GGoodrum wrote:what if we just used one of these clamping circuits to simply limit the voltage that each cell can reach to 3.7V. Then we can use any one of many existing SLA chargers, that also have the CC/CV charging profile, but at a pack voltage level.
Malcolm wrote:Couldn't have put it better. There are so many decent SLA chargers available that can do most of the job (and do it cheaply) that it seems overkill to start messing about with lab power sources and replicating stuff that's readily available. But how do you shut off the charging circuit if one cell takes too long to reach the cutoff voltage?
brandonh wrote:Some RCers already use CC/CV supplies to charge their A123s, and there's even a YouTube video showing how it's done with a Mastech 5020E supply.
I think there's an issue with your overvoltage-threshold-in-series idea. I'll assume that a CC/CV charge is being used, with voltage set to (#batteries*3.6v). In an out-of-balance pack, one cell will need longer to reach full voltage. That lagging cell might be at 3.4v, while the other cells are at the 3.6v target. Since the charger starts in CC mode (not at Vlimit yet), the other cells rise in voltage until the total of their voltages plus the lagging cell voltage equals the charger limit (#batteries*3.6v). At Vlimit, the charger switches to CC mode, with current through the string dropping quickly - many cells won't accept as much current. The cycle stops when all cells have reached at least the 3.6v threshold. But while the lagging cell voltage is rising, the other cells are all being overcharged! The cycle does nothing to balance the pack.
This is the whole reason for the shunt regulator "voltage clamp" design; it enables each cell to rise in voltage to exactly 3.6v, then accept no more current. The pack comes out of every charge cycle balanced.
Malcolm wrote: Great work Gary! I'm definitely interested in a version for the LifeBatt cells (12s please!).
Regarding charging: As the extra time/expense needed to add balancer plugs is relatively small compared to the cost of these packs I definitely think it makes sense to add them right from the start. It leaves options open for the future.
The clamp idea sounds good, but what's wrong with an existing solution such as the Astroflight Blinky: http://www.astroflight.com/store/store- ... 74RtU2x0b7
Would this not be able to dump enough current to cope with balancing a pack of more than 1p (2.3 Ah)?
Am I right in thinking that if I could find a fairly simple and practical way of switching cells in a pack from series to parallel that A123 cells and LifeBatt cells should balance themselves perfectly well without any help?
The idea is to run wires from each cell to a multi-pin socket (possibly two). The cells themselves would not normally be connected to each other. Instead, you have two plugs that fit each socket; one is wired with jumpers to give a parallel (balancing) configuration, and the other wired to give a series (run) configuration. If you fit the parallel plug whenever the pack is not in use you can keep the pack perfectly balanced until you need it.
You also have the option of charging the entire pack as a single parallel string at 3.7V, although I'm not sure how useful that would be.
Granted it's not a very practical idea for A123 cells, but for LifeBatt cells with their screwed terminals it seems workable. For a 12s pack I could use two 12-pin trailer plugs and sockets. As long as I use 10 gauge cable, solder the connections to the sockets and keep the runs as short as possible it shouldn't add too much resistance.
fechter wrote:If you use the shunt regulators on each cell and one full voltage charger, the shunt resistors won't dissipate any power unless a cell is out of balance. Normally, I think the shunts wouldn't be doing much since only a slight difference in current should keep the cells in balance.
Jozzer wrote:I think 10 amps is more than enough for most Ebikes, and more to the point, it is more than most of the chargers that are suitable for our use (old SLA/NiMh chargers) put out.
For motorcycle type applications, where more charging current would definitly be desireable, the board could be fitted with a heatsink, and mounted in open air or fanned, or a new board made with components chosen for higher loading...
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