Deep under voltage recovery in a battery pack

A method for charging a battery system comprising:

• determining that a charger is coupled to a battery system, the battery system including one or more cells and a charge transistor, where:
• the battery system is distinct from the charger,
• the charger monitors a voltage level of the battery system, and
• the charger is operable to charge in two or more modes of operation and switch between the two or more modes of operation based on the voltage level of the battery system monitored by the charger;
• determining that a voltage level of the cells is less than a first low voltage level,where the voltage level of the cells is distinct from the voltage level of the battery system monitored by the charger;
• charging the cells at a reduced rate including adjusting a voltage applied to a charge transistor gate terminal, including:
• regulating the voltage level of the battery system monitored by the charger to a level that is less than a second voltage level, where the charger does not switch between the two or more modes of operation, and
• supplying a minimum operating voltage for at least one component of the battery system;
• and
• when the voltage of the cells reaches the first low voltage level, substantially fully enabling the charge transistor to allow the charger to switch between the two or more modes of operation.