In this article I want to write about a fault I found in my dual battery system in my 100 Series Landcruiser which was installed before I brought the vehicle.

The main reason I wanted to check the system first was that we were only able to recharge the auxiliary battery to 11.6 to 11.9 volts no matter how much driving I did. To give you an idea how poor this is: 12.1 volts is somewhere near half charge!

To test how well the system was working I first tested how much charge voltage we were getting into our start battery, this was to test check that our alternator was putting out enough voltage to charge the batteries. This test revealed 14.3 volts, which was within acceptable limits for this particular vehicle.

Time to move on to see how much charge we had at the auxiliary battery. With the vehicle idling we noticed that we were only receiving 12.65 volts at the auxiliary while receiving 14.25 volts at the start battery, this shows a voltage drop between the two batteries of 1.6 volts!

As alternator based systems have trouble fully charging auxiliary batteries as it is, a 1.6 volt drop was always going to make it difficult for the auxiliary battery to charge up anywhere near acceptable limits.

The voltage drop could occur from two likely sources, either voltage drop through the positive cable and dual battery isolator of the dual battery system, or the earth system.

The earth system seemed, on visual inspection to be fine, a short (30cm) 4 B&S battery cable connected the negative terminal of the auxiliary battery to the vehicle body and all looked well fitted. With this in mind, I started our voltage drop test with the positive side of the dual battery system.

To test this we placed the positive lead of my multimeter onto the positive terminal of the start battery and the negative lead of the multimeter to the positive terminal of the auxiliary battery. This shows the drop in voltage though the positive cable and the isolator. The amount of drop will vary depending on how much current is flowing through the circuit, however, at 0.03 volts drop in this instance there was no sign of a problem here.

The next test is the negative side of the system; we placed the positive lead of the multimeter on the auxiliary battery negative and the negative lead of the multimeter on the negative terminal of the start battery. This test revealed our problem, a 1.6 volt drop between the negative terminals of both batteries. This more than explained our lack of ability to recharge the auxiliary battery.

As you can remember from earlier in this test, we were quite happy with the visual inspection of the negative side of the dual battery system. So where was the problem? It turns out that where the earth cable from the auxiliary battery was bolted into the vehicles body went into a thread that had not been used on the vehicle before, and was heavily painted. This paint caused a massive voltage drop in the system, and lead to our problems. To fix this I tapped out the thread to remove the paint and removed the paint from where the bolt head and washer seated on the body. Another trick I used was to use a star washer between the body of the vehicle and cable lug to ensure the washer dug in and gave a good connection through any remaining paint work.

This reduced our voltage drop from 1.6 volts down to 0.12 volts drop. This improved our charging voltage from 12.65 volts to 14.20 volts and increased our ability to charge the auxiliary battery from 11.9 volts to 12.83 volts. The particular battery is fully charged at 13.2 volts on a CTEK battery charger to give you some idea of the improvement in recharging.

Another potential problem to be aware of is a poor earth lead from the start battery to the vehicles body. This could have the same effect on the system. In my opinion the only way to professionally install a dual battery system correctly is to run a separate earth cable directly from the auxiliary battery to the start battery. This helps eliminate any future earthing problems and reduced our voltage drop by a further 0.05 volts bringing up our ability to charge the battery to 12.88 volts instead of 12.83 before we installed the dedicated earth cable.

This is a great test anyone with a dual battery system and a simple multimeter can do at home to ensure their dual battery system is performing to its maximum ability. This is very important to the performance of your dual battery system, as the level of voltage delivered to the second battery has two major effects apart from the obvious one of not fully charging the battery.

The first effect you will notice is that the current flow from your alternator to your auxiliary battery will be a lot less than at higher voltages. It is the differential in your alternators charge voltage to the auxiliary battery’s voltage that will determine how many amps flow into the auxiliary battery and thus, how quickly it charges. Higher voltages will see much quicker charging times and of course much more useable amp hours of power from your auxiliary battery. The second effect you will notice over a longer period of time is how long your auxiliary battery lasts. Basically if a battery is kept under 12.4 volts of charge for an extended period of time the lead plates will sulphate and the life of the battery will be significantly reduced. Many of you will have heard of auxiliary batteries in dual battery systems lasting only 18 to 24 months. This is the cause, leaving your battery at a low state of charge will significantly shorten its life expectancy.