What to do about short run time

Bought a Dyson V8 Absolute (not included in selections) a year ago. It worked great for awhile, then started to act like the battery was not fully charging. I purchased a third party battery which worked okay for awhile, but then the run time was dramatically reduced (5 minutes or less). I cleaned it thoroughly and no change. I stripped completely down to the motor and the same thing happened with no load. I recently watched a video that showed that much of the power control circuitry is inside the battery. In fact, the on off switch is actually in the batter, not the vacum head. I am about to purchase another battery if no one has a better answer. I see this appears to be a common issue with Dysons and cleaning is the first recommendation. I have had Dysons for a few years and know how to clean them completely. That is not the issue.

Thanks!