There are people who claim to have opened a power unit and soldered new cells into place - however, it's also the case that breaches in the power unit's casing can allow atmospheric moisture in, which appears to adversely affect the performance of the Power Unit (bear in mind it's not just a battery, it houses the main processing chip that converts lever commands, digitised at the Interface, into commands for the derailleurs).
An early indicator of damage to the PU's case integrity is often unpredictable derailleur behaviour - in the case if the v2 Power units, there were occasions where the seam in the power unit opened slightly and in almost all of these cases we looked at, one factor that alerted the rider to a possible problem, was either that the derailleurs shifted inaccurately, or required frequent re-setting to maintain good shifting.
There's also a space problem inside some of the PUs - v2 PUs particularly, used a cell that is no longer available & the closest available is too big for the casing, if enough of them are ganged together to meet the voltage / current requirement of the PU.