Indeed. The battery was bad and wouldn't hold charge, but the charging circuitry probably would still try and charge the battery and it eventually, catastrophically failed. Owner is lucky they were not sitting there at the time.

If your battery doesn't hold a charge anymore, don't keep it plugged in. There should probably be a PSA for battery handling at this point.

Thanks for this! My partner's laptop has started giving battery-related warnings and I - even as someone vaguely technical - hadn't really considered it might become actively dangerous.

If I'm totally honest, I simply and somewhat cynically figured it was a ruse by the manufacturer to bilk some money from us on an unneeded replacement, given its software's likely counting daysSincePurchase!

Modern lithium-ion batteries have a really nice voltage regulator and smart charger embedded within which can estimate cell wear, count charge/discharge cycles, and tell you how much absolute charge is remaining.

It's at least possible for the manufacturer to be using this information instead of simply counting the hours.