DAMAVO ® It is a professional manufacturer of USB power sockets and USB chargers. Use our industry knowledge to solve the problem that phone won't charge when car is off; Welcome to exchange and discuss
First,If the phone won't charge when car is off, we can use the following methods to check:

If your mobile phone can be turned on, please confirm whether there is poor contact or damaged charging equipment
(1) Please check the interface between the charging cable and the mobile phone, and whether the charging cable is fully inserted;
(2) Please check whether the data cable, charger and socket are in poor contact, and it is recommended to plug and unplug again;
(3) Please check whether the connector and Jack are clean. If dust is found, it is recommended to clean it with a brush and then recharge it for confirmation;
(4) Please check whether the data cable, charger and socket are damaged or have abnormal noise. If damaged, please replace them.

Secondly We do not recommend that phone won't charge when car is off. Generally speaking, it is OK to fully charge the mobile phone when the car is off. However, as long as the switch is turned on, other electric equipment in the vehicle will also be in the energized state. Over a long period of time, it is easy to cause the battery to lose power or even fail to fire, and it will also cause losses to the battery.
Not only the mobile phone is charged, but also other electrical equipment should not be used for a long time after the car is turned off, such as air conditioning, multimedia, lights and other external electrical equipment. Try not to let the battery work overload. Battery power loss causes not only the trouble of not being able to start, but also the damage of the battery.

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