Advice: Joining Batteries In Parallel

If your caravan was fitted out by its manufacturer with numerous 12V appliances, from wireless phone chargers to a compressor fridge, there’s a good chance it was also fitted with two house batteries joined in parallel.
But what does this mean, exactly? And can you create your own parallel battery ‘bank’

PARALLEL UNIVERSE

Running two deep-cycle batteries in parallel means they are connected to each other in order to increase the capacity of the 12V system, in amps, without affecting the voltage – i.e., 12V DC energy remains available to power the system.
Two 100Ah deep-cycle batteries joined in parallel would provide 200Ah of 12V power, three would provide 300Ah of 12V power, and so on.
In theory, yes, a layperson could parallel their own deep-cycle battery setup; however, there are a number of traps which make it a good idea to have it done by a professional.
For example, failing to use cable of adequate size to join the batteries, as well as using appropriate fuses, will detrimentally impact the performance of the paralleled batteries, defeating the purpose of the exercise.
BMPRO ambassador David Bayliss, who has decades of experience working with RV battery systems, says it’s also crucial to use batteries of the same chemistry.
“AGM, calcium and gel batteries have different charging voltage requirements, so if you were to parallel an AGM with a calcium battery, for example, it would be impossible to get them to charge properly to act together as one unit,” he said.
Unfortunately, batteries degrade over time and their capacity reduces as a result. For this reason, it’s important to ensure the batteries being paralleled are of the same age. This will ensure the batteries operate as one bank, with all battery cells balanced and in the same condition.
“It’s not really possible to properly charge an ‘unbalanced’ bank of paralleled batteries,” David Bayliss said. “This is due to the internal resistance of the battery cells, resulting in one part of the battery bank receiving more current than another.”
The take-home message here is, if you want to create your own parallel battery setup in order to increase the amount of 12V appliances you can run in your RV (or perhaps run some 240A appliances through an inverter), it is important to start with batteries of the same chemistry, capacity (100Ah, 120Ah, etc.), and age.

USING A CAR BATTERY

Is it possible to parallel a deep-cycle battery with a car battery? Not if you expect the system to work safely and effectively.
A deep-cycle battery is designed to deliver a certain amount of current over an extended period of time, while a car’s starter battery is designed to deliver a short surge of current in order to start the car. These inherent differences make them incompatible for running in parallel.

THE LITHIUM OPTION

By the time you’ve purchased two new 100Ah batteries of reputable quality and paid a professional to have them installed in parallel, your wallet is likely to be around $1000 lighter.
If it’s in your budget, it would be worth considering forking out the extra $1500 (or thereabouts) for a quality lithium battery of equivalent amperage.
Not only are they considerably lighter than their lead-acid counterparts, they offer numerous benefits, from charging efficiency, depth of discharge and overall longevity, that over time will offset their higher purchase price.
Furthermore, while it is possible to wire lithium batteries in parallel, for most RV applications, a single 200Ah lithium battery would be sufficient.