Charging Lithium

 A year ago, I made the leap to lithium (LiFePO4). My lead AGM gave up the ghost, the result of a failed solar panel allowing it to go flat for too long. It was also long in the tooth. I wrenched my shoulder maneuvering it out of its hold, which was horribly located, so the decision was significantly based on its being half the weight. 

 I'd read up on lithium. I picked one that had been vetted by a battery expert I trust. The BMS (battery management system) would cut off charging below freezing, which can be an issue for me. We've all been told that charging below freezing is bad, resulting in lithium plating. But ...

1. 32F has no particular significance to a system that does not contain water. It sounds like just a number that is easy to remember.
2. I have read in several places that, in fact, it is a soft number, and that what matters is tapering off the charge rate at low temperatures. That at some low rate, charging in the cold is non-damaging. 

One source said 0.05C (5% of battery capacity) may be considered safe. My panel can only charge 0.025C on the best of days, and typically more like 0.010C in the winter, when this would matter. 

 C= charging rate/capacity. Many of the studies were done at C=1 to 3, meaning that a battery could be fully recharged in 20-60 minutes. That's FAST for solar systems. I doubt many sailors can recharge the bank in less than 5 hours, and most, more like 10-20 hours (C=0.1 to 0.05).

 

 

It's never clear how far you can extrapolate data without more  testing, but it seems clear from the above graph that at very low charging rates, the effect of temperature on plating (low temperatures) and side reactions (high temperatures) are much less than at common charging rates. While it is convenient to repower you EV, cell phone, or device in an hour or less (C=1 to 3), that has no application to sailors charging from solar (C=0.1 to 0.05 or less). I'm guessing that curve is pretty flat, and that even if it is not, the cycles at C=0.025 at 32 F are probably better than C=1 at 20C. 

 

How about those apples? Nothing for sailors to worry about.