What are the problems with LiFePo4 float charging? How should I charge them?
LiFePO4 batteries have a very low internal impedance so constant voltage charging does not work well. Essentially you provide the battery with current and it controls the voltage.
You do not say much about your application but these batteries will give maximum life when they are kept in the 50-80% charged range so float charging is a bad idea. Given that they show very small changes in voltage with charge level it will not be easy to do this simply.
If you look at your datasheets there should be a charge curve. This will show that at around 90% charge the voltage starts to rise quickly. If you design your charger to pick up the voltage at this point and then to shut off it will give the easiest approximation to this.
The big question is when to turn the charger back on. This will depend on your application but my best suggestion without getting into Amp Hour measurement is to use a delay which you can set up to suit your application and the battery capacity.
The LiFePO4 technology is not that sensible against overcharging like normal Lithium Ion batteries, which tend to destroy themselves. The Problem about float charging is
- your charger produces power dissipation,
- a full cell tends to convert the applied power into heat, which accelerates aging,
- the electrode is likely to produce pure Lithium out of the Lithium ions which lowers the capacity.
To sum this up, float charging LiFePO4s is not as dangerous as overcharging normal Lithium Ion batteries, but has an bad influence on the aging of the cells. The best practice is to charge the cells with CC, then CV. If the current drops below a certain limit (e.g. 5% of C) turn the charging off.
The LFP battery will have a no-load voltage that varies a bit with SOC. Let's say your LFP sits idle at 13.30 volts when it's 80% charged and you want to keep it there. You could float it at 13.30V and no charging would occur beyond compensating for the slight self-discharge that would otherwise cause voltage to drop slightly (over some months). Ditto for other SOC levels. This approach would be suitable for a battery connected to some load but you want your charger to carry that load, not the battery. Again, you would set the charger to float the battery at that voltage (13.30 in the above example) and the charger will carry the load without charging the battery. But, if there will be no load on the battery, there is no point to floating it because the self-discharge rate is very small. Even a six month idle period will not drop the charge much; maybe a percent or two. An LFP SOC monitor will usually be a coulomb or AH counter and will not record the self-discharge.