Classic Prius Replacement 12-Volt Battery
Actually, "shorted cell" was just an educated guess, as this is the most common failure mode for auto batteries.
Lead/acid batteries put out about 2.1 volts per cell; hence, 12.6 volts typical (2.1 volts x 6 cells) for an automotive battery not under load or being charged.
Lead/acid batteries use sulfuric acid as the electrolyte; and when lead plates break down with usage, they sluff off lead-sulfate (PbSO4), the crusty white powder you see around fill-caps and leaking out of failed, "flooded-cell" batteries with split cases.
The battery fails when the lead-sulfate builds up and shorts all of the plates together in one of the cells. This chemical process is happening in all of the cells, all of the time. It just takes a couple of years for the build-up to bridge the gap between the plates. Anyway, whichever cell shorts out first usually causes the battery voltage to drop enough to cause starting problems (in normal cars). If you subtract 2.1 volts from 12.6, you get 10.5, which is usually what batteries measure when they fail in this mode.
The shorted cell isn't a hard short, because the lead-sulfate is not as good a conductor as solid lead or copper, etc. Therefore, besides getting reduced voltage (10.5 volts) and capacity (one less cell), you also get high resistance through that cell, which limits the amount of current the battery can provide -- a double-whammy.
Most PCs use 12 volts DC internally (along with other voltages), and I assume that the Prius computer components are no different. So, 10.5 volts from the battery is like a 96-volt AC brown-out from your 110-volt wall socket to your PC, which leads to unreliable operation and the ultimate computer crash. Luckily, this is easily (though, not inexpensively) remedied with a new 12-volt battery.
In the old days, battery checkers had spikes on them; and you stabbed through the top of the case to check the voltage on each cell. That would give you positive confirmation of the failure. New equipment just measures the voltage with the battery under load, and gives a generic failure indication. It probably really doesn't matter exactly what is wrong with the battery; it just doesn't work anymore, and you can't repair it -- It has to be replaced.
The AGM battery design is a little different from conventional, flooded-cell batteries, with distinct advantages and a few limitations. However, at heart, it is still a lead/acid battery and subject to failure, with time and usage.