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Li-Ion BMS video: Top vs bottom balancing

4287 Views 5 Replies 5 Participants Last post by  EVfun
For those of you who use Li-Ion cells, but don't have the Li-Ion BMS book, I am recording a series of videos on Li-Ion BMS issues.

The first set discusses the 8 reasons why only top balancing is appropriate for battery packs for energy use.

I'll announce the next videos as they come out.

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There is more than one way to do things. Bottom balancing seems to be a viable option that DeWalt has chosen to patent. I thought it was interesting, a cell level BMS with shunt loads used to actively bottom balance. JR must be having a love/hate relationship with this :D

I have to disagree with your last reason for top balancing. If your controller has a user settable minimum input voltage bottom balancing will prevent any cell from being killed. You will kill one or a few cells if they are top balanced and the BMS fails to shut off or turn down the controller. The controller's low voltage input limit won't be able to catch a single cell pulled to zero volts if the rest of them are still a little above the minimum voltage. With a bottom balanced pack no one cell will be pulled under because that is where they will be together. They will all be dropping so the controllers low voltage input limit will kick in to limit amps and protect the pack.

That said, I have a top balanced pack at this time. Having tighter bounds on what is happening when I'm in bed (charging) is more important to me than protecting cells while I'm there in control. My smallest cells (by some slight amount I haven't seen) aren't regularly reaching the highest charging voltages to shut off the charger.
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FYI their balancer also top balances. Also worth noting is the very high failure rate of Dewalt packs.
How do you do that? The 2 balance points are opposed to each other. The shunts would have to be used at the end of each discharge and at the end of each charge on every cycle. That is the only way cells that may vary by 3% in capacity could be made to agree on both dead and full.

DeWalt packs are out there in millions of tools, some being abused every day. I know the Li cordless tools where I work are routinely run to dead and more than one battery is discharged by many of them each day. Tools using these packs have been available for a few years now. I expect some packs to die. This application is a really good life cycle test for consumer LiFePO4 cells and management systems.
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