LifePO4 Cycle & Voltage Recommendations

jkmusic777 · May 2023

I have 2 JavaEnergy LifePo4 200AH Plus batteries. I use a Midnight solar 150.
Factory Data:
Charge @ 14.6, 29.2
Low Cutoff - 10 to10.8, 20 to 21.6
For more cycles they say 80% DoD.

I float at 27.2 - not sure it can be turned off... I charge at 28.8. I re-bulk at 25.6.

Among other things, what I am questioning are the recommended cycles. According to MS, they said the system should be discharged to 20% before charging for efficiency. This is confusing. My system is an emergency system only-- although I will run a freezer on it. Or, I could just keep it charged and never use it unless an emergency occurs. They said mini-cycles will shorten the life of the batteries. If I happen to run this to 20% overnight,... assuming I could... then its not an emergency system. A cloudy day or two and its done without a generator. 80% of 29.2 is 26.6, 20% is 25.6. Thats a tiny spread. Does this mean I must do this daily? or should it just run as it is, whatever I hook to it...and simply charge as needed? If I need to operate at 26.6 to 25.6 ... should float be set to 25.6? What about re-bulk to start full charge again?

Appreciate the help.

Horsefly · May 2023

A few points to consider:

Charging at 14.6V is taking it to the 3.65V per cell (4 cells per 12V battery, 8 for a 24V battery). That is the absolute maximum that the manufacturers recommend. Most people who have succeeded at keeping their LiFePO4 batteries healthy charge to a much lower level, like 3.45V per cell or 3.5V per cell. That would be 13.8V-14.0V for a 12V battery. So if you can, I would turn the bulk / absorption voltage down to 13.8V or 14.0V.
Float at 27.2V (I guess you are talking about a 24V battery?) is 3.4V per cell, which is OK but a little high. 3.35V per cell or even 3.3V per cell is probably better for float. That's somewhere in the range of 26.4V to 26.8V for your 24V battery.
You can't use battery voltage to even guess the SoC of LiFePO4. The reason is that the charge / discharge curve for LiFePO4 is almost completely flat in voltage between about 20% and 85%. So don't assume 80% is 26.6V and 20% is 25.6V. Just doesn't work. To measure SoC you really need a shunt. A Victron smart shunt is pretty dependable.
I don't agree that "mini cycles' shorten the life of the battery. Everything I've read and experienced says just don't don't risk overcharging or over discharging. Unlike lead acid, LiFePO4 doesn't care if it doesn't get fully charged very often. So if you can keep it in float for weeks or months, you are golden.

OutsourcedGuru · July 2023

What I find interesting is information like this on Victron's own website. https://www.victronenergy.com/solar-charge-controllers/smartsolar-mppt-75-10-75-15-100-15-100-20 ...

"The SmartSolar charge controller will even recharge a severely depleted battery. It can operate with a battery voltage as low as 0 Volts, provided the cells are not permanently sulphated or otherwise damaged."

I routinely see people in the sales channel trying to up-sell to lithium with this idea that lithium batteries can go down to zero volts unlike the 11-ish volt low point for lead-acid. But then I hear people suggest that you shouldn't take a 12V lithium battery below, say, 10.5V, or you'll limit the lifespan. If this is the case then why on Earth would you pay significantly more if you're not actually getting more than a deep-cycle lead acid battery?

My own 6V * 4 lead-acid set of batteries (series) routinely outperform my neighbor's with 12V * 6 lithium (parallel).

LifePO4 Cycle & Voltage Recommendations

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