LVC circuit for cheapskates?

[fechter]

The 'standard' LVC circuit for lithium batteries monitors every cell and triggers the cutoff when any cell gets below the minimum allowable (around 2.7v?).

I'm thinking there should be no reason I couldn't build a LVC circuit that monitors pairs of cells (assuming an even number of cells in the pack). Normally, one of the two cells in a pair will hit the cutoff before the other, so the cutoff voltage would need to be a bit higher then 2X the single cell value. So if I made 'pair monitors' that cutoff at ~5.7v, even if one cell craps out way before the other one, the low one will still be protected.

This would cut the cost of the parts to about half of every cell monitoring.

Any reason why this wouldn't work?
This approach is used by Toyota and Honda on their Nimh hybrid batteries. They monitor groups of 6 or more cells.

 

[disadvantage]

I think it would work fine, but I'm not sure you'd want to do it.

It makes sense to me to go ahead and spend $5 on the parts to protect a $50 cell.

 

[Malcolm]

Sounds like a good plan to me Fechter, I really like the idea of a bare bones system. I found a little circuit that uses a precision Zener regulator (LM431 I think) to activate a piezo buzzer or LED. I was planning to use one of these for every two or even three cells on a lightweight A123 pack. It would be great if you could come up with something similar that could be daisy chained, or would that add too much complexity?

The sharp knee on the discharge curve means this approach should work great on LiFePO4 in general and A123 especially. Gary Goodrum and Doc are obviously the experts on these cells, but 2.7V sounds like a pretty conservative value for the minimum allowable. I think the A123 BMS refuses to charge when a cell drops below 2.5V?

 

[Doctorbass]

I think the A123 BMS refuses to charge when a cell drops below 2.5V?

That's right, more precisly at 2.496 to 2.498V on most of the BMS

Doc

 

[Malcolm]

Thanks for putting me straight Doc

 

[safe]

12 Volt Battery

I think the cheapest way to go would be the 4 cell, 12 volt, "Smart Battery" style system that I was working on in a different thread. Using just enough MOSFET's to force "On State" or "Off State" the entire 12 volt unit and then using the PIC to do some minimal balancing across the cells. You might even forget the balancing entirely.

Of course this might not fit your definition of "cheap enough".