratings table

I merged all data for common power cable sizes into one reference table below. For cables not in the ABYC E-11 table (e.g. ‘auto cables’), I estimated ratings from quadratic curves fitted to the ABYC data.

Area DC Ampacity (A) DC Resist.
Cable (mm2) 30°C 60°C (ohm/km) Terminal
0.5 mm2 0.5 8 6 36.04 RED
20 AWG 0.52 8 6 34.65 RED
2 mm auto 0.56 9 7 32.18 RED
2.5 mm auto 0.64 9 7 28.16 RED
0.75 mm2 0.75 10 7 23.92 RED
18 AWG 0.82 10 8 21.88 RED
1 mm2 1 13 10 17.94 RED
3 mm auto 1.13 14 10 15.95 RED
16 AWG 1.32 15 11 13.70 RED
1.5 mm2 1.5 16 12 11.96 RED / BLUE
4 mm auto 1.84 18 14 9.79 BLUE
14 AWG 2.1 20 15 8.63 BLUE
2.5 mm2 2.5 21 16 7.18 BLUE
5 mm auto 2.9 24 18 6.21 YELLOW
12 AWG 3.3 25 19 5.42 YELLOW
4 mm2 4 34 25 4.49 YELLOW
6 mm auto 4.59 38 29 3.93 YELLOW
10 AWG 5.32 40 30 3.41 YELLOW
6 mm2 6 53 40 2.99 YELLOW
8 AWG 8.5 65 49 2.14
10 mm2 10 79 60 1.79
6 AWG 13.5 95 71 1.35
16 mm2 16 105 79 1.12
4 AWG 21.3 125 94 0.85
25 mm2 25 141 106 0.72
2 AWG 33.7 170 128 0.51
35 mm2 35 173 130 0.51
Cable ratings chart sorted by wire size (white = IEC/ISO cables, yellow = AWG cables, grey = ‘Auto cables’). Ampacity for 30°C and 60°C (‘engine room) ambient temperatures. Most data are from ABYC E-11 found at Blue Sea Systems. Auto cable cross-sectional areas are from the manufacturer Tycab. Blue values are from quadratic interpolation of the ABYC data. Red values are extrapolated. I have also added a column for insulated crimp terminal selection.

The most common insulation for copper power cables seems to be PVC, which is rated for 75°C conductor temperature (V-75). Usage up to 90°C (V-90) is limited. I selected ABYC data for 75°C insulation.

Ampacity should be derated for ‘engine room’ conditions (e.g. inside the engine bay of a vehicle). The ABYC data decrease ampacity by 25% at 60°C ambient temperature. Tycab recommend a larger derating, 40% at 60°C, perhaps because they provide ampacities for V-90 insulation.

When routing cable inside an engine room or engine bay, be careful not to run PVC insulated cables near exhausts, cyclinder heads, radiators and other hot parts > 75°C. PVC insulation will melt at high temperatures!

To use the above chart, one needs to identify the cable size and insulation:

  • Sometimes, the insulation of a power cable is marked with the conductor area (mm2 or AWG) and temperature rating of the insulation (°C or maybe °F).
  • When buying cable off the reel, a label on the reel should identify the conductor area (mm2 or AWG) and insulation type (e.g. V-75).
  • For scrap pieces of cable, I have to judge whether the insulation is PVC or not and then estimate the copper area using a wire stripper. PVC insulation is ‘opaque’ (never clear), ‘firm’ (not soft), fairly tough (when cutting and stripping) and softens when heated lightly (but doesn’t burn, melt away or shrink by a large amount).

Beware that the size of ‘auto cables‘ refers to the total cable diameter, including the insulation. Plastic is cheaper than copper and the cross-sectional conductor area of auto cables may not be well standardised. For Australia, Electra auto cables and Tycab auto cables have the same cross-sectional area as in the above table. Avoid buying cable from auto parts stores. Better cable and better prices are found at electrical trade stores and sometimes on Ebay.

Also note that ‘gas cable’ is double-insulated cable for hazardous applications. It’s better than single insulated cable, but is slightly more expensive.

Voltage losses and cable selection charts

Resistance in a power circuit results in voltage losses and less power reaching the load (DC power = voltage × current). For example, the compressor in my Evakool fridge may have starting problems and runs slower when voltage is low.

Voltage loss is a function of current and resistance (Ohm’s law: volts = amps × ohms). Cable resistance is a function of cable length and cross-sectional area (ohms = ohms/m × m; see the cable ratings table above).

Below are some cable selection tables for both 12 V and 24 V systems. I calculated these tables in five steps:

  1. Calculate maximum cable resistance (ohms/km) from voltage drop, cable length and current.
  2. Calculate minimum copper conductor cross-sectional from cable resistance (using power function fitted to ABYC data, see above).
  3. Calculate ampacity for 75°C insulation and 60°C ambient temerature (using quadratic function fitted to ABYC data, see above).
  4. Upgrade results smaller than 0.5 mm2 to 0.5 mm2.
  5. Upgrade results with ampacity less than the current in Step 1 above.

These cable selection tables specify the minimum conductor area. An appropriate cable can then be selected by reference to the cable ratings table above.

mm2 1 2 5 10 15 20 30 40 50 75 100 A
1 0.5 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
2 1.0 0.5 0.5 0.8 1.5 2.3 4.0 5.3 6.0 10.0 16.0 25.0
5 2.5 0.5 0.8 1.9 3.8 5.6 7.5 11.3 15.0 18.8 28.2
10 5.0 0.8 1.5 3.8 7.5 11.3 15.0 22.5 30.0
15 7.5 1.1 2.3 5.6 11.3 16.9 22.5 33.8
20 10.0 1.5 3.0 7.5 15.0 22.5 30.0
25 12.5 1.9 3.8 9.4 18.8 28.2
m Single m Twin
Cable selection chart for 12 V systems and 2% voltage loss (sensitive loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.
mm2 1 2 5 10 15 20 30 40 50 75 100 A
1 0.5 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
2 1.0 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
5 2.5 0.5 0.5 0.9 1.9 2.8 4.0 5.6 7.5 9.4 16.0 25.0
10 5.0 0.5 0.8 1.9 3.8 5.6 7.5 11.3 15.0 18.8 28.2
15 7.5 0.6 1.1 2.8 5.6 8.4 11.3 16.9 22.5 28.2
20 10.0 0.8 1.5 3.8 7.5 11.3 15.0 22.5 30.0
25 12.5 0.9 1.9 4.7 9.4 14.1 18.8 28.2
m Single m Twin
Cable selection chart for 12 V systems and 4% voltage loss (normal loads). Equivalent to 24 V and 2% voltage loss (sensitive loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.
mm2 1 2 5 10 15 20 30 40 50 75 100 A
1 0.5 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
2 1.0 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
5 2.5 0.5 0.5 0.5 1.0 2.1 4.0 5.3 6.0 10.0 16.0 25.0
10 5.0 0.5 0.5 0.9 1.9 2.8 4.0 5.6 7.5 9.4 16.0 25.0
15 7.5 0.5 0.6 1.4 2.8 4.2 5.6 8.4 11.3 14.1 21.1 28.2
20 10.0 0.5 0.8 1.9 3.8 5.6 7.5 11.3 15.0 18.8 28.2
25 12.5 0.5 0.9 2.3 4.7 7.0 9.4 14.1 18.8 23.5
m Single m Twin
Cable selection chart for 24 V systems and 4% voltage loss (normal loads). Read minimum cross-sectional copper area (mm2) at the intersection of current (top row) and cable length (one of the two left-rows). For single core cables, it is assumed the chassis return has zero resistance.

Cable selection tips

When selecting electrical cables, biggest is not best. Bigger cables are more difficult to work with (e.g. routing, making connections), heavier and more costly. Use electrical engineering know-how and always select the thinnest cable that is fit for purpose. For example, you will not find many thick cables in OEM installations because they are not necessary for most applications.

When routing cables, shortest is not best. Leave a generous amount of extra length to allow for future work. Investing in a little extra cable is cheaper than having to replace a whole section of cable that is found to be too short or better than having to join two pieces of cable.

Exceed the ampacity and risk an electrical fire! Use appropriate fuses, especially for smaller cables with low current ratings.

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DIY 12 Volt LED light strip bar

January 4, 2015

12 Volt LED lights are expensive. 12 Volt LED light strips are cheap. I bought a light strip and made my own custom light bar for camping. The materials cost about AUD$20.

LED light bar design

This project upgrades my previous 12 V LED camping light. The objective was to install a rugged LED light bar in the back of my ute (pick-up).

My home made 12 V LED light bar in action with two LED strips, about 540 Lumens.

My home made 12 V LED light bar in action with two LED strips, about 540 Lumens.

I bought a 5 m white LED (WLED) strip light roll and some switches on ebay. Here are the specifications for the strip:

LED 5630 SMD
Colour temp. (K) 5000–5500
Viewing angle (deg.) 120
LED density (LEDs/m) 60
Intensity (lumens/m) 840–900
Current (A/m) 1
Specifications for ebay 5630 LED strip light. The intensity is more realistically about 290–320 lm/m at this current.

I wasn’t sure that one strip would be sufficient, so I installed two side-by-side. Some basic soldering skills are needed for attaching new leads after cutting the strip. I have actually found one strip = 290 Lumens/m × 0.93 m = 270 Lumens quite sufficent.

Small diameter power cables can be used when currents are small. For my design, 1 A/m × 0.93 m × 2 strips = 1.86 A. The following table can be used for sizing power cables:

AWG 2 A 4 A 8 A 16 A Current
1 m 22 22 18 16
2 m 22 18 16 12
5 m 18 14 12 8
10 m 14 12 8 6
15 m 12 10 6
20 m 12 8 6
Twin-core
American Wire Gauge (AWG) cable sizes for 4% (0.48 V) voltage drop, twin-core cable. Read the cable size at the intersection of current along the top row and cable length on the left column. Calculated from American Wire Gauge data on Wikipedia.

Here’s the same table with ISO (mm2) sizes:

ISO 2 A 4 A 8 A 16 A Current
1 m 0.5 0.5 1 1.5
2 m 0.5 1 1.5 4
5 m 1 2.5 4 10
10 m 2.5 4 10 16
15 m 4 6 16
20 m 4 10 16
Twin-core
Metric (mm2) cable sizes for 4% (0.48 V) voltage drop, twin-core cable. Read the cable size at the intersection of current along the top row and cable length on the left column. Derived from the previous table.

For bigger strip lights there are two important points to consider:

  1. The maximum current rating of LED strips is low (typically 5 A) and maximum strip length is short (typically 5 m).
  2. There could be substantial voltage losses along the strip.

The basic solution for the above problems is to use short LED strips in parallel.

LED light bar assembly

The strip lights I bought are waterproof. However, I wanted better protection from shifting cargo in the back of my vehicle. I made a lighting fixture out of thin timber strips (10 mm deep), with a plywood backing (3 mm) and a clear polycarbonate plastic cover (2 mm). The timber I cut myself from scrap and the plywood was leftover from other projects. Polycarbonate and acrylic (‘perspex’) can be found at a fibreglass supplies shop.

Detail of home made LED light bar with wiring cover removed. This end is thicker to accommodate the diameter of the switch.The plastic cover has a foam gasket. The wiring cover has a rubber gasket (recycled bicyle tube, contact-glued on). I contact-glued aluminium foil inside the fixture to minimise reflection losses.

Detail of home made LED light bar with wiring cover removed. This end is thicker to accommodate the diameter of the switch.The plastic cover has a foam gasket. The wiring cover has a rubber gasket (recycled bicycle tube, contact-glued on). I contact-glued aluminium foil inside the fixture to minimise reflection losses.

The two LED strips in my light are wired in parallel, for equal voltage and equal intensity. Here are two different wiring diagrams including switches.

Wiring diagram for two parallel LED strip lights (only 3 LEDs shown in each strip). Switch 1 (master) controls both strips. The optional switch 2 (dimmer) controls the second strip. For electrical safety, there is an in-line fuse between the supply and switches (2A mini blade fuse in my case).

Wiring diagram for two parallel LED strip lights (only 3 LEDs shown in each strip). Switch 1 (master) controls both strips. The optional switch 2 (dimmer) controls the second strip. For electrical safety, there is an in-line fuse between the supply and switches (2A mini blade fuse in my case).

wiring2

Wiring diagram for two parallel, individually-switched LED strip lights (only 3 LEDs shown in each strip).

The self-adhesive backing on the strip lights did not hold for long. I re-glued the strips with contact glue and secured the strips with four cable ties (zip ties).

The thin, flexible plastic cover did not seal well against the timber. I made a gasket from foam tape to improve dust and insect resistance.

According to my solar regulator, this light bar draws 0.7 A at 12.4 V with one strip (design 0.93 A) and 1.4 A at 12.3 V with two strips (design 1.86 A). The intensity is overrated in the first table above.


Understanding LED strip lights

January 4, 2015

I bought a 12 volt white LED strip light on ebay. After some testing and further research, I figured the luminous intensity was overrated. This post should be helpful for understanding LED strip lights.

LED strip light package

An LED strip is basically a package of Surface-Mount-Device (SMD) LEDs with current limiting resistors. The strips are flexible and easy to install with a self-adhesive backing. Waterproof strips have a clear, silicon-like layer over the electronics.

Section of ebay LED strip light (both sides). The strip is 12 mm wide. There are two groups of three LEDs (yellow). The SMD resistors are black. A splice can be seen in the lower left. The strip can be cut and new leads soldered at the copper tabs. Contrary to the logo on the back, the adhesive back is not genuine 3M quality.

Section of ebay LED strip light (both sides). The strip is 12 mm wide. There are two groups of three LEDs (yellow). The SMD resistors are black. A splice can be seen in the lower left. The strip can be cut and new leads soldered at the copper tabs. Contrary to the logo on the back, the adhesive back is not genuine 3M quality.

Examining my strip light, I see repeated groups of three LEDs, each with one 39 ohm resistor. I guess the wiring is series within-groups and parallel between-groups.

Possible LED strip light wiring. R = resistor.

Possible LED strip light wiring. R = resistor.

An LED strip is a passive device. It contains no electronics to regulate the power and brightness. If the supply voltage drops, then brightness drops and vice versa.

Currents are highest near the supply end. Firstly, the current for all the LEDs passes through the supply end. Second, the copper is thin and there could be substantial voltage losses along the strip, meaning that LEDs nearer the supply operate with higher voltages and currents.

You can feel that the strip is warm at the supply end because more heat is produced at higher currents. Long strips in series can melt.

Strip light specifications

Here are the specifications for the White LED (WLED) strip I bought:

LED 5630 SMD
Colour temp. (K) 5000–5500
Viewing angle (deg.) 120
LED density (LEDs/m) 60
Intensity (lumens/m) 840–900
Current (A/m) 1
Specifications for ebay 5630 LED strip light. The intensity is more realistically about 290–320 lm/m at this current.

The above lumens are overrated. The following table compares the ebay LED with specifications for two other 5630 LEDs from Samsung and Philips. The lumens/milliamp calculated for the ebay LED is impossibly high. The ebay seller seems to have calculated strip values using the typical LED intensity, but the LEDs are operating at a lower current.

ebay Samsung Philips
Intensity (lm) 14.5 20 32
Current (mA) 17 50 100
Voltage (V) 3.8 3 3.1
lm/mA 0.85 0.40 0.32
Comparison of 5630 SMD LEDs (single LEDs). I calculated the ebay specification as follows: current = 1/60 A, intensity = 870/60 lm. Voltage was calculated by subtracting the drop across the current limiting resistor from the supply voltage (12 – 39 × 0.017) and dividing by three. Voltage seems a bit high. Other LED data from Samsung SPMWHT5225D5WAR0S0 and Philips LUXEON 5630 datasheets.

If I assume the Philips lumens/milliamp result, I estimate 0.32 lm/mA × 17 mA = 5.33 lm and 60 × 5.33 lm = 320 lm/m for the ebay strip light. Alternatively, I could assume the Samsung current and then estimate intensity as 14.5×17/50 = 4.83 lm (the datasheet showed that intensity is approximately proportional to current) and 60 × 4.83 lm = 290 lm. Both of these estimates seem reasonable in side-by-side comparisons with my 97 lumens Fenix HL21 headlamp.

Some reasons for running the LEDs at lower currents are: to reduce heat, reduce voltage losses and allow longer strips. Using the Samsung LED for comparison, at 60 LEDs/m the current is 60 × 20 lm = 3 A/m and a 5 A strip would be only 5/3 = 1.67 m long. Lower currents and less heat is safer and easier for home made strip lights.

WLEDs come in different colour temperatures. “Natural” or “daylight” LEDs (5000–5500 K) are effectively brighter than “warm white” (3200 K). This is because human colour vision is more sensitive to wavelengths in the middle of the daylight spectrum. The ebay listing did not include the Colour Rendering Index (CRI). My impression is that the LEDs are rather blue compared to 5000-5500 K daylight.

SMD LEDs cast light across a very wide angle (viewing angle typically 120 degrees). If installed inside fixtures, the fixture should be shallow to achieve a wide floodlight. Avoid vertical mounting (e.g. wall mounting), because up to half of the light will be directed upwards.

It is best to have the SMD LEDs horizontal and facing down

It is best to have the SMD LEDs horizontal and facing down

Saving money

Here are some cost-saving measures to expect from ebay LED strip lights:

  • LEDs from low grade bins and mixed bins (variable intensity, colour, forward voltage).
  • Thin copper (more resistance, more heat, shorter maximum strip length).
  • Spliced strips (possible failure points).
  • Poor quality adhesive backing (strip lights come unstuck after a short time).
  • Low currents (see previous discussion).

Cheap ebay LED strip lights may be false economy if more LEDs are required to achieve satisfactory brightness.


Simple, low cost solar and battery monitoring

July 22, 2014

Battery power can be stressful. How much capacity is being used? How much is remaining? How much is being recharged? Installing cheap panel ammeters have greatly improved my understanding of my auxiliary (‘aux’) battery system.

monitor

Basic solar charge controller upgraded with a DC ammeter. This is from my old system (3 A solar panel, 5 A meter).

Battery monitoring

Voltage is a poor indication of battery capacity in working solar power systems where voltage varies with charging and loads. The battery would need to  be rested before measuring voltage for estimating state of charge. Then voltage should be corrected for temperature.

It is more informative to measure the current going in to the battery and the current going out. If the solar charging current is higher than the average load, the battery will be storing the difference. If the average load is higher, the battery is supplying the difference.

If I am running out of battery capacity, I can use the ammeters to guide me in taking corrective action. The input ammeter can be used to improve solar panel positioning and to estimate daily charging input. The output ammeter can be used to decide which loads to disconnect or reduce.

Current measurement

I use analog DC panel ammeters because they are simple, do not consume power and are cheap (you can find them on ebay). Low-current ammeters (less than about 50 A) do not require an external ‘shunt resistor’ and are very easy to install (in series). I once tried cheap digital ammeters, with no instructions, and burnt both of them.

Ammeters can be installed on the positive cable (or negative if you prefer) and on both the input and output sides of the aux battery. The ammeters I have are marked with a ‘bar’ symbol at the negative or ‘downstream’ terminal. If the needle moves in the wrong direction, then swap the connections.

I recently built a custom power distribution panel using recycled connectors and plugs. You can buy DC power outlets and some have features like USB charging and maybe a voltmeter – just add an ammeter in series.

Custom power distribution panel with DC ammeter (10 A, which should cover any loads I plan to plug in). It’s made from timber and plywood and easily modified should I need to add more outlets. A voltmeter would be useful, but I already have one in my new charge controller.

Custom power distribution panel with DC ammeter (10 A, which should cover any loads I plan to plug in). It’s made from timber and plywood and easily modified should I need to add more outlets. A voltmeter would be useful, but I already have one in my new charge controller.

Save money

Real battery monitors have microprocessors and can estimate remaining battery capacity at variable discharge rates via Peukert’s equation. Battery monitors are expensive gadgets. A simple volts and amps monitoring system is cheap and simple.

I now have an expensive and unnecessary Morningstar Prostar 30-M solar charge controller, which measures voltage and current. However, Morningstar do not recommend (see Tech Notes) to connect inverters and compressor fridge/freezers to the load terminals (installing a ‘clamping diode’ is a possible workaround). It is cheaper to buy a good basic solar charge controller, separate ammeters to monitor the charge and load currents and perhaps a voltmeter for precise battery voltage.


AGM and wet deep-cycle batteries compared

May 31, 2014

Many times I have read that Absorbed Glass Mat (AGM) lead-acid batteries are ‘better’ than traditional wet (flooded) lead-acid batteries. This article examines the characteristics and performance of the AGM and wet batteries. The ‘best’ choice depends on the budget and the application. AGM’s are sealed and can charge three times faster. Wet deep-cycle batteries cost less and can last 1.4 times last longer. Where either technology is safe, most users would not notice the performance differences between wet and AGM batteries and cost-savings can be substantial.

Products compared

I downloaded products specifications for Ritar DC series AGM (China), SunStone ML series AGM (China) and Trojan Signature Line wet deep-cycle batteries (U.S.A.).

I compared ratings at 12 V. Many of the Trojan batteries evaluated were 6 V and I then considered two 6 V in series. For two batteries in series the mass and voltage are doubled and the capacity (Ah) is unchanged. And for two batteries in parallel, the mass and capacity are doubled and the voltage is unchanged.

Capacity versus mass

More lead = more capacity, as the following graph shows. The linear regression fitted to the entire dataset can be used to predict mass and identify fraudulent ratings. A battery that plots below the trend (actual mass less than predicted) is over-rated.

Mass versus 10-hour capacity for Ritar DC series AGM, SunStone ML series AGM and Trojan Signature Line flooded lead-acid batteries. Linear model fitted to all data points.

Mass versus 10-hour capacity for Ritar DC series AGM, SunStone ML series AGM and Trojan Signature Line flooded lead-acid batteries. Linear model fitted to all data points.

Charging rate versus capacity

More lead = higher acceptance = faster charging. For the two AGMs evaluated, maximum charging rate is 0.30 times 10-hour capacity. For the wet batteries, maximum charging rate is 0.14 times the 10-hour capacity (Trojan actually specifies 0.10 to 0.13 times the 20-hour capacity).

Maximum charging rates for Ritar DC series AGM, SunStone ML series AGM and Trojan Signature Line flooded lead-acid batteries. Maximum charging rates are commonly summarised as a fraction of capacity (= the gradient of the rate-capacity line).

Maximum charging rates for Ritar DC series AGM, SunStone ML series AGM and Trojan Signature Line flooded lead-acid batteries. Maximum charging rates are commonly summarised as a fraction of capacity (= the gradient of the rate-capacity line).

A second consideration is charging efficiency, because not all of the input current is stored. Some of the energy is lost to heat and gassing. Charging efficiency for AGM batteries is about 95% and greater than 85% for wet batteries. Multiplying charging rates by efficiency increases the differences between AGM and wet batteries. For the two AGMs evaluated, the effective maximum charging rate is 0.29 times 10-hour capacity versus 0.12 times for the wet batteries.

These results agree fairly well with my own testing. My Trojan T105s bulk-charge at about 0.13 times 10-hour capacity. My old Ritar DC12-100 charges at about 0.24 times 10-hour capacity.

Note that maximum charging rates vary between manufacturers, depending on the design, construction and safety margins. For Trojan AGMs (not included in this study), maximum charging rate is about 0.2 times 20-hour capacity.

Cycle life

Cycle life determines lifetime cost. AGM batteries are sealed, maintenance free and tend to have shorter cycle lives than wet deep-cycle batteries, as the following graph shows. The larger Trojan Signature Line batteries are exceptionally rugged and can deliver 50% of rated capacity after 1200 cycles. Next, the Ritar DC series are true deep-cycle AGM batteries and can deliver 60% of rated capacity after 850 cycles. Last, the SunStone ML series are really for standby use and can deliver 60% of their rated capacity after 500 cycles.

life-capacity

Cycles to 60% of rated-capacity for Ritar DC series AGM and SunStone ML series AGM batteries and 50% of rated-capacity for Trojan Signature Line flooded lead-acid batteries. Average depth of discharge was 50% for all three manufacturers. Observe that cycle life is unrelated to capacity, except for the smaller Trojans, which must differ in design and construction to the higher-capacity models.

Be careful when comparing cycle life ratings that the depth of discharge and the percentage reduction in final capacity are the same. Increasing depth of discharge will reduce battery life. Increasing the capacity end-point will reduce battery life (e.g. the Trojan cycle life ratings would be a little bit lower at 60% of rated capacity, rather than 50% as specified).

Summary

I have experience with both Ritar DC series AGM and Trojan Signature Line wet deep-cycle batteries. The following table summarises what I think are the important differences between these two battery technologies.

Main pros and cons of AGM and wet deep-cycle batteries.
AGM Wet
Sealed, spill-proof, no explosive hydrogen gas vented. Spill-resistant with the right caps, but must be allowed to vent gas.
Charging nearly 3 times faster than wet batteries. Charging slower, but less load on the charging system.
Advanced construction and higher cost than wet batteries. Lower cost than AGM.
AGM battery life varies between different designs. Maintenance not possible. Cycle life can be 1.4 times greater than AGM. Maintenance prolongs life, especially in hot climates.

Two Trojan T105s are doing a good job in my dual-battery plus solar system:

  • Excellent value for money (low cost, high capacity, long life).
  • Slower charging is no issue because Australia is a big country and I drive long distances when travelling which allows the batteries plenty of time to charge. When camped, the charging rate is limited by my solar panels, which can deliver a maximum of 12 A.
  • Maintaining the T105s is not a great inconvenience. They don’t use much water.

AGMs are required where there are safety concerns or unusual operating conditions:

  • Inside vehicle, where gases can’t be vented.
  • Marine applications (salt water + battery acid = Chlorine gas).
  • Short drives, where the batteries have to charge quickly.
  • Serious off-road driving, where wet batteries can spill (my Trojans spilled some acid at Cape York, Queensland).

Alternator current testing

October 22, 2013

This post extends a previous article about testing a dual battery system. Not satisfied with voltage measurements alone, I purchased and assembled a high current DC ammeter. It is useful for measuring alternator output, load currents and charging currents.

I assume that have a basic understanding of electrical systems and know how to use a multimeter. I have outlined multimeter basics for the novice in a preceding post.

Disclaimer: Electrical systems and batteries can be dangerous. I am not responsible for any losses, damages or accidents you may incur by following these instructions. If you don’t understand then don’t do it!

A cheap high-current DC ammeter

I use cheap analog panel ammeters for measuring large DC currents. There are plenty to choose from on ebay. For very high currents (more than about 50 A), an external shunt resistor is required.

Firstly, I had to assemble the ammeter following the schematic below. I used 6 AWG cable (maximum current rating 75 A) and large ring terminals for the high-current path. I used nuts and bolts to attach the ring terminals to the mega fuse and shunt (expensive fuse holders are not required).

Wiring diagram for high current DC ammeter. 6 AWG cable in red and thin automotive cable in blue. Arrows show direction of current flow (from alternator to positive terminal of the main battery).

Wiring diagram for high current DC ammeter. 6 AWG cable in red and thin automotive cable in blue. Arrows show direction of current flow (from alternator to positive terminal of the main battery). The polarity of the ammeter refers to the current direction and not the ground circuit of the vehicle. The fuse above is not really necessary. There is no fuse between the alternator and main battery in the usual configuration.

I used thin automotive cable for the sense wires and 2 A mini-blade fuses (the lowest current fuses I had) to protect the meter. Check the continuity of these sense cables is perfect (it should be less than 0.1 ohms). Any resistance will bias the ammeter reading. Everything was installed in a custom box, made from scrap medium density fibreboard (MDF) and plywood. The fuses, shunt and internal wiring can be accessed by removing the lid of the box.

Measuring load currents

To measure alternator current, the ammeter is connected in series between the vehicle’s alternator (the current source) and the positive terminal of the main battery. A digital multimeter can be connected to the main battery terminals to measure voltage.

High-current ammeter in action: c. 34 A and 13.83 V.

High-current ammeter in action: c. 34 A and 13.83 V.

I then started the engine and let it warm up. I increased rpm to about 1500 rpm and started switching on accessories, recording the alternator current and voltage at each step. My old vehicle has a hand throttle (which is handy!). Here are some results:

Accessory Volts Amps Increment
None 14.27 6 0
Headlights low (50W) 14.18 18 12
Headlights high (60W) 14.20 20 14
Blower max. 14.16 31 11
Rear demister 14.11 34 3
Alternator load testing results with engine speed at 1500 rpm.

Note the 6 A current used by the computer, fuel pump, fuel injection etc. The headlights and the fan blower are high current accessories. I forgot to test the windscreen wipers.

If the combined load is not huge, there could be sufficient alternator output with the engine idling. Alternatively, accessories could be tested one-by-one.

Measuring charging currents

I was most interested in charging currents for my dual battery system. For high current, bulk charging, the battery state-of-charge (SoC) should be less than 80%.

Main Aux
V 12.64 12.1
degC 24 22
SoC 100 78
State-of-charge measurements before alternator testing. The main battery was a wet low maintenance battery. The aux was a wet deep-cycle battery and partly discharged.

Here are the charging currents with all accessories off:

Engine rpm Main Volts Alternator A Charging A
800 14.29 29 23
1000 14.28 32 26
1500 14.27 32 26
2000 14.26 32 26
2500 14.26 32 26
Aux battery charging currents for my dual-battery system. Charging currents estimated as alternator current minus 6 A (no load current). Engine speed was 1500 rpm. Aux battery charging voltage was quite steady at 12.8 V.

The maximum aux charging current was 26 A. Battery capacity was 207 Ah (10 h rate)and the charge rate was 26/207 = 0.13 times battery capacity. This agrees with capacity/8 for flooded batteries. Doubling the battery capacity might increase the maximum charge current to 2 x 26 = 52 A. I haven’t tested if this is true.

There was a 1.5 V voltage drop between the main and aux batteries, which is more than 0.4 V expected for the cables and 1.0 V in the dual-battery system design. The large measured voltage drop occurs because the aux battery is bulk charging and its voltage is depressed relative to the main battery.

I also tested a 100 Ah AGM deep-cycle battery, for which the charge rate was 17/100 = 0.17 times battery capacity (or perhaps 0.24 because the capacity of that old battery was around 70 Ah). AGM batteries accept higher charging rates (current/capacity) than wet batteries.

Alternator performance testing

Alternator performance is tested with a heavy load, equal to or slightly greater than the rated alternator maximum current. I tested with the aux battery charging (32 A), headlights on high (14 A), blower on full speed (11 A) and demister on (3A). The total load was 66 A (including the no load current 6 A measured above). Here’s a plot of alternator current versus speed:

Alternator current versus alternator speed. The maximum alternator current was 60 A. Alternator speed = 2 × engine speed (I measured the diameters of the drive and alternator pulleys and calculated the ratio of circumferences as 2:1).

Alternator current versus alternator speed. The maximum alternator current was 60 A. Alternator speed = 2 × engine speed (I measured the diameters of the drive and alternator pulleys and calculated the ratio of circumferences as 2:1).

At low speeds, the alternator can’t satisfy the combined load and the shortfall would usually be supplied by the main battery. At higher speeds, alternator current increases and then plateaus above 3000 rpm at 60 A. This is the alternator maximum current rating.

Conclusions

Based on my testing, the charging current can be much less than the maximum alternator current. Smaller aux batteries are unlikely to overload the alternator:

  • maximum wet battery size = (60-6)/0.13 = 415 Ah
  • maximum AGM battery size = (60-6)/0.24 = 225 Ah

Multiple accessories would need to be switched on to create a large combined load. A worst case scenario could be driving on a cold night, in the rain and with a deeply discharged aux battery (charging current plus lights, blower, wipers, demister). Another possibility is driving on a hot day with a deeply discharged battery, lights (e.g. dirt road) and air conditioner (blower on). However, bulk charging of smaller batteries at 20 to 30 A is rapid and the charging current will soon decrease.


Testing a small 12V solar power system

May 15, 2013

I have a dual-battery system in my vehicle and solar panels for camping. This article describes how to test a small 12 V solar power system.

The system I tested includes two 120 W solar panels in parallel, a Pulse Width Modulation (PWM) charge controller and 225 Ah wet deep-cycle battery. I have no experience with and no procedures for Maximum Power Point Tracking (MPPT) controllers.

I assume that have a a basic understanding of electrical systems and know how to measure voltage with a multimeter. I have outlined multimeter and electrical basics for the novice in a preceding post.

Disclaimer: Electrical systems and batteries can be dangerous. I am not responsible for any losses, damages or accidents you may incur by following these instructions. If you don’t understand then do not do it!

Step 1 Wait for a clear, sunny day

The system should be tested at maximum current, with the solar array in direct sunlight and facing the sun.

Step 2 Measure State of Charge

The solar charge controller must not limit the current flow during testing. There are two different ways to load the system:

  • Start with a full charged battery and connect a load slightly greater than the solar current.
  • Use a lightly discharged battery, around 80 per cent State of Charge (SoC), as the load.

Car and Deep Cycle Battery FAQ provides State of Charge (SoC) tables for different battery chemistries. It is recommended to let a battery rest (no current flow in/out) for two to eight hours before measuring SoC. I tested with a lightly discharged battery:

Rested voltage 12.19
Temperature 20.6
SoC 83%
State of Charge (SoC) measurements.

Step 3 Measure charging voltages and current

A multimeter and ammeter are required. Most multimeters are not rated for high and continuous DC currents. I have previously used cheap analog DC ammeters. My new solar charge controller measures current.

Several voltage measurements are made at various points in the circuit:

  1. Solar panel voltage, controller input, controller output and battery voltage.
  2. Voltage drops in the positive line.
  3. Voltage drops in the negative line.
  4. Solar panel open-circuit voltage (i.e. panel disconnected from the controller).

Solar current should be constant. I recorded current three times during testing and calculated the average.

You will need to pop open the waterproof cover on the back of a solar panel to access the terminals. For parallel solar arrays, it is only necessary to measure voltages at one panel. A long jumper lead is required for measuring voltage drops from the panel. Any thin, insulated cable will do since there is no current flowing during a voltage measurement.

To safely disconnect the panels before testing open-circuit voltage, they can be turned face-down on the ground or covered with any opaque material.

The following table details measurements for my system. Average solar current was 11.6 A.

Measurement Total volts Pos. volts
Neg. volts Per cent drop Design
Solar panel 14.64  c. 15
Controller in 13.32
Controller out 13.19
Battery 12.84
Panel to ctrlr. drop 1.30 0.65 0.65 -9% 0.9-1.3
Controller drop 0.14 0.13 0.01 -1%
Ctrlr to battery drop 0.36 0.31 0.05 -3% -2%
Panel to battery drop 1.78 1.09 0.69 -12%
Solar panel open-circuit 20.6
Solar power measurements at average 11.6 A solar current. Morningstar Prostar-30M controller. Note that voltages are additive. Voltage drops in the positive and negative leads are added together to give total voltage drop. Differences between terminal voltages are equal to the sum of voltage drops between terminals (e.g. 14.64 – 12.84 = 1.30 + 0.14 + 0.36 = 1.78).

First, compare voltages versus design. The battery cable was 6 AWG (= 13.3 mm2) yet the voltage drop from controller to battery exceeded two per cent. The battery was not full and charging voltage drops to what the battery can accept during bulk charging. Overall, 12 per cent of the solar array voltage was lost and mostly in the solar cable. I have 15 m of 6 mm2 cable.

Second, compare solar panel measurements versus rated values. Current was 13 per cent below expected 6.7 x 2 = 13.4 A. Rarely have I seen more than 12 A from these Chinese panels, they perform like 110 W panels. For comparison, my old 54 W Kyocera panel frequently produces rated current and often a bit more. Japanese panels are better but way too expensive.

Rating Measured
Solar irradiation (W/m2 1000
Temperature (degC) 25 c. 40
Maximum power (W) 120 × 2
Voltage at max power (V) 18 14.6
Current at max power (A) 6.7 × 2 11.6
Open-circuit voltage (V) 22 20.6
Specifications for my Chinese solar panels. I have two panels connected in series so power and current are multiplied by two.

Measured solar panel voltage was less than rated because hot solar cells output lower voltages. The open-circuit voltage suggests cell temperatures were about 40 degC, which is reasonable for a solar panel in direct sunlight on a warm day. Secondly, PWM controllers operate near battery voltage and not at the maximum power point.

Resist the temptation to multiply amps by volts because the result will disappoint (e.g. 14.64 x 11.6 = 170 W). Amps are more important for small 12 V systems with PWM charge controllers.

Low charging current could happen if the solar panels are shaded or the sun hidden by clouds. Reasons for low charging voltage at the battery include: low panel voltage and current, excessive voltage drops (e.g. small cables, bad connections), heavy load on battery and a flat or bad battery. By checking voltages at several points in the system, one can isolate the source of problems. To troubleshoot controller performance, compare results with previous tests and/or retest the system with a different controller

Step 4 Check regulated voltage and current

When the battery is nearly full, the solar controller should start cutting back the solar current. A green LED on my controller starts flashing and there is a slight buzzing noise in PWM mode.

A fully-charged battery (Step 2 above) is advantageous here because you will not have to wait long for the controller to start regulating voltage and current.

The following table shows regulated voltages from my testing. Current decreased gradually during absorption (PWM) and was about 0.5 A in float mode.

Mode Measured
Volts
Expected Volts
(25 degC)
PWM 14.1 14.15
Float 13.4 13.70
Regulation voltages for Morningstar Prostar-30M controller set for flooded battery. Regulated voltage is temperature compensated (> 25 degC).

Wrong charging voltages could result from an incorrect battery selection (if selectable on the controller) or a bad charge controller. Under- or over-charging will reduce battery life and should be corrected.