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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.

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  • \$\begingroup\$ Huh. I didn't realize electric cars were like that. That follows large electric radio control aircraft logic where the controls and radio run off a small battery and the motor runs off a large battery. You don't really worry about the small battery running out since the large battery that powers the propeller always runs out first. FYI, 420mA@12V is quite a lot for something battery powered all the time. It would suck if your cameras really drew that all the time. That would kill the battery in a car in about a week. Maybe you should get motion activated cameras instead. \$\endgroup\$
    – DKNguyen
    Commented Jun 11, 2021 at 3:20
  • \$\begingroup\$ @DKNguyen Thanks for the reply. The 420mA would need to power all of the car's systems long enough for them to take over and engage the HV system. Not JUST the cameras. Do you still think that would be enough? \$\endgroup\$
    – Oreo Collas
    Commented Jun 11, 2021 at 3:34
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    \$\begingroup\$ You should probably get a small float charger and keep it plugged in. Deep-discharging the 12V battery will eventually kill it. It's possible you could find a solar charger but the ratings may be optimistic, especially since you seem to be in the PNW. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 11, 2021 at 3:35
  • \$\begingroup\$ @SpehroPefhany Thanks for the advise. This is something that I do plan on getting but is outside of the scope of this question as I would like something that I could rely on even if I were stranded without a convenient outlet. \$\endgroup\$
    – Oreo Collas
    Commented Jun 11, 2021 at 3:38

1 Answer 1

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Usually there is a DC/DC converter for HV-battery/12V-battery installed in hybrid/electric vehicles. The problem is that the HV-system is disconnected when the car is parked/locked for safety reasons. The systems for starting and connecting the HV-battery are supplied by the 12V-battery. Once the HV-battery is connected the DC/DC converter charges the 12V-battery and supplies the low voltage systems.

Bypassing this safety mechanism would certainly not be the greatest idea. Also notice that hybrid/electric vehicles often have a lower energy 12V-battery since in normal operation the low voltage systems can be supplied using the HV-battery+DC/DC.

I would focus on either reducing the power that is drawn from the dashcams or having an additional power source for them or maybe even both. There are almost certainly some guides on the internet for permanently installing and powering dashcams. At least this seems like a more reasonable way for solving the problem compared to fiddling with the dangerous HV-battery.

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  • \$\begingroup\$ You are correct, the car does have a built in DC/DC converter that is turned off when the car is off for safety reasons as well as to preserve the HV battery. I'm not looking to permanently or continuously bypass this as that would be very unsafe and would also risk draining and damaging the HV battery. I'm looking for a way to temporarily bypass it for about 10 seconds in case it is needed in a situation where I am otherwise stranded with no other way of getting to a safe and/or populated area. \$\endgroup\$
    – Oreo Collas
    Commented Jun 11, 2021 at 19:10
  • \$\begingroup\$ I do have plans on taking care of the current draw caused by the camera but that does not help if the battery is run dead by other means and is outside of the scope of my question. \$\endgroup\$
    – Oreo Collas
    Commented Jun 11, 2021 at 19:13

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