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I have recently gotten a hybrid car (2013 Ford C-Max). Due to working from home I do not drive the car regularly so the accessories on the car will end up draining the 12v battery which prevents all of the main systems on the car from starting even though the HV battery (200v I believe) is fully charged. Since the 12v system controls the HV system I’m left stranded.

I find it odd that a car such as this does not have a way to kick start the 12v system using the HV battery so I would like to put something together to accomplish this. I wouldn’t want to link them together constantly as this would run the risk of draining the very expensive HV battery and the car does not need a lot of current like jump starting a regular combustion style car requires so I was thinking that a simple DC/DC converter wired up to a momentary switch should in theory not only get the job done but should also be pretty simple to achieve. Being that I have little to no experience with voltages over 120v and that this is an expensive car with delicate electronics I would like to ask for help and advise on how I can achieve this safely.

I did find this https://www.digikey.com/en/products/detail/cui-inc/FSC-S5-12U/1699037 that looks like it will do what I want if I add a momentary switch and a relay but it’s only rated for 420mA. I don’t suspect that I would need a lot of amps but I’m not sure it 420mA would cut it.

Four Side Notes:

  1. The accessories that are draining the battery are two dash cameras. The accessory outlets on this car are hot all the time so they do not turn off when the car turns off. I kinda like this cause if something happens to my car while it is parked I would want the cameras to catch it. I have been unplugging them when the car is safe in my driveway but it’s a bit of a hassle and I don’t want to forget to plug them in one day when I need them.

  2. I have considered getting https://www.amazon.com/Galley-Power-LLC-Voltage-Disconnect/dp/B0147DYUN8 which in theory should only disconnect the cameras if the car has been sitting for an extended period of time but I have never used anything like this so I do not know how well they work and this would not fix the issue if my battery died from something else. This is outside of the scope of my question but if anyone has used this please let me know what you think. 🙂

  3. Someone has recommended getting a charger so I can plug my car in while it’s at home and this is a good idea that I may also implement but would not help if I’m not near a convenient outlet.

  4. I did get a mini jumper pack that works perfect for this but sure enough, I could not find it today when I needed it so a built in solution would be really nice.

Thanks for all the help/advise/suggestions in advance! 🙂