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Lithium cell balancing

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BruceM:
I've noticed that LiFePO4 cells direct from China have come down quite a bit.  So I've been researching how to best handle cell balancing for a 120VDC battery bank (39 cells in series operated from 119V to 132V. (3.4 to 3.05V)  I've got a few years to get ready for the switch.

I wondered if Starfire or anyone else might chime in on homebrew designs?  I am familiar with the various commercial  balancing products, but they generate a lot more EMI than I'd like.  The modest current, passive balancers are lowest EMI but still it is connecting a microcontroller with SPI interface to all the cells.

mike90045:
I would prefer not to use balancers, but to order some extra batteries with the initial order, and then sort/bin the batteries, so you build a bank with all matched batteries, and top or bottom balance the bank.   (and hold in reserve the low capacity & high capacity cells )
Then just rely on the BMS to monitor cells in case one fails.

BruceM:
Some folks do nothing but manual  monitoring and balancing every few weeks, even months for the first year or so.  I think I'd like something automated.  I'm not getting younger.


 I'm now thinking that some passive top balancing (fixed voltage load switch type) and modifying my big linear PV regulator to be fixed voltage might suffice.  The commercial ones are very cheap but the trip voltage is preset too high for my planned scheme.  The ones sold for bike packs are only 35ma of discharge, only when the batteries are full and still on the charger. That's not much time or current.  For my off grid PV system, it would be most of the day every day, so might work out even at 50ma.

I've never used lithium cells at all so lack the practical experience with them despite doing a lot of reading on BMS/balancer designs. 

My experience with my own linear 12V battery regulators is very positive; the batteries last a long time and I can swap in a new battery without trouble.   I could implement the same thing for lithium cells, though it might be much more than is needed, and with 38 or 39 regulators needed, it's quite an expense and build!




mike90045:
This is a well written, correct article about setting up a Li bank
http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/
  enjoy

BruceM:
OK on the basics but he is not mentioning battery balancing hardware, even though the photo of the 4 cell pack clearly shows a balancing cable hooked up for a battery balancer. 

Daly makes a decent 4 cell LiFePO4 basic one for cheap, I just don't care for the added EMI of that type with a microcontroller.  There are also plenty of  balancers with current and low voltage protection as well, for 12, 24, and 48V packs.  It more serious money for a balancer for 39 cells in series, and again comes with EMI issues I don't want.

Because my bank is full every day, most of the day, I think a simple passive top side shunt regulator or top voltage switched load resistor should suffice.  Because of the number (39) required, I'm trying to resist the urge to get fancy.

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