DIAGNOSIS The battery is overheating or cannot supply the high current demand of max mode due to age or a weak cell.
FIX Let the vacuum and battery cool down completely for an hour. Try max mode again. If it still shuts off, run a full discharge cycle. Use it on low mode until it dies, then charge it uninterrupted for 6 hours. This can recalibrate the battery gauge.
IF THAT DIDN'T WORK The battery pack is worn out. Its capacity has degraded and it can't handle the peak load. Replacement is required.
HEADS UP Max mode cuts runtime to about 10 minutes. It puts extreme stress on the battery and motor. Use it only for short, heavy-duty spots.