A computer is programmed to upon determining that a battery charge level of a battery of a vehicle is below a first threshold, determine a first fuel quantity for an engine to charge the battery to a second threshold; and actuate a vehicle component according to the first fuel quantity. The computer may transmit a message specifying a second fuel quantity that is a sum of a preset fuel quantity and the first fuel quantity. The computer may, upon determining that an ambient temperature is below a temperature threshold, provide an instruction to the engine to charge the battery to the second threshold.