Balancing Batteries

When connecting batteries in parallel to get more capacity, you have to be careful to balance the individual strings of batteries so that they all do the same amount of work.  Otherwise, some will become more discharged than others and suffer damage from hard lead sulphate forming on the plates that cannot be dissolved by charging.

Having batteries of different sizes is also not a good idea as the small ones will discharge to a greater extent than the bigger ones and so suffer again.

So what did I do?  I bought a bunch of different sizes and types of battery and connected them all up together... Go figure.

So, how do we bodge this so that the batteries all stand a chance of surviving?

 

The main block of batteries are gel types and 180Ah in size.  They have a rating of 1,000 cycles at 50% depth of discharge (the deeper you discharge a battery, the fewer times you can do it).  The Marathon ones I've just bought are AGM types and 104Ah in size.  They are intended for computer uninterruptible power supplies (UPS) and as such are not expected to be discharged every day (if ever).  They probably have a rating of 250 cycles to 50% discharge.  But if discharged by only say 25%, they might last nearly 1,000 cycles.

So, to balance the different batteries lifespans, you have to try to balance the amount of work each does. To do this your main tools are size of battery and the wiring that connects them.  Thinner wires resist the flow of current and so large loads will drain batteries connected by fat wires faster than ones connected by thin ones.

In the diagram above you can see how I've wired the batteries together.  The big gel ones are directly connected to the solar chargers and the AC inverter by very heavy gauge 35mmsq cables.  This lets the gel bank do the bulk of the work when under high load. The weaker Marathon batteries are connected in 3 groups that use a star wiring pattern.  This means that each group is connected by wires that are individually quite thin (6mmsq).  But the wires are deliberately a bit long and are all the exact same length (hard to draw so take my word for it).  The other important fact is that they are all connected together at one point (I soldered them together after weaving the ends into a sort of knot. This means that, as far as possible, the wire resistance for each branch is the same and so each group of batteries will do the same work.

The whole group of six batteries is then connected to the main bank by a single pair of 6mmsq cables with a 30A fuse.  The fuse is important as 6mmsq cable can only carry about 50A and a fault could cause a few hundred Amps to flow.  If the main battery dried up or got a short circuit somewhere inside it, a very large current could flow and start a fire.  Actually, each of the groups should have a fuse but adding fuses in every leg makes it difficult to balance the resistances as the fuses and connectors introduce variances.

So, by having the Marathon battery bank 1.7 times the size of the gel one and using thin wires to connect it, I hope to keep the workload low enough on the Marathon bank to make it live for as many cycles as the gel ones.

When running loads, I've noticed that the gel bank does do more work (delivering about 2/3rds of the current).  But as the gel bank starts to run down the Marathons start to assist more.  This is because gel batteries are designed to resist deep discharge by being acid starved.  They don't have enough acid to fully convert all the lead in the plates.  As the acid runs out, the gel battery increases its internal resistance and so does less work until the output voltage falls to a point where the protection in inverters causes them to turn off.  

Because the resistance of the gel battery increases as it gets discharged, the balance of resistance between the gel bank and the Marathons will change.  At some point, the Marathons will have the lower resistance path and so assist the gel bank by doing more work.  So it's more important that before for me to stop discharging the bank before it gets much below 50% as beyond that point, the Marathons will start to be drained quite quickly.  But even then, that shouldn't be a big problem as they are 1.7 times bigger than the gel bank.

Tonight we're at 50% as the last couple of days has been very gloomy (it rained all day today).  Hopefully tomorrow will bring some sun.

I'll have to watch the charge in the morning as the reverse is true for charge balancing.  Because the gel bank is low and has high resistance, the Marathons will accept more charge current than the gel battery (at first).  The charger can deliver up to 73A and could cause the 30A fuse on the link wire to blow...