Battery Charging Principles
Hi Everyone. I have some thoughts so please don't shoot me down! as all I would like to do is spread some 'knowledge' I have gleaned researching [videos & also research papers on LifePO4] & then installing my own GRID TIED & UPS capable system [8kw Inverter, 14kWh battery & 12 PV panels]. Please let me know if anything I say is rubbish!!!
1) Be careful with what DoD figures we use. The stated DoD figure will COMPLETELY vary depending on manufacturer and, crucially, when the battery was produced. In that, the nominal 20% [maybe as low as 13%] DoD is a basic characteristic of the LifePO4 cells to protect the cells voltage. SO, if you see a battery that says 100% DoD that is because the Battery is actually BIGGER than the DoD advertised, shuts off at 80 or 87% but its saying you get 100% of the advertised power. It's all about marketing I suppose.... So a 100% DoD 10kWh battery is actually more like 12kWh. I believe, this is how the newer batteries are advertised.
2) Concerning above and charge Temperatures. ALL of the batteries produced use the same LifePO4 cell manufacturers. They all use the same cells! I am told that these cells have their own BMS control functions..... so it really isn't the Inverter or Battery manufactures fault about the 12degC problem. [probably why one never gets an answer from them about it...]
3) Concerning DoD and percentages. Actually the requirement for life of the Cells is stable Voltage and Current. You don't want to kill the battery with low voltage [hence lower DoD limit] and you don't want to frazzle the battery with high voltage as the battery gets to high SoC.
Personably I and others on this thread have noticed that above approx 95% the Voltage starts to ramp up a little..... BUT a study in china suggests that charging to low SoC [80%] may actually be bad for battery life and as long as charge current and voltage is stable and constant there is no reason to not fully charge LifePO4 batteries. As a 'hunch' I have set my Night GRID charge to 93% and only 70A which is 0.25C. Obviously the PV charges to 100% because you can't stop it......
4) In terms of the temperature charging issues. I have had the same rage as my battery is outside. But after some reading and youtube [DIY Solar Power with Will Prowse is good!] I have decided to build a insulated, heated and cooled enclosure for the battery. With correct construction it should easily be kept warm with a small heater but MUST be cool in the summer as battery life MASSIVELY reduces if used above 40degC. So storage in unheated garage is not great AND storage in a vented loft [cold winter & hot in summer?] or indeed a small cupboard with little ventilation is just as bad as outside!
Ideally CELL TEMP should be kept around 25degC.
Note that Battery performance reduces at low temperatures so the 'lost power' from a 400W heater may be partly regained with better efficiency?
5) Also, its not great to have a LifePO4 battery held at 100% [stretched ballon concept!] so ideally use the power / not charge as much rather than keep it constantly at 100%.
Thanks. Important stuff for anyone who owns or operates several £'000 worth of Lithium-ion batteries; i.e. many here. The Wikipedia entry on Lithium Ion Batteries, and its reference list, tells the story in some detail. My personal summary of it all is : keep temp between +10 and +30C. At low temp, limit charge rate severely (discharge rate not so critical - the big danger is Lithium plating on the anode, which is irreversible and not only degrades capacity but can cause internal shorting and total, violent destruction). After reading it, I also recommend a temperature controlled battery box. About 40W is plenty of heat input if the box is at all insulated. You can buy cheaply small 240V- supplied mats for warming pet beds and home-brew fermentation vessels. I wouldn't trust these to support a battery's weight, and would rather heat the air than the battery base, so place in the airspace under the cross-supported wooden planks the battery stands on. In theory the Battery Management System embedded by the battery manufacturer will prevent any problems. Meanwhile back in the real world we still see battery fires and can only guess at the battery degradation that is unseen but presumably more common, so perhaps some cheap common-sense precautions are worthwhile.