Self-heated Lithium Batteries

[JockandRita]

We chose Fogstar Drift Eco, (no heater pads) as the batteries are stored internally, and our Victron Smart Shunt tells me all (I think) I need to know re charge/discharge, etc, with the 2 x Victron Solar Chargers and Battery Sense, providing further info.

In our van there is always some cycling going on, around 2-3% of capacity, due to the victron kit, small CBE monitoring functions and the Ablemail topping up the starter battery. The solar chargers go on and off everyday, and are crap in deepest winter, so the battery can get down to 65%SOC if left (couldn't drive it much last year over winter due to hips), is that storage?

Although early days yet, that appears to be my experience so far, ie, when we came home last Sunday afternoon, the batteries were at 95% SOC. Today they are at 85%, (83% earlier this morning).

So far so good, but will definitely keep an eye on their temperature as winter progresses and prior to us heading for the sun.

Cheers,

Jock.

 

[AdrianChen]

I'm not sure either, but I think it's to do with the inaccuracy in the measurement, and the fact that lithiums really do not like being over-charged, unlike lead-acids. If you are charging to a measured "100%" then that's probably say 97% to 103% in fact. So a 50-50 chance of overcharging. If it was limited to 90%, or even 95%, that danger would be averted.

Yes I thought it might be something to do with storing a battery with potentially overcharged cells, so yours is the best explanation I've heard. So I guess a simple rule of thumb like 60-80% SOC avoids any chance of that.

Ours stay in use with a small amount of cycling going on, so hopefully charging to 14.25v (cell balancing starts at 14.2v on mine apparently) is going to be fine over a reasonable time frame.

I should just turn everything off when not using the van, but I do like the remote monitoring - OCD I guess.

 

[Gromett]

When a LiFePO4 battery (and other chemistries) is charged. The anode expands by up to 10% due to lithium intercalation where Lithium Ions move into the Graphite anode.
Over time the anode degrades due to fractures and cracks in the structure. Keep a cell in a high SOC will mean the degradation through this fracturing is accelerated.

This is the primary reason for not storing LiFePO4 batteries at 100%. High temperatures and 100% SOC "may" also cause a thermal runaway which although much rarer and not as exciting can still happen in LiFePO4 cells.

 

[Gromett]

I have posted this a few times in the past. But for those who didn't see it. This talk by Professor Jay Whitacre is well worth a watch as it explains a LOT about how LiFePO4 cells work.