Sorry I wrote it wrong again. I thought lithium batteries were charged individually in an application like this. Discharged in series is a given.
When I read the NYT article about the ANA battery fire, they said the crew noted a normal voltage, then zero voltage. I believe they referred to the zero voltage as a short circuit, not that the battery charger had stopped charging "as designed". If the battery charger had stopped charging, the voltage would have dropped slightly to actual open cell voltage, assuming no load on the battery. Airbus puts a little connecting line on the EICAS to show that the battery charger is connected, but you still look at the voltage across the terminals. It has been 8 years since I have flown a Boeing, I believe they all just show battery voltage. Above 28 volts the charger is working.