Understanding 12V Ground In Dual Battery Systems

Often reading manufacturer directions, they direct people to run a ground from the battery terminal to the device. What does this really mean for you doing your own vehicle wiring?

Additional Battery Ground

When you install a battery into your vehicle you ground the negative to the body. The cable size will depend upon the rated amp hour of the battery being installed. If you install a 100ah battery, meaning it can continuously draw 100ah safely until discharged, then your ground cable should be about 50% larger (150 amp cable). This does two things. Firstly, it provides a lower resistance as the return path. Second, it gives a little room for short duration spikes.

All batteries installed in your vehicle must have a ground to body.

Never run your ground solely from the main battery to the second battery, without also ensuring each battery negative is individually grounded using the above method.

Why Ground Each Battery?

Because electricity completes its circuit on the shortest path.

Using the above understanding, if you grounded only your main battery, then ran a ground from main negative to secondary battery negative, and then connected loads to your secondary positive, you are now exceeding your main grounds capacity as you have effectively connected your negatives in parallel, yet use each positive uniquely in isolation.

Simply, you added your second batteries amp hour capacity to your main batteries single ground cable. This is how you burn your vehicle to the ground.

Don’t confuse running a device from battery negative with every battery still uniquely grounded. Electricity travels the shortest path, which means for any load on any battery positive, that load will return on that batteries ground, and not via another. That is how electricity works. Shortest path to complete circuit. Running a negative to negative cable is simply taking out insurance.

Using Busbars

Adding cable after cable to a battery negative is not feasible, and this is what many think after reading manufacture directions. In fact you’re just looking to burn your vehicle to the ground doing this. Alternatively, adding terminals to random bolts near a load for its ground, just as bad because you have no easy way to fault find a potential issue.

Busbars are the industry standard solution.

You take a single cable from your battery to your busbar and connect it. Depending on the busbar, you then connect all further loads to that busbar, whether screw or bolts.

Caveat — for vehicles with a smart alternator, you should make your busbar ground from the chassis ground ensuring you’re on the body ground side of the shunt installed (not battery direct) which measures load to your starter battery to initiate the smart alternator for charging.

Apply this for your positive too, just make sure you buy insulated busbars for positive use. Even ignition wire if you have multiple devices requiring ignition connection.

Busbars are the industry standard to make multiple quality connections that can all be traced to a single point of origin. They’re secure connections versus trying to connect everything directly at the battery terminal.

You can buy busbars for positive fusing, or fuse blocks with common pos / neg bus.

This is how you do things properly and not risk burning down your vehicle. Busbars can be placed where you mostly need them, engine bay, cabin or rear tub / canopy.

Busbars keep your vehicle wiring safe.

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