Systems and methods for thermal battery control

A thermal battery system comprising:

• a thermal storage device including a first phase change material having a first phase change temperature and a second phase change material having a second, different phase change temperature;
• a coolant valve adjustable between a first position and a second position to selectively couple the thermal storage device to an engine coolant circuit and regulate an amount of a coolant circulating through the thermal storage device;
• a temperature sensor for estimating a temperature of the thermal storage device;
• a heat recovery loop, the heat recovery loop disposed at least partially within an exhaust passage of an engine system and at least partially within the thermal storage device, the heat recovery loop including a fluid, circulating there-through, for transferring thermal energy from the exhaust passage to the thermal storage device;
• and
• a controller with non-transitory computer readable instructions for:
• estimating the temperature of the thermal storage device when the coolant within the thermal storage device has stopped for more than a threshold duration;
• and
• determining a state of charge of the thermal storage device based on the estimated temperature and one or more chemical properties of the phase change materials.