Unit shuts off after running for 15-20 seconds.

Power issues can be hard to pin down. Although they are usually the result of the battery or connections degrading over time, it could be much simpler, and be caused by overheating.

You could start by removing the battery and wiping the connections over. It’s possible there’s a power consumption issue, with there just not being enough residual charge to run the machine. A new battery should fix this.

Dyson V6 Battery

My other thought is that it’s overheating. If too much power is pushed through the motor, which MAX power does, it could stop after a few seconds if it becomes hot. Rule this out by giving the machine an overhaul. Wiping down the individual parts, cleaning the filter, etc.