Saturday, January 29, 2011

Active Battery Balancing

A while back, I had a problem with the AGM battery bank getting out of balance. This would have damaged it by having one 12V battery under charge all the time, while the other one over charged and gassed (eventually drying up and being killed).

Luckily, I found a fairly cheap (£29) battery balancer that has fixed this.
This one is from Rapid Electronics, made by a British company called Camden Boss.
You connect the balancer to the 24V + terminal and the 0V - terminal and the yellow wire to the 12V middle point. It then works when each battery is over 12.8V (when being charged) and shunts up to 1A of current across either battery, to keep the mid-point at exactly half the full terminal Voltage, ensuring that the batteries charge evenly and fully.

I actually ordered two of these but one hasn't turned up yet, on back order from the maker.

It gets a bit warm when it's working but as the batteries get to be equal in charge, it gradually stops taking power and cools down. So it wastes a bit of solar power by dumping it as heat, but that's better than a fried battery :/

When you've discharging the batteries, they are below 12.8V each and so the balancer does not work and draws no power.

In order for the balancer to work across all the batteries in the AGM bank, I installed the equalisation network that I said I was going to install months ago (but was too lazy to actually do :D ).
It has fuses in the links so that if there is a problem with one string of batteries or the balancing current is too big, the fuse will prevent a melt-down. It doesn't matter so much that the wires are not going to a star point as the current across the links should be zero (or close to it over time). The battery balancing module only moves 1A so that doesn't cause any real Voltage drop either.

The whole thing seems to be behaving itself perfectly and the six batteries now charge much more evenly and fully than before.



    Its wiring diagram uses n-1 number of CEQ12A (where n is the number of batteries).

  2. I have a question on your equalization scheme. At first blush it looks like if the upper left (say), battery had an internal cell short, then it wouldn't trigger either of the 10A fuses as the two upper right batteries tried to gamely shunt current through the upper star connection to it. Do you also have a fuse in each positive lead of the three upper batteries to prevent this?

    1. Yep, good point. I don't think I did but you should have a fuse on every battery close to the terminal to limit the damage of any short in the wiring (or internal fault in the battery bank).

      Adding fuses to multiple parallel batteries has its own issues, as the differing micro-resistances between the paths (from the fuses and their contacts) can create battery imbalances. Another reason to not use high power batteries in parallel and keep to single cells in single series strings.