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Li-Poly cell balancing

A lot has appeared in the model press on the subject of balancing the cells in a lithium-polymer battery pack. Basically, this was because these cells operate between well defined voltage limits and could be damaged by both overvoltage and undervoltage. My own feeling was that balancing was unnecessary if you were only using 2 cell packs. With greater numbers there was an increasing possibility of one cell becoming out of step. With enough cells it would be possible for one cell to be well outside the limits without this being obvious from the overall pack voltage.

I built a couple of balancers from the circuits on David Theunissen's Fly Electric site. These work by discharging the high cell(s) to match the low one. I used these several times and didn't reveal any problems, or find any improvement.

Two things then happened more or less simultaneously:
1. I acquired a balancing charger as part of a helicopter package (E_Sky Lama).
2. I bought a DVM purposely made to check Li-Poly batteries from Hillcott Electronics.

The balancing charger works from mains power or 12 volts and has output connectors (BST) for 2 or 3 cells.

Balancing charger Connectors for 2 or 2 cells

The DVM works by looking at each cell in turn and displaying the voltage. It saves a lot of messing around with connectors and is very convenient. It works with both 2 and 3 cell packs. With 2 cells, it displays a 'no' for the missing cell.

Cell1 Cell2 Cell3

The DVM cycles through the cells. Note that the decimal point tells you which cell is being checked

The most immediate use is that it tells you whether the battery pack is charged or not. If you have a lot of packs that one is invaluable. It displays the voltage to 2 decimal places (10 mV). This is fine for most purposes though some people may want something more accurate. What is less reassuring is that it consistently shows 20 or 30 mV difference between the cells with the first one displayed always being the lower. It also tends to show a greater difference after being used for a few seconds. So, if you need extreme accuracy look elsewhere.

Some of the things learned

1. Batteries which have been charged on a non-balancing charger show very similar voltages before and after use.
2, Batteries which have been charged on a balancing charger show quite a large difference after use.

I have a problem here because it would seem to show that a balancing charger is not a good idea. I added balancer leads to several packs that had been in use for some time so that I could check them. Charged on my usual (non-balancing) charger I got differences of 10 - 20 mV after charging and a similar difference after discharging the pack by flying it.

I charged both new and old packs with the balancing charger and got a similar 10 - 20 mV difference, but I got anything up to 300 mV (nearly 1/3 of a volt) difference after flying them! This is rather odd and I am working on ways of confirming this. I used the balancer to equalise a couple of these before charging and got similar results.

My one doubt in all of this is the accuracy and repeatability of the (very cheap) DVM. As it uses an LED display, it clearly puts a fair load on the battery. This is something else that needs checking. I'm working on a device which will commutate through the cells like the above but attached to a decent DVM/voltmeter.

I have recently scrapped a couple of 2 cell Li-Poly packs at just 14 and 15 cycles. These were cheap packs and had poor performance from the outset. One cell in one pack began to swell (actually, the better of the two packs), so I disposed of both packs. The point here is that both packs had ONLY been charged on the balancing charger, This is far from conclusive as I only have one balancing charger, but we shall see.

To be continued...

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