Apple Watch won't update after last update drew battery down too much.

My Apple Watch series 5 is less than a year old. The last update drew the battery down so badly that it now won't hold a charge -- it boots in a loop, and fails to get past 1% charge. The new update doesn't have enough time to load before the watch reboots again. It isn't the charger because it always has enough power to boot. It's been like this for more than 24 hours now. On charger all night and all day. Still 1% charge, reboots after about a minute.