Foxy

Cell balance and Lipo cutoff...

When we run our RC vehicles, eventually our lipo cutoff kicks in and tells us our batteries need to be changed. It is rarely the case that each cell of a 3 cell battery will have discharged at exactly the same rate (though we hope they stay pretty close, if not, then that would indicate a problem with one or more cells). So, when the cutoff kicks in, maybe one cell is now at 2.8v, maybe another is at 3.1v, maybe the third is also 3.1v. If we were to charge all three cells with the same power ('fast' charge or NON-balance charge), obviously, the highest voltage cells would fill up first, then the charger will stop, because it doesn't want to 'overfeed' the cell that is now full (that is a highly simplified analogy, it's not exactly like that, but for the layman, that explanation is good enough). So, we now have some small differences between the cells. At this point that they are only a few millivolts out, it's not really a problem, but the next time we run the car, they will become a little more out of sync, and a little more, and the same goes for each non-balance charge. Eventually it will reach a point where the cell voltages are so different, that one or more cells is being discharged way too low causing permanent damage to that cell and rendering the pack useless.

Where the 'balance' charge differs, is that when it sees the pack is starting to get full, it starts to charge each cell individually, stopping when a cell reaches peak (usually 4.19 to 4.21 volts) and moving onto the next one (or it may do it in a round-robin way, it depends on the charger). It keeps going in this fashion until it has topped up all cells and the pack is considered once again 'balanced', with all cells at as close to the same voltage as possible. If a cell gets badly out of balance, even 2 hours on the balance charge may not be long enough to correct it (most chargers have a time limit to prevent heat build up in the equipment). When the balance charge seems to go on forever, this is probably good indication that your pack is dead or dying. If this happens to a 3S or more pack, and you are handy with electronics, you could consider removing the bad cell and effectively creating a good pack with one less cell than before.

Of course it is therefore a good idea to use balance charging mostly, and I do. Mostly. It depends on circumstances. If you are in no rush, charging at home the day before a race or a big bash, always balance charge. If you are at the bash spot and about to run out of batteries, feel free to put one you just used on the charger on a normal (often called 'fast' or NON-Balance) charge, perhaps even at 2C to get it done quick (a non-balance charge at 2C will usually see the battery fully charge in about 25 minutes). You won't hurt it to do this once or twice, especially if you know the specific battery is well matched and is a well-behaved ack. I guess I don't need to say don't do this with a battery that you know is not balancing well. Even with your good packs, make sure that when you get home you balance charge it (or storage charge it - more on this later) properly. So my rule of thumb is; in the field, normal/fast (non-balance) charge at 1.5C or 2C (7.5A to 10A charge rate based on our example), and when at home, take the opportunity to balance and/or storage charge, ensuring the health of your packs.

So why is it so important to balance the cells? It's because of the permanent damage that can occur to cells that drop below a minimum voltage. The safe minimum voltage for a lipo cell is actually approximately 2.8v (remember they are nominally 3.7v per cell), but most people sensibly choose to set their cutoff at 3.0 or 3.1v per cell, some even go much higher, though I question the value of reducing your runtimes so much for the sake of a little extra longevity of a 40 buck battery. That sounds fine, I hear you say, and in theory it is, however, in practice, your ESC isn't actually cutting off at 3v per cell, it's cutting off when the pack total reaches the stated cutoff. 3v per cell in a 2 cell pack is obviously 6v, so that's what your ESC is looking out for, 6v total. If it were a 3 cell pack, 9v total, etc etc. Now let's see how this changes the game...

Let's take a real world example.... Johnny has had his Traxxas Stampede VXL for a few weeks, he got a couple of lipo batteries with it, but he doesn't know much about lipo and the cheap charger he bought isn't exactly plying him with information, so he thinks he's good doing a 'fast charge' every time. Who wouldn't? Faster is better! But what's been happening to his batteries is the following... Battery A is a 5000mAh 2S 25C pack. Unknown to Johnny, one of its cells isn't well matched to the others, as is common. So each time Johnny runs the car, one of the cells gets a bit lower than the others. Johnny's VXL speed control is programmed to cut off the power from a 2S battery at 6v, as we said above, but it isn't keeping an eye on individual cell levels, all it cares about is total voltage. Each time Johnny runs and then charges the batteries without balancing them, this mismatch gets a bit bigger. Yesterday, Johnny ran his truck as normal, but noticed that he got a little less runtime than usual. He doesn't think twice about it, goes back inside, puts the pack on charge only to be told 'Error' by the charger. Uh-oh, one of Johnny's cells has died. But Johnny had his lipo cutoff set correctly, what went wrong?

Johnny's problem comes from the weaker cell. Each time Johnny ran the truck, the cells were getting more out of balance, and this imbalance was not being corrected during the charging process because Johnny doesn't know to balance his packs. The last time he ran, the auto cutoff worked perfectly at 6v, but what nobody could see, was that in actual fact, those 6v were made up of 3.5 in one cell and 2.5 in the other. The cell that dropped to 2.5v is now dead, having dropped below the danger line of 2.8v.

That's why we balance charge when we can, cos ESCs are stupid and lipo cells are sensitive.

A quick note on Storage Charging to finish off this chapter on charging.

Most lipo chargers these days include a program called 'storage charge', and it absolutely should be used for exactly that, storage. When you know that the pack is not going to be used for a few weeks or more, you really should storage charge it. This is a charging program that will set the cell voltages to their nominal level, usually around 3.7v per cell. Lipos like to be stored at this 'mid-charge' point, and will hold this voltage for a surprisingly long time, many months. Why do we do this? Two reasons, to prevent the over discharging that we mentioned earlier, and also to prevent swelling from long periods sitting full.

We already talked about why we don't let packs get below 2.8v per cell, and the same is valid even when packs are not in use. Despite the fact that lipo batteries hold their charge for a very long time, they do slowly discharge, like all batteries which are not used. Of course this allows for the possibility that if they sit for long enough, they will discharge below the minimum level and cells will go bad. That's why we definitely don't store them empty, as it would not take long for this low limit to be reached.

The other way (storing them completely full), is less of a risk, but still invariably causes damage over time. This is actually the way that I have lost some of my lipos over the years, I tend to be lazy and not discharge them if I don't use them. This causes them to swell and damages the fine layers of material in the cells, again eventually rendering them inert. Many say that they have stored fully charged batteries for a long time, myself included, without any issues, it seems to depend on the pack. Better packs seem to survive longer, but at the end of the day, there's no need to risk it, just storage charge them when not in use and refresh that storage charge about once every two months in periods of extended disuse.