Limiting Batteries
- Thread starter Lowfly
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LakeHouse
Knowledge Collector
Are you using Lithium Iron Phosphate (LFP, LiFePO4) batteries? If so, ignore what you've read about only charging them to 90%. Charge them to 100% every day that you can.
What's going to be far more important to the longevity of the batteries is keeping temperature in check (I'll guess this isn't a huge problem in Scotland), what voltage you're charging to (I charge to 3.45V/cell, or 55.2V on a nominal 48V system) and what voltage you float at (3.375V/cell, 54V).
What's going to be far more important to the longevity of the batteries is keeping temperature in check (I'll guess this isn't a huge problem in Scotland), what voltage you're charging to (I charge to 3.45V/cell, or 55.2V on a nominal 48V system) and what voltage you float at (3.375V/cell, 54V).
I will need to find out what voltage they use to represent 100%
what temperature range would you keep them in? and what would be the lowest % you would let them fall too
but all the same and just wanting to explore the system, how would I limit the charge?
LakeHouse
Knowledge Collector
This is exactly the problem: The LFP charge curve is so flat that you can't measure 90% via voltage. The best you can do is track energy flow in and out and estimate SoC, but that method drifts over time and your estimate gets worse and worse.
Ideally, anywhere from 10C to 25C. Don't charge below 0C. Try not to let them get above 30C if you can avoid it, but you're probably not going to do any damage until something closer to 40C.
At least in theory, 0%. I set my BMS to cut off at a minimum cell voltage of 2.7V; this will protect any cell from low voltage damage. I set my inverter to stop drawing power at 48V because I don't really ever want to reach low-voltage cutoff on the BMS, for a few reasons: 1. The system doesn't automatically recover from this because my SCC won't start charging if it doesn't detect a battery. 2. The BMS is a last-line safety device, and I don't really ever want to plan to use it.
As far as I can tell, you can't limit charge by SoC unless you have communication between the battery and the inverter. Your inverter is, at best, guessing on the SoC.
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@LakeHouse
thanks for all that - interesting regarding temperature and charging, there is a lot of agreement that below freezing charging is very bad for these batteries. the documentation says they are good down to -10c, and they are also design to go on outside walls. Ours have been filled in the loft above the garage, and we're in Scotland, -10 is rare but these batteries will often be subject to below freezing.
Do they keep themselves warm ? nothing in the manual says they do!
And also yes to charging them up to 100% - from what I am learning it is not getting to a 100% that is damaging, but how you get there
I note with mine the charge rate slows down dramatically after 90%
thanks for all that - interesting regarding temperature and charging, there is a lot of agreement that below freezing charging is very bad for these batteries. the documentation says they are good down to -10c, and they are also design to go on outside walls. Ours have been filled in the loft above the garage, and we're in Scotland, -10 is rare but these batteries will often be subject to below freezing.
Do they keep themselves warm ? nothing in the manual says they do!
And also yes to charging them up to 100% - from what I am learning it is not getting to a 100% that is damaging, but how you get there
I note with mine the charge rate slows down dramatically after 90%
LakeHouse
Knowledge Collector
Usually not, but there are exceptions. Really depends on the battery. Probably best to check the manufacturer’s literature.
If it only gets below freezing at night (I don’t really know all that much about weather patterns in Scotland, so maybe this isn’t applicable), you could get a lot of mileage out of some insulation. Overheating isn’t much of an issue because, unlike lead-acid, they don’t generate much heat when they charge and discharge.
If they're designed to handle outside mount, and were sold to you in Scotland - I'd expect them to have internal low-temperature protection. The battery itself will cut off if it's too cold, stopping it being damaged from being charged. The devil-in-the details parts are
- Can you still draw from it (discharge)
- Does it wake up on its own once warmed up
- Does the inverter object, and start emailing China with error codes - or itself shutdown until manually reset
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