In article , Mark
wrote:

I have a back-UPS 400 with a useless gelcell. However, touching
terminals of a fully charged and healthy gelcell to the battery
leads causes mega amps to flow, so something is likely fried.
Interestingly, the dead battery has about 10.5 volts on it,
in-circuit, no current!

This is the classic symptom of a 12-volt lead-acid battery with one cell
shorted. Hooking a 12-volt battery to a 10-volt battery will draw lots
of current, as observed. Replace the old gelcell battery with the new
one; do not keep the old gelcell in the circuit.

Joe Gwinn