The following are the main reasons why the 12V auxiliary battery of an electric vehicle frequently runs out of power: 1. Some system settings only supply power to the auxiliary battery when the vehicle is started: Certain electric vehicle system configurations are designed such that the high-voltage battery only charges the auxiliary battery when the car is running, and stops supplying power once the vehicle is turned off. Therefore, sometimes the auxiliary battery is already severely depleted, but the owner turns off and locks the car to charge the electric vehicle. At this point, the charging process cannot replenish the auxiliary battery, resulting in the auxiliary battery being completely drained after sitting overnight. 2. Excessive charging duration: Some electric vehicles can recharge the auxiliary battery even when turned off during charging. However, once charging is complete, even if the charging cable remains connected, the auxiliary battery will not receive additional charges—meaning each charging session only replenishes the auxiliary battery to full capacity once. Meanwhile, the auxiliary battery continues to power various electronic control modules and electrical components in the vehicle, slowly discharging until it is completely depleted. 3. Electric vehicle in an extremely low-power state: Sometimes, when an electric vehicle is in an extremely low-power condition, recharging may no longer be able to replenish the auxiliary battery. This is because the pathway for the converter to charge the auxiliary battery itself requires power from the auxiliary battery to maintain contact closure. Once the auxiliary battery lacks sufficient power to keep the contacts closed, the high-voltage battery cannot recharge the auxiliary battery.
