A fast response would be appreciated. My battery box is at 35f. Is it ok to charge the batteries at this temp. I know not to charge below freezing. But will it damage my batteries AT ALL to charge at 35f?
From what I've read - you're quite cold but within most specs I've seen. One trick is to lower the charge current to a trickle (per cell) compared to what you might do at regular temps. Even if its "OK" to charge right up to 32F/0C per specs, I think of it like charging up to 4.17v every day - you can shorten the life of the cells if you do this often. Also, even if it's within specs, I understand that the efficiency (power in vs power back out) degrades significantly as you approach the extremes.
Under the low temperature situation, there is one rule to protect the lithium-ion battery from irreversible damage when charging up the battery. “Don’t charge them when the temperature falls below freezing (0°C or 32°F) without reducing the charge current.”
What’s the Reason behind This Important Rule?
When the battery is charging up at moderate temperature, the lithium ions inside the battery can move normally to the negative terminal of the battery. Below freezing, the lithium ions aren’t efficiently captured by the anode. Instead, plating of metallic lithium can occur on the anode which means there’s less lithium available to cause the flow of electricity and the battery’s capacity drops. Besides, if the battery is charged at an improper charge rate, it may cause battery failure suddenly. When the temperature is below 0°C, then the charge current must be reduced due to the anode’s lowered diffusion rates. A much slower charge rate can reduce the damage. A research suggests that the charge rate that is allowed at the temperature of –30°C (–22°F) is about 0.02C. But the charging time would be so long with such a low current, which is an impractical solution. With the development of battery technology, there are certain lithium-ions that are able to charge down at the temperature of –10°C (14°F) at a lowered rate.