My power supply arrived today, I set it to 25.1v, hooked up my 4 6S lipo packs in parrallel and away it went. It started off at 8 amps (rated at 6a) at around 23v then progressively got lower as the voltage went up. At 24v it was 5a, 2a at 24.5 and I turned it off when it was reading 700ma at 25.0v. Turning the voltage adjust pot up to around 26v while charging momentarily increased the current but obviously I can sit there tweaking that to keep the where I want it.
I was under the impression these power supplies were CCCV, but that's not terribly constant current
Any suggestions ? Is this no good for charging my lipos as intended ?
Actually, this is might be normal. I say might, because although it is normal for cells to slow down the amount of current it lets in, as it gets fuller, it depends on the rate. Normally, the cell voltage rises at a steady rate, as it becomes fuller. There is a point, however, where the rate of this voltage rise suddenly changes to a much faster rate. At that point, the cell is about 85% full. For LiFePO4 cells, this "knee" in the curve is about 3.65-3.70V. For LiPos, this number is 4.2V. What needs to happen to get the last 15%, or so, into the cell, is hold the voltage at this point (CV mode...), and let the current taper off. Once it is below about 25-50 mA, the cell is about as full as it is going to get.
With multi-cell packs, however, it gets more complicated. If all the cells are perfectly balanced, they will all reach this crossover point at the same time, so you can treat the pack like it is one big cell. Once the cells get out of balance a bit, you end up having a case where the cells start reaching the crossover point before the others, and the cell voltage starts to quickly rise, while it also starts limiting the current it lets in. The thing about current is that all the current has to go through all the cells, so if one cell is limiting the current, it is also limiting it for the rest of the cells, which still can accept a higher amount of current, but now can't get as much. The net result is that these cells don't end up with a full charge, which further increasesthe amount of imbalance.
Compounding this further is if you are simply connecting the main pack leads in parallel, and not the individual cells, because now you have cells that are not only not in balance with the other cells in its own pack, but between packs as well. The first thing I would do is check the voltage for each cell in each pack, and see how close they are. If there is more than about .05V difference between them, I'd use a balancer on the pack. Next, you can reduce the amount of potential imbalances during charging by connecting all the balance plugs on each pack together in parallel. This ends up giving you 6 blocks of 4 paralleled cells. When cells are put in parallel, they automatically equalize to the same voltage. Then when you charge, it will be like one big pack.
If you are not regularly using balancers, or a BMS board, you need to check the individual cell voltages for all your packs on a regular basis. If they get too far out-of-balance you can end up with one cell hitting the cutoff way before the rest, which can cause catestrophic results. If Lithium-Cobalt cells are allowed to get above about 4.3V, they will explode in a fireball, and will burn white-hot until all the cells in the pack have cooked off. That's the big difference between LiPos and LiFePO4-based cells. Tha latter can be overcharged to extremes (10V+...) and they will not explode, or catch fire. Most RC chargers these days either individually charge each cell, or use integrated balancers and check the voltage level for each cell during charging, shutting down if it detects an overvoltage condition. By simply using a 25V supply, you don't have any such protection, so you need to take extra precautions.