General > General Technical Chat
LiFePO4 balancing.
paulca:
On this particular BMS you get to set quite a lot. The preferred balancing method is to do it during or close to absorption. So when the cell voltages are below say 3.50V no balancing occurs. When cells are above that they become balance candidates. I am not sure, but similar at the bottom end might be nice too to prolong the "single low cell" cuts pack.
Siwastaja:
--- Quote from: paulca on March 09, 2023, 08:15:09 am ---I find the balancing is most important near the top and near the bottom.
--- End quote ---
You can't balance at both top and bottom. You choose one (usually top), and then the pack is out of balance at the other end, because capacities differ. That's life.
As I explained above, you could design a high-power, redistributive, high-efficiency balancer which rebalances the pack during each and every cycle, so that it drifts from top balance to bottom balance. But this would be colossally expensive, for a small gain in added capacity.
Remember that balancing is only for maximizing energy. Cell-level LVC and cell-level HVC is needed anyway. It's hard to guarantee perfect balance (for safety). Hence, safety is always provided by the HVC and LVC; when any cell hits any limit, charging/discharging the pack is stopped. In a perfectly top-balanced pack, HVC happens in every cell nearly at the same time (so it's basically random which cell triggers first).
paulca:
Dumb idea, but...
A single "auxilary" cell. It is not part of the main pack.
Under discharge when a cell gets within 10mV of LVC the aux cell is used to support it, using whatever concuction of relays/mosfets to float that aux cell in paralell with the low cell and rotate across them supporting the low cells while we drain the last bit out of the high cells.
Under charge, once the main pack has cells exceeding "Nominal charge" voltage when a passive resistive balancer would cap it, instead the aux cell is placed in parallel with the high cell to pull it down. Completing the charge when all cells including the aux cell are at nominal charge voltage.
Siwastaja:
Good idea, but WITHOUT disconnection of the "aux cell". I did exactly that with the LFP EV pack mentioned above; out of the 26 (IIRC) cells, I think two had significantly less capacity than others, bringing the usable capacity of the whole pack down. I simply added some tiny 2Ah cylindrical cells (of the same chemistry, of course - voltage curve needs to be similar enough) in parallel to get their capacity up. Worked fine.
All you need to do is to run the pack to empty once and see which cell(s) caused the LVC, and increase their capacity by adding the tiny aux cell (permanently in parallel).
Dynamic disconnection/connection is a lot of work and expensive components, and if something goes wrong and your logic connects the cells together at wrong time, it's catastrophic, as large currents will flow unless the voltages are equal to maybe +/- tens of mV.
But make sure you have a problem with capacity differences first. If you buy decent cells, they are very close to each other in capacity, so there is not much to gain.
This is a more general comment: make sure you understand the problems you are solving (or if they exist at all) before thinking about how to solve them.
paulca:
--- Quote from: Siwastaja on March 09, 2023, 12:00:26 pm ---Dynamic disconnection/connection is a lot of work and expensive components, and if something goes wrong and your logic connects the cells together at wrong time, it's catastrophic, as large currents will flow unless the voltages are equal to maybe +/- tens of mV.
--- End quote ---
Yes. My thought experiments around cross linking between cells etc. Tended to include, "I'm a software engineer and no, I don't trust even my own code during development with real cells.", not without safety interlocks.
I was considering using a chain of MCU->74H logical gate array-> Mosfet drivers -> Mosfets. The logic array hard coded to prevent any invalid mosfet configuration with basic NAND gates.
It is far, far too much effort for an "hobby" class nano gen off grid system. Might be a different story if you are developing the next bleeding edge multi-cell EV pack and have 1000 cells to balance and a product with a £10-15k market value.
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