Alternator current testing

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.

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2 Responses to Alternator current testing

  1. brad says:

    Nice writeup
    In ‘Measuring charging currents’,
    “Engine speed was 1500 rpm. Aux battery charging voltage was quite steady at 12.8 V.”
    Main volts was 14.26, so the difference is cable/connector loss! You said “The large measured voltage drop occurs because the aux battery is bulk charging”, I think this is wrong. Perhaps I’m misinterpreting something.

    • bulumakao says:

      Good point. There is no large cable/connector loss in the design. I reasoned that the voltage has to drop to whatever the aux battery is charging at. The alternator doesn’t know about the aux battery and still aims for around 14 V at the main battery. Perhaps someone else can explain better. The main point of this post is that we can make our own voltage and current measurements to evaluate the performance of charging systems.

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