Earlier this year, I wrote a post about being surprised that my Anker A1336 72 Wh power bank, which I very much like by the way, has an interesting charge and discharge behavior when it gets warm. While it can be charged and discharged at 100W and can even be charged and discharged at the same time, as soon as it reaches around 42 deg Celsius on the inside, it stops charging other devices and continues to charge at only 22W. At the time I was wondering a bit why thermal protection was kicking-in at such low temperatures. However, when I recently researched the topic a bit more, I found out that this is actually the proper and expected behavior.
Here’s an interesting article on the topic which basically confirms that lithium-ion batteries can be discharged in a temperature range between -20 and 65 deg. Celsius. Below 0 C, significant capacity reduction should be expected.
The charging envelope is significantly narrower!: 0 to 45 C. As my Anker battery does not have a fan, 45 C can be reached rather quickly during charging, especially when using a 100W power supply. I was surprised the temperature limit is so low, but OK, according to the article above, you really don’t want to push the battery above this limit.
So this explains things very nicely and I have started using a small external fan to cool the power bank during rapid charge/discharge/charge cycles to keep up charging speeds. This is particularly helpful in warm weather situations in combination with air that is hardly moving.