Hi,
From what I've found thus far there are 3 issues that affect permissible charging, temperature, current and the state of charge. (SOC)
All of these affect the charging in a
gradual, linear-ish way,
non of them (including temp) change drastically at a specific point ie drop off a cliff suddenly.
Now people like things to be binary, they want a yes or no answer to the question 'is it ok to charge my lithium
battery right now?' This is an impossible question to answer accurately when there are three 'grey variables' to consider and take into account. In order to give black and white advice to a very grey area the 'you should not charge lithium below 0deg advice was widely adopted'.
Considering the above it's easy to see that following this advice means we're being over cautious at -1 deg and under cautious at +1 deg, after all there's very little difference between 1 deg and -1 deg yet we're told one is permissible and the other isn't.
Combine charge current and SOC in their most undesirable state and someone could easily end up charging at 40A (or more) from a B2B at say +2 deg temp into a
battery with a 80% or greater SOC and he/she would feel safe in doing so when it appears that it would damage the
battery irreversibly.
However say 10A of charge at -5 deg would be fine as long at the
battery's SOC was below 70%.
