I have a customized car with 2 engines. I have no mechanical door opening system for the doors as I have removed it in favor of a remote-controlled electrical solenoid. I have recently installed a capacitive switch to be able to open the doors without the remote. Since I have the switch hooked up to the rear engine battery (that is not the car's primary drive engine), the battery tends to drop over time through non-use. I was wondering if there is a way to hook up some sort of ups system in the car so that the switch will work even if the battery drops too low (this has happened). I have a solar trickle recharger on order to try and help this problem, but I would like a more stable system to be able to open the doors.

  • \$\begingroup\$ A diode and a battery? \$\endgroup\$
    – winny
    Jan 16, 2017 at 13:13
  • \$\begingroup\$ how much voltage do your switches need? If it's less than 10.5V, you can't really do anything. An empty battery is still empty, even when you apply some circuit around it. \$\endgroup\$
    – Christian
    Jan 16, 2017 at 13:15
  • \$\begingroup\$ Yeah, a large diode system allowing the secondary battery to charge from the alternator sounds like it would work. I'm very curious about how you end up with 2 engines and what the second one is for though! \$\endgroup\$
    – pjc50
    Jan 16, 2017 at 13:32
  • \$\begingroup\$ I was wondering the same thing as @pjc50... Anyway, Lead-Acid batteries are supposed to have very low self-discharge (it takes from 6 to 12 months to go from 100% to 50% capacity). So the statement "the battery tends to drop over time through non-use" makes me wonder: How much time, exactly? What is the battery capacity? Is there some standby load on this battery? If yes, how much? \$\endgroup\$
    – dim
    Jan 16, 2017 at 13:44
  • \$\begingroup\$ What is the operating voltage of the switch? \$\endgroup\$
    – User323693
    Jan 16, 2017 at 14:10

1 Answer 1


You should charge both batteries when either or at least the main engine is running. Add a Schottky diode in series between each battery and the altenator output. If you can, adjust the altenator output up a few 100 mV to compensate for the Schottky diode voltage drop.

Another possibility is to use a relay to short the batteries together once the main engine is running. Deciding "running" isn't as easy as it may sound, but the advantage is that there won't be any voltage drop between the altenator and the charging batteries.

Once both batteries are being regularly charged, you shouldn't need separate power to cover the case when the back battery is dead. Regularly deep-discharging a lead acid battery, especially one intended for a car, is not good. That will ruin its capacity, and make it vulnerable to low temperatures.

  • \$\begingroup\$ We were worried about doing that and what the effect would be if both alternators were running at the same time. Wouldn't we need some way to ensure that too much current isn't being pushed to a single battery? Or would the load from both batteries be sufficient on their own? \$\endgroup\$ Jan 16, 2017 at 14:37
  • \$\begingroup\$ @Matt: Then put a Schottky diode from each alternator to each battery, a total of 4 diodes. That way both batteries get charged when either engine is running, but the two alternators are still isolated from each other. The batteries will also be isolated when the engines are off. \$\endgroup\$ Jan 16, 2017 at 14:48

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