3 ms·

I wish I could find this information for LFP batteries. There’s a lot out there, but it’s all mixed with advice for LiPo; I can’t find anything I trust to tell

I wish I could find this information for LFP batteries. There’s a lot out there, but it’s all mixed with advice for LiPo; I can’t find anything I trust to tell me the optimal voltages for maximum lifetime.

user_7832 2y ago
Is it possible to reach out to the manufacturer, or the place you;re buying your batteries from?
speedgoose 2y ago
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.
- 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.
serpix 2y ago
Lfp voltages are all over the place and even after charging to 90% the voltage drops back down. They might not care much at all being charged to high percentages. There is no reliable way to know the SoC other than coulomb counting and regular 100% top balancing.
8xeh 2y ago
LFPs are indestructible and last forever. For general advice, they like it cooler[1] than other kinds of lithium batteries. Try to keep them between 15-25°C. Other than that, charge LFP batteries to whatever percentage you want whenever you want. They are simply better than every other kind of lithium chemistry. Their practical downside is lower cell voltage (3.2 vs 3.8), and slightly lower energy density. I've spent the last 7 years building LFP-powered lawnmower-sized robots, and in practice, the lower voltage and doesn't matter. And, unlike other chemistries, they don't catch fire when you stab them. For other kinds of lithium batteries, to maximize lifetime, keep them between 40-60%, avoid big charge swings (10-90%), and keep them warm (above 25°C). [1]: At 35°C, tests show they lose about 5% more capacity after 2,000 cycles compared with 25°C. But even that isn't really a problem, because they don't get to 20% capacity loss until 4,000 cycles. See: https://iopscience.iop.org/article/10.1149/1945-7111/abae37 https://iopscience.iop.org/article/10.1149/1945-7111/abae37, fig 3.
- Filligree 2y ago
  A practical concern is that the batteries in question will be powering our cottage, which is entirely off-grid. It’s also unused for half the year, and for much of that half the solar input is effectively zero; for part it’s literally zero. Not enough to even run the Victron gear, never mind a heater. Spring temperatures often drop below -5C. So we’ll winterise it by shutting everything off, BMS included to the degree it’ll let us, and the question is what SoC they sold be at for that to work.
  - 8xeh 2y ago
    Cold batteries do struggle a bit to deliver power and charge when they're cold, but they're just fine being stored in low temperatures. LFPs are really good at maintaining their charge. Even below -5°C, they should be fine, particularly if they're not being used. Also, I wouldn't worry about the BMS too much. Unless it's really dumb, when there's no charging or discharging happening it will put itself to sleep for most of that time. Technically it will use power, but likely only a few hundred micro watts to a few milliwatts (nothing that will make a difference on a 5 kWh+ battery). Then when the sunlight comes back in the spring (or whenever the snow melts off the solar panels), it'll see some input voltage and start feeding the battery again.

user_7832 2y ago

Is it possible to reach out to the manufacturer, or the place you;re buying your batteries from?

speedgoose 2y ago

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.
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.

serpix 2y ago

Lfp voltages are all over the place and even after charging to 90% the voltage drops back down. They might not care much at all being charged to high percentages. There is no reliable way to know the SoC other than coulomb counting and regular 100% top balancing.

8xeh 2y ago

LFPs are indestructible and last forever. For general advice, they like it cooler[1] than other kinds of lithium batteries. Try to keep them between 15-25°C. Other than that, charge LFP batteries to whatever percentage you want whenever you want. They are simply better than every other kind of lithium chemistry. Their practical downside is lower cell voltage (3.2 vs 3.8), and slightly lower energy density. I've spent the last 7 years building LFP-powered lawnmower-sized robots, and in practice, the lower voltage and doesn't matter. And, unlike other chemistries, they don't catch fire when you stab them. For other kinds of lithium batteries, to maximize lifetime, keep them between 40-60%, avoid big charge swings (10-90%), and keep them warm (above 25°C). [1]: At 35°C, tests show they lose about 5% more capacity after 2,000 cycles compared with 25°C. But even that isn't really a problem, because they don't get to 20% capacity loss until 4,000 cycles. See: https://iopscience.iop.org/article/10.1149/1945-7111/abae37 https://iopscience.iop.org/article/10.1149/1945-7111/abae37, fig 3.

Filligree 2y ago
A practical concern is that the batteries in question will be powering our cottage, which is entirely off-grid. It’s also unused for half the year, and for much of that half the solar input is effectively zero; for part it’s literally zero. Not enough to even run the Victron gear, never mind a heater. Spring temperatures often drop below -5C. So we’ll winterise it by shutting everything off, BMS included to the degree it’ll let us, and the question is what SoC they sold be at for that to work.
- 8xeh 2y ago
  Cold batteries do struggle a bit to deliver power and charge when they're cold, but they're just fine being stored in low temperatures. LFPs are really good at maintaining their charge. Even below -5°C, they should be fine, particularly if they're not being used. Also, I wouldn't worry about the BMS too much. Unless it's really dumb, when there's no charging or discharging happening it will put itself to sleep for most of that time. Technically it will use power, but likely only a few hundred micro watts to a few milliwatts (nothing that will make a difference on a 5 kWh+ battery). Then when the sunlight comes back in the spring (or whenever the snow melts off the solar panels), it'll see some input voltage and start feeding the battery again.