Using 12v battery to charge B300s

Product Discussion Expansion Battery
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I have two 12v-100Ah LifeP04 batteries left over from another project. I tried hooking one of them up to a B300s and while it was working great, after a few hours I went to check the battery voltage and the Positive post and clamp was way too hot. The Negative post temp was room temp along with the battery as a whole. I earlier measured the current draw with an amp clamp meter, and it showed 7.9 amps, which seemed reasonable.
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This can’t be good nor safe since it’s a lithium battery. After disconnecting and letting it cool down, the voltage showed the battery was down to only 70% SOC, according the company’s charge chart. I emailed support from the company but haven’t heard back all week.
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Anyone familiar with using LifeP04 batteries that could shed some light on this? Maybe the battery’s own BMS is what was heating up?

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So you say the +'ve terminal of the 12v-100Ah LiFePO4 battery was hot as it was draining a few hours at 7.9A and when from 100% SOC to 70% SOC?

7.9A at 12V is about 100W or so which is reasonable. 100W for 3-4 hours is 300-400Wh which is close to 30% of the capacity of the 12v-100Ah battery. So everything checks out.

I don’t think there’s anything wrong with the B300, as it won’t cause your other battery to heat up. 7.9A isn’t a very high drain rate at all, as your battery should be capable of 1C discharge, around 100A with the right cable hook up.

It is possible that the contacts are not good at the terminal, increasing resistance and causing heat build up. Check the positive terminal connection and make sure it is clean and tight.

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