Quote:
Originally Posted by bwilson4web
I'm starting to look for A123 charging protocols. I didn't find a clean description at their web sites. So these are my initial thoughts.
I'm looking at two, battery management chips: - LTC6803 (Linear) - includes integrated MOSFETs for discharge balancing. However, discharge balancing limit appears to be 80 ma for a single cell, 0.25W. With up to 12 cells, two are needed.
- BQ76PL536 (TI) - external balance MOSFETs, current is design selectable. With a limit of 3-6 cells, three are needed configured 5 cells each.
Near as I can tell, these are series, voltage measurement chips with modest temperature monitoring. The LTC6803 has a modest cell balancing, up to 80 ma, one cell at a time, 0.25W capability. The BQ76PL536 drives an external MOSFET which increases the discharge current that can be used to discharge a cell.
I've been thinking about cell balancing and two approaches come to mind: - Discharge cells to a lowest dV uniform state and then when all are at the same low, state, let charging continue to when the first cell achieves a limiting, dV. To achieve reasonable times, the discharge needs to handle a hefty current, 1-5A. Once initially balanced, the discharge draw-down would be fairly short so 1A may be feasible.
- Top limit, uniform using a shunt regulator. This regulator can quickly become pretty complex if one tries to keep it efficient. A simple, series Zener stack would work but the total charge load would remain constant until the last cell reaches the Zener limit. Again, as the cells become balanced, the time during which individual cells complete their charge becomes smaller but at the end, the Zeners are handling the total heat load. Tapering the charge current at the cell voltages increase could mitigate the Zener load at the end.
You've had a lot more experience with balancing LiON cells. Any insights to share?
Also, looking at the solder tabs, have you considered spot welding? It would help if we had test articles but spot welding two or three 'dots' per tab would both block air and ensure metal-to-metal bonding. It would be mechanically simpler and only intra-tab insulation would be needed.
Thanks,
Bob Wilson |
If you read the first post in
my BMS thread you can see how I am doing things with my setup. I haven't really looked into the BMS ICs yet. I'm trying to get my current version working before moving on to using those.
As for balancing, I don't believe active balancing is really necessary. When initially assembling a battery pack you should choose how you are going to balance. You either balance at the top (fully charge each cell), or balance at the bottom (discharge each cell to a set voltage). There are advantages and disadvantages of both techniques. In general, bottom balancing seems to have greater advantages IMO.
The guys who have been running lithium cells for a while now have been monitoring their packs and have noticed very very little unbalancing over time. Personally with my PHEV kit, I have top balanced my pack mainly because it is easier. In about a year I plan to hook up my charger to each cell and see how much energy I'll have to put into each cell to top them all off. I am guessing it will be very uniform.
For charge protection things are pretty simple. The celllogs monitor all the individual cell voltages and when the first cell hits 3.65V the BMS shuts the charger off. This completely eliminates the CV portion of the charge. This charges my 39Ah mottcell cells (the cells in my PHEV kit) to roughly 95%. So, there is a small capacity penalty, but that also leads to longer cell life. This eliminates the need for any shunting and is thus more efficient too. It also elminates the need for a smart charger since the brains have been moved to the BMS.