If your BURST Toothbrush is not holding a charge the first step with the Toothbrush is to try a reset on your BURST Toothbrush. This is done by plugging the unit in to charge, Once charging go ahead and hold down the power button, After a couple of seconds all of the lights will illuminate on the unit. You can then release and the lights will go off after a second and the unit will then start charging. Let it finish the charge and use the unit as normal.
If this reset has not resolved the issue then submit your warranty claim through the BURST website here: https://www.burstoralcare.com/register
A Video on How to reset your BURST Toothbrush: