Cheap LiFePO4 BMS?

[IanD]

11 hours ago, nicknorman said:

However it is not exactly the SoC vs voltage that is relevant, it is the internal resistance. Which yes of course is a lot lower for Li. But anyway the effect is still minuscule and virtually unmeasurable. What exactly is the problem that is trying to be addressed? It in my experience there is no significant SoC imbalance and even if there was, so what? Such a SoC imbalance would only occur in mid SoCs and as upper or lower knees were approached, it would all come out in the wash. There isn’t a problem to address.

The LFP internal resistance per battery forms a resistive ladder divider with the interconnect resistance; the voltage differences will be small (a few mV or maybe tens of mV at high currents?), but the flat SoC/V curve for LFP means this could mean significant SoC differences during charge and discharge.

 

If it happens (a big if!), the cell closest to the connections will hit the lower knee first on discharge and the upper knee first on charge, but this would only show up a relatively high currents as a cell imbalance. If the batteries are then left sitting there for long enough then they'll eventually equalise anyway, but this could take a very long time.

 

It won't be a problem in many cases -- but it might be in some, and the problem can easily be avoided using diagonal connections, so why not do this?

Edited January 16, 2023 by IanD

[peterboat]

34 minutes ago, IanD said:

The LFP internal resistance per battery forms a resistive ladder divider with the interconnect resistance; the voltage differences will be small (a few mV or maybe tens of mV at high currents?), but the flat SoC/V curve for LFP means this could mean significant SoC differences during charge and discharge.

 

If it happens (a big if!), the cell closest to the connections will hit the lower knee first on discharge and the upper knee first on charge, but this would only show up a relatively high currents as a cell imbalance. If the batteries are then left sitting there for long enough then they'll eventually equalise anyway, but this could take a very long time.

 

It won't be a problem in many cases -- but it might be in some, and the problem can easily be avoided using diagonal connections, so why not do this?

My drive batteries are 15p x 2s they are done diagonally, not as neat cable wise but it's what we have always done for LAs

[rusty69]

1 hour ago, peterboat said:

My drive batteries are 15p x 2s they are done diagonally, not as neat cable wise but it's what we have always done for LAs

Wow 15p, that's cheap. I paid 20 quid each for mine.

[magnetman]

15p X 2shillings so it's actually £1.50 I think.