On another note, when I have built my battery, I didn't know what case to put my cells into. It was summer, and most of the members like dogman were fighting hot temps, and suggested to use aluminium casing with good venting to protect the battery. Now in winter we want best isolation and possibly active heating
I ended up with a 4mm PVC casing completely closed and sealed with silicon in the summer. Still performs well in -5C temps. I rarely use my bike in colder weather.
I see for most lithium packs is not recommended to charge them below 0C. Is this because the risk of overcharging them? Should we consider charging them to lower voltage in very cold?
liveforphysics said:
For seeing voltage related shifts, and possible over charge events, temperature has crazy effects on cells
Charge one up to 4.0v at 70degF, throw it in the deep freezer, pull it out and see 3.7v, measure Ri and see perhaps 10mOhm. Heat that same cell up to 200degF, see the voltage climb to 4.2, measure Ri at 2.5mOhm. The neat thing is, this cell still has the exact same amount of charge in it, despite what a voltage reading would lead you to believe. Things get kinda scary if you charge one all the way up to 4.2-4.3v in a freezing cold garage, then start heating that pack up inside and watching voltage keep climbing up... When you have a frosty cold LiPo pack charged to 4.1-4.2v, you've just over charged that cell by 10-20%(
here)
I always thought the colder the cells are, the slower they naturally degrade. Seems like LiPO doesn't like sub freezing temps. I know that there is some electrolyte freezing issue with them, but that should be way below freezing temps, and that probably damages the cells permanently, not just degrade them.
recumpence said:
Sub freezing weather is definately bad on Lipo cells. They will tolerate it for a while. But, your longevity will decline noticeably.
Matt(
here)
So what's situation, they only like stored in cold weather, not in use? The same applies to LiFePO4, or only LiPo? Is the cold temp damage due to internal resistance increase and thus more internal heat generated under heavy load? So if loaded lightly there would be no big degradation?
Now one of my favorite quote from LFP battery genius:
liveforphysics said:
The low voltage damage to the battery only occurs from the state of charge of the battery dropping to the point that the Lithium under goes a chemical change of state, which damages the cells.
This means whatever the pack drops down to under discharge is not important, as long as that resistive thermal energy isn't building up to exceed the cells safe temps.
You only need to care about resting voltage for concern about hurting the batteries from over-discharge.
Best Wishes,
-Luke(
here)
I was reading recently that a 2.7v LVC on TS cells in cold weather sucks, because even under light load and 60%SOC it trips the LVC.
Can we load a LiFePo4 cell (for short moments)in cold weather even under 2.0v and get away with that knowing that we have plenty of charge left in(taking into account charge that goes into heat under voltage sag)? Of course this is just hypothetical question, because at that big sag we probably just exceeded max current, because these cells are C rated so that they won't drop below LVC, and we are likely to damage the battery by heat. However I wouldn't encourage anyone to do this, because it's pretty hard to get a precise SOC in cold weather as stated above.
Is seems to be a good idea to use CA and measure burnt capacity and leave some headroom.
Or should we just derate Lithium cells in cold weather because of internal resistance increase? Like 3x less C-rate according to Doc's example?
Your professional input is very appreciated.
Take care of your batts,
Zsolt
