Well this has always been an interesting topic. Mainly because there is a lot of confusing or conflicting information about how to set up a multi-battery system out there. Some of the current trends we are seeing towards new product innovation also puzzles us. Before we get into some different designs or methods of installing two or batteries in your vehicle, it is probably best we go over a few principals first.

When two batteries are hooked together they act, if done properly, as one battery. When done as a parallel set up, that is + to + and – to -, the voltage stays the same but the CCA and reserve capacity doubles, and their resistance is reduced by 50 percent. That means if you have two 1000CCA batteries with 180mins reserve capacity, and 10mohm internal resistance, they will act like one battery with 2000CCA, 360mins reserve capacity, and only have 5mohm resistance. All of which are good things.

However in order to make sure that they function properly, the batteries need to be “twins” of each other. Each should be bought from the same company, with the same date code, same size, same rating, same everything. (There are a few exceptions we will talk about later). Further the closer their at rest voltage is the better. By that I mean when you put a volt meter on the each of the batteries they should all be the same to the second decimal place. An example would be 12.72 volts and 12.72 volts or 12.81 and 12.81. Most EV builders will go one decimal place further when putting together large packs.

There is a good reason to do this. Your alternator decides what amperage to create by turning itself on and off up to 333 times in one second. Each time it turns off it reads the voltage or state of charge in your battery(s). It then calculates how long it must turn back on for to bring the total system voltage up to a preset level. When you have more than one battery in the system the alternator is then taking the average of the batteries. So if battery A is sitting at 12.5 and battery B is sitting at 13.0, the alternator will think that there is 12.75 volts in the battery (it does know there are two batteries). When this happens there are two problems created. Battery A will not get a high enough charge, and battery B will be overcharged. This will cause premature failure in both batteries.

This is also why using a battery isolation circuit that does not charge both batteries at once is not a good idea. We are seeing these become popular in the 4x4 crowd. They operate by leaving one battery completely isolate from the main battery and charging system. Then are added to the system when extra power is required. Couple of major issues with this type of design. First the way they are added to the system they are not capable of adding there full share to the load. In tests we have seen alternators contributing 150amps, the main battery contributing 125amps, and the second battery only adding 40-50amps to the system. Lot of extra work and money for very little benefit. Second issue is when the alternator goes to recharge the two batteries, they are unbalanced so you end up with under/overcharged problem noted in the last paragraph. Last and certainly not least, once you start cycling (using) a battery it is not healthy for it to sit around doing nothing. So if the second battery ends up sitting for weeks or  months between uses, its ability to function and durability are negatively effected.

For optimal performance and durability multi battery systems should be wired so all batteries are tied together in such a way that the alternator and system sees them as one large battery. The key to this is to have power going in one side of the battery bank and going out the other. Likewise the opposite side of the batteries has the negative return path. So in a two battery system Battery A would have the positive from the alternator and the vehicle ground attached to it. Battery B would have the positive to all the loads in the system and if equipped the negative from the alternator. We have some illustrated examples below to help clarify this. 

Dual Battery Figure 0: Here we see the alternators positive and negative leads going to opposite batteries. Likewise the vehicle’s positive and negative leads are going to the opposite battery as the alternator’s. This works well if the alternator has an isolated (insulated) ground circuit. Typically this is only found in big truck and industrial type alternators. It can sort of be achieved in an automotive alternator by painting (insulating) the mounting surfaces of the alternator and the bracket it bolts to. This is not perfect, but since electrcity takes the path of least resistance, often has the same effect as a true isolated ground.

Dual Battery Figure 1: Here are two of the more common ways of adding dual batteries. Since the alternator’s ground is not isolated they are not as desirable as the layout in Figure 0. This system will still work very effectively, give almost perfect  performance and longevity. Standard cables are available for making the inter-battery connections, so cost of wiring is reduced. The only disadvantage is, you lose the use of the two extra available side post battery connections. They can be handly when hooking up extra loads such as winches or plows.

Dual Battery Figure 2: Same basic layout as Figure 1 with the exception the batteries are turned opposite of each other. Standard battery cables are available to make the inter-battery connection for most standard automotive group size batteries. Big advantage to this setup is you gain the use of the second battery’s two side post terminals.

Dual Battery Figure 3: This is the common setup used when batteries are on opposite sides of the engine bay. Often found in OE applications with the negative paths of each battery sharing a ground surface, typically the engine block.This is done to save manufacturing costs not because it is the best way. The system will perform better and have a longer lifespan if wired as pictured in Figure 3. This setup will require custom cables and with the price of copper today, it is not relatively cheap, and needs to be worked into your project budget.

Dual Battery Figure 4: This is a common setup where the two batteries are completely isolted from each other. common when you have a load used while the key is off. An example would be an RV or stereo at a tailgate party. The only issue with isolators is you lose 1/2 to 1 volt through them. meaning if the laternator is puttting out 14.9 volts the batteries may only see 13.9 volts of it.

Dual Battery Figure 5: Here is our approach to vehicles equipped with huge continous loads that peak at higher draws than the vehicles alternator can produce. i.e. Cars with major systems. If this may be you, please click the figure above for a full explanation of how this layout works.

Dual-Multi Battery Figure 6: Same basic setup as Figure 5 with the exception of extra batteries added in the rear. But using several batteries in parallel in the rear, you create a monsterous cap. Remember when we said everytime you add two batteries together their resistance is halved. Well by using 4 good quality batteries you can achieve almost the same response time as a digital cap, but have huge capacity. By huge I mean like 5000-10000 Farads.