Dual battery system design

I recently installed a dual battery system in my vehicle. Fridge and Solar have already written a fairly good overview of dual battery systems and alternator charging. This article adds some more details for designing and installing a dual battery system.

After writing this post, I have learned that simple relay isolators are not suitable for modern cars where alternator voltage is regulated by the computer. Do some research if you have a new vehicle. You might need a DC-DC charger. My 19 20 year old Mazda has the voltage regulator inside the alternator.

What you need (and don’t buy kits)

Wiring up a dual battery system is quite simple: the battery isolator is installed on the positive cable and fuses are placed at both ends. For many Voltage Sensitive Relay (VSR) isolators, a switch can be installed on the ground line.

Wiring diagram for dual-battery system with Voltage Sensitive Relay isolator. The main battery is connected to the vehicle's alternator charging system.

Wiring diagram for dual-battery system with Voltage Sensitive Relay isolator.

VSRs are popular for several good reasons: high current capacity, automatic connect/disconnect and simple installation. A VSR will connect the aux battery when charging voltage is sensed at the main battery. Switch the engine off and the VSR will disconnect the aux battery. A VSR should always keep the starting battery charged.

Some VSRs can be operated with a manual override, else a manual switch can be connected in parallel with the VSR. This allows starting the engine with the aux battery if the main is flat. Single-sense VSRs monitor only the main battery. Dual-sense VSRs monitor both batteries and will allow charging the main battery from the aux side.

I bought a Projecta dual battery kit. That was not a good purchase as several items were not adequate, not included or not required:

  • 4 m of red battery cable was supplied. I needed 5 m at least.
  • No fuses and fuse holders were supplied.
  • There were insufficient terminal lugs to install the fuses.
  • I did not use the three battery terminals in the Projecta kit.

I reckon you can buy all the components separately and save. Here’s an essential shopping list:

  • VSR isolator. A simple component but prices vary considerably.
  • Red and black cable. Large gauge battery cables can be very expensive. Car audio cables can be cheaper but they have softer insulation. I bought some cheap red car audio cable on ebay and taped the ends black for negative.
  • Mega fuse holders and mega fuses.
  • Extra ring terminals for the heavy gauge cables. Electrical wholesalers sell these.
  • Split corrugated tubing to protect the cables. I have also found heater hose and ‘pond tubing’ useful for protecting cables.
  • Battery terminal boots. The VSR terminals are at the same potential as the positive terminals on your batteries and the system is grounded to the chassis. For safety, the terminals on the isolator should be insulated, the same as the starting battery. A best practise is to remove both mega fuses before doing any mechanical work in the vicinity of the isolator.
  • Switch for the battery isolator ground (optional).
VSR mounted in the engine bay. Note the battery terminal boots and switch.

VSR mounted in the engine bay. Note the battery terminal boots and switch. The cable on the left is protected by ‘pond tubing!’.

Cable sizing

Larger diameter cables deliver higher currents with lower voltage losses. Fat cables are expensive and more difficult to install however. Selecting the minimum cable gauge depends on the length of the cable run from the main to aux battery:

Alternator maximum current, A Cable size Max. current
<40 <60 <80 <100 <150 AWG/
mm2 A
2 2 12 4 30
4 3 2 10 6 40
6 4 3 2 8 10 55
10 6 5 4 3 6 16 75
15 10 8 6 4 4 25 95
16 12 10 7 2 35 130
15 12 8 1 50 150
16 10 0 50 170
Dual battery cable sizing table, calculated for a 0.5 V drop (in one line) at maximum alternator current. Read maximum cable length in metres at the intersection of alternator current and cable size. Cable data from Wikipedia. The current ratings are only really important for the smallest cables (see realistic charging currents below).

I have a maximum 60 A alternator. The length of the cable run from the main to aux batteries is about five metres. I used 6 AWG cable as per the table above. Fridge and Solar recommended 6 AWG cabling for all installations, which may be inadequate for long cable runs and/or high output alternators.

Relay sizing

The relay should be rated higher than the maximum alternator current. My relay is rated for 100 A constant current. A bigger relay will only cost more and not perform better.

High surge currents are possible when connecting a fully charged battery to a flat battery. In the worst case I may have a completely flat aux battery (10.5 V under load) and a fully charged starting battery (12.65 V). Through 10 m of 6 AWG cable (round trip), I estimated a surge current of 166 A. The actual surge current will be much less because of internal resistance in the batteries. Over-sized cables and relays are really only for very high-current accessories (e.g. electric winch).

Fuse sizing

A blown fuse is better than something melting and catching fire. The fuse rating should be close to the maximum alternator current, even a bit less, and certainly no greater than the maximum current rating of any components and cables in the system. I’ve installed 100 A mega fuses (the smallest available?) and haven’t had one blow yet. It seems there are no large surge currents.

Battery selection and sizing

Firstly, different battery chemistries require different charging voltages. Absorbed Glass Matt (AGM) Valve Regulated Lead-Acid (VRLA) deep-cycle batteries are usually interchangeable with with wet low maintenance batteries. Gel cell VRLA batteries are not and will not be considered here.

I have written a post comparing AGM and wet (flooded) batteries. I have also measured charging currents.

For long service life it is best not to discharge deep cycle batteries below 50% State of Charge. I have a fridge that consumes an average 2 A.

Daily energy consumption is 2 A × 24 h = 48 Ah.

For one day, required storage capacity = 2 × 48 Ah = 96 Ah.

For two days, required storage capacity = 2 × 2 × 48 = 192 Ah of battery capacity.

For extended camping, many people use alternative charging systems rather than huge battery banks. Some carry generators. I use solar panels.

A second concern in battery sizing is the alternator load. Larger batteries will accept higher charging currents and place greater loads on the alternator. For AGM batteries, maximum charging current is about 0.29 times 10 hour capacity. For wet batteries, it’s about 0.12 times 10 hour capacity. Assuming no accessories load:

AGM battery capacity at alternator overload = 60 ÷ 0.29 = 207 Ah (not very big).

Wet battery capacity at alternator overload = 60 ÷ 0.12 = 500 Ah (huge!).

Charging current for an ‘over-sized’ aux. battery will be less than the maximum. It will equal the alternator current minus any accessories load.

Also note that smaller aux batteries will charge at lower currents. Smaller cables and fuses might then be possible.


Do-it-yourself installation saves money and if something fails on the road you will know how to troubleshoot and fix it. Do-it-yourself requires some trial and error to achieve a neat installation however.

Fuses must be installed very close to the positive battery terminals. Mega fuses are bulky and I made mounting brackets for the fuse blocks.

Mega-fuse mounted in the engine bay. The fuse at the aux battery was mounted on the battery box.

Mega-fuse mounted in the engine bay. The fuse at the aux battery was mounted on the battery box.

VSRs can also be quite bulky. My VSR was supplied with a mounting kit, which I modified to suit my installation (see above picture).

My aux battery is in the tray of my ute (pick-up). I made a custom battery box out of Medium Density Fibreboard (MDF), with a plywood lid. The batteries are held down by webbing straps. For installations inside the passenger compartment the aux battery must be very firmly secured to the of the vehicle. You do not want a heavy lump of lead flying around the cabin in the event of a rollover!

Battery box.

Battery box.

It’s easy enough to drill holes through the panels if needed. Wiring grommets are available in auto parts shops. Blanking plugs can be installed if the cables are ever removed.

Cables running through panels.

Cables running through panels with some silicon to improve the seal. It looks a bit messy but is normally hidden behind a seat.

Fridge and Solar recommended a cable negative return from the aux to the main battery. Actually, the chassis path has far less resistance than any practical size of copper cable. You save cable and the installation is easier using a chassis return. Just avoid ground connections through bolted on or hinged sheet metal (e.g. a ute tray). Use the shortest possible cable and ground it to any permanently integrated part of the vehicle body (e.g. floor, wall, etc.).

Terminating large diameter cables requires specialised equipment. I had an auto electrician solder the ring terminals onto my cables. That was costly and I later bought a new soldering iron: 60 W, with a large tip.


4 Responses to Dual battery system design

  1. arnoldsantos2008 says:

    How about reviewing this http://www.trueam.com battery isolator?

  2. Alo says:

    Hi Stephen,

    Great article.

    Is the switch to the earth made to override the VSR?


    • bulumakao says:

      The switch to earth is to turn the VSR on/off. I’ve also done this for Victron relays. The Victron relays also required an external switch for the override function (“start assist”). Each VSR can be slightly different so READ THE MANUAL.

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