I have an android head unit that suffers (like most stereos) a power drop/fail on starting the car. While the unit is engineered to retain OS power for a few seconds,

  • the dvd player has to restart, and
  • (more importantly) the bluetooth restarts, momentarily dropping the connection with the ODB adaptor and resetting trip calculations in Torque

So I'm thinking of adding a smallish SLA battery (6Ah) for 3 purposes:

  • Keep the stereo powered for engine starts
  • Keep power to the head unit (it has an off mode that disables everything incl screen, but keeps android running)
  • Enable running for shortish periods for "normal volume" music etc (1-30 min)

I'm not wanting a huge outlay or vehicle modification, so am considering:

  1. cutting the existing stereo power and making a power triangle:
    • diode and 1A fuse from car battery to aux for charging (but not overcharging)
    • diode and 5 or 10A fuse from aux batt to stereo to for power
    • diode from car batt to stereo to stop the aux powering other circuits
  2. Earthing negative.

Will this work well? Will an SLA overcharge and gas off at car voltages? How much does a SLA demand at different charge/voltage levels? (Mainly my google foo is not helping, and gives me what a charger "should" do - interested in charging fuse size and battery longevity.)

Also, I'm having trouble finding 10A diodes - It seems I could use a bridge rectifier for my scenario as well - yes?

Note: I'll get my ammeter out and update with my expected current usage scenarios tomorrow

  • I haven't tried it, but have you considered running the stereo completely off of a battery, then having a car charger recharge it? Maybe that's what you are suggesting and I'm not following exactly. Jul 3, 2015 at 14:32
  • @Paulster2 - yep that's pretty much it. But if the car is charging the battery, what's to stop the battery powering the car? That's where the diodes come in.
    – Stephen
    Jul 4, 2015 at 12:44
  • I was just talking about using a charging type device which is run off of switched power. This would be an autonomous unit which is only running while the car is running. The only time you'd have a problem with secondary battery (which isn't connected into the main power) is if the car sat for a very long time. Alternatively, you could use something like this which is a complete kit. Jul 4, 2015 at 13:53
  • Another solution is to use a battery isolator. Direct connect to the secondary battery, which is recharged by the alternator. Pretty slick setup. Jul 4, 2015 at 13:56
  • I'm looking for a system that isn't expensive or overly bulky. So I'd like it small (hence SLA), and would prefer to blow the charging fuse (1A) rather than destroy the life of the battery by high amp charging. So perhaps I need a charger, but I think I still need a power pass through from the car too, since the max power is 10x charging amps. But at what voltage will a battery "demand" more than C5-C10 (.1-.2 of capacity amps)?
    – Stephen
    Jul 5, 2015 at 20:47

1 Answer 1


You can try adding some high grade capacitors in between you battery and the starter. This would reduce/ stop the main power surge you are experiencing. Also the car was designed for a smaller batter assuming a standard stereo installation. You are obviously running competition grade equipment and this would most likely require that you add a much larger truck battery that can take the surge. If your battery is failing it will lose cranking amps causing you to notice the power being diverted to the starter more than before or normal.

To completely fix the power draw you could always add a motorcycle or boat battery next to the original battery to and connect them in parallel. This would allow the extra energy produced by the alternator to trickle into the stereo's battery and keep it charged. A deep cycle battery would allow you to play the stereo with the car turned off for longer periods of time without causing damage to the battery as it would on a conventional battery. Hope this helps.

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