sb_sfo
"... two inside the battery and two external, would prevent any serious battery incident."
With this degree of redundancy we may think she (batteries) are operating a little bit above safe levels (during recharge at BOS and TAK). I assume in both cases the Systems were recharging her. To higher current levels in Logan. (rear batt used to start APU and being fast charged by APU gennies)
This may lead to a much better scenario: No major design issues, no defective parts, just the need of tweaking. Critical devices normally presents this in their teething. A Li Ion battery is such.
Finger crossing.