Peplers In Rye: Parallel lead acid batteries with solar PV

Monday, 18 April 2011

Parallel lead acid batteries with solar PV

Just a quick update to my previous post about wiring up our off-grid solar PV system. The friend I bought the batteries off has swapped the dead one for one the others he had, so I now have a working pair:

I'd originally been planning to put them in series for a 24V system, but I've reconsidered and am currently going to keep it as 12V, partly for simplicity and partly because that gives me greater redundancy in future if parts should ever fail and prove difficult to replace.

There are several important considerations when wiring batteries in parallel:

It's best to have them the same make and capacity.
It's best to have them the same age and condition.
It's important to wire them so that the load and charging currents are spread equally across them.

I'd already got these first two points sorted. The last point requires making sure that the length of wire the current must travel to a load or from the charge controller is the same for each battery. If you don't do this, the battery that is nearer to the load/charge connection will discharge and charge faster than the other one, potentially resulting in premature failure.

The solution for two batteries in parallel is simply to connect together the positive terminals with a length of wire, connect together the negative terminals with another equal length of wire, and then make sure the positive and negative connections for the load and charge controller DON'T go to the same battery. This way, the current must always flow through one length of wire to reach either battery, so keeping them equally charged. Here's a pic, with the +to+ and -to- links on the right, and load/charge connections coming in from the left: