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Old 08-03-2012, 11:25 PM   #48 (permalink)
bwilson4web
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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:
  1. 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.
  2. 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
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Last edited by bwilson4web; 08-04-2012 at 12:43 AM..
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