If we assume the two event aircraft were the only ones that fried their batteries, what was the condition of the two flights where it happened? In cruise flight, both batteries should get little to no use, and little charging.
There were no other system malfunctions.
787's use electric wing and engine anti-ice. Pilots on other aircraft use these sparingly, as they consume a lot of fuel (bleed air). Very occasionally, you have to leave the wing anti-ice on for a long time.
Electric engine and wing anti-ice would use massive amounts of electricity, all of which would have to be dumped into the aircrafts' grounding/bonding system.