3 ms·

I was told that LFP is difficult because the battery management system (BMS) cannot reliably guess the state of charge using the voltage. The voltage is a mess

I was told that LFP is difficult because the battery management system (BMS) cannot reliably guess the state of charge using the voltage. The voltage is a mess on LFP. Even though a lower voltage prolongs the battery, you have to charge to 100% regularly to help the BMS. Thankfully the LFP chemistry doesn’t seem to mind high state of charge as much. As a popular source, Tesla recommends to daily charge the LFP batteries to 100%.

andruby 2y ago
> The voltage is a mess on LFP That's a weird statement. The voltage curve for LFP [0] is just a lot flatter. That's nice for your electronics, but makes it hard to estimate state of charge based on the voltage alone. So the BMS needs to keep track of total energy used to calculate state-of-charge. [0] https://www.jackery.com/blogs/knowledge/ultimate-guide-to-lifepo4-voltage-chart https://www.jackery.com/blogs/knowledge/ultimate-guide-to-li...
- Filligree 2y ago
  Excellent link, thank you!
- speedgoose 2y ago
  True, it’s very flat. And If you add some major temperature changes over many days and a descent amount of non full charging cycles, it’s more challenging. Maybe I wasn’t told it was a mess, but it’s how I remembered it.
- Kirby64 2y ago
  The problem is that total energy used is never fully accurate so coulomb counting can only be so good. Top and bottom balancing is needed to reset the algorithms, otherwise state of charge is largely a guess especially over time.
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fy20 2y ago
The 100% charge thing is also for balancing, as most BMSs are pretty dumb and only use passive balancing. When an individual cell reaches a set voltage, a resistor is turned on which bleeds the excess power preventing that cell from overcharging. When the pack reaches 100% theoretically every cell should then be at the same voltage.
- Filligree 2y ago
  If that's the case, then if you don't regularly charge to 100% you can end up depleting any cell that's out of balance. Good to know.

andruby 2y ago

> The voltage is a mess on LFP That's a weird statement. The voltage curve for LFP [0] is just a lot flatter. That's nice for your electronics, but makes it hard to estimate state of charge based on the voltage alone. So the BMS needs to keep track of total energy used to calculate state-of-charge. [0] https://www.jackery.com/blogs/knowledge/ultimate-guide-to-lifepo4-voltage-chart https://www.jackery.com/blogs/knowledge/ultimate-guide-to-li...

Filligree 2y ago
Excellent link, thank you!
speedgoose 2y ago
True, it’s very flat. And If you add some major temperature changes over many days and a descent amount of non full charging cycles, it’s more challenging. Maybe I wasn’t told it was a mess, but it’s how I remembered it.
Kirby64 2y ago
The problem is that total energy used is never fully accurate so coulomb counting can only be so good. Top and bottom balancing is needed to reset the algorithms, otherwise state of charge is largely a guess especially over time.

deleted 2y ago

[deleted]

fy20 2y ago

The 100% charge thing is also for balancing, as most BMSs are pretty dumb and only use passive balancing. When an individual cell reaches a set voltage, a resistor is turned on which bleeds the excess power preventing that cell from overcharging. When the pack reaches 100% theoretically every cell should then be at the same voltage.

Filligree 2y ago
If that's the case, then if you don't regularly charge to 100% you can end up depleting any cell that's out of balance. Good to know.