I am doing a hobby project and I don't know much about engineering; therefore I decided to ask pros.

I have a 12v electric device that runs on Car battery only and I need to make it work if there is no power available. I need to get correct components that would make my plan work.

I need to:

  1. add 8 x AA rechargeable batteries as a reserve power -- I bought this

  2. add option to charge those batteries when there is 12V power available from the car battery [need help]

  3. connect solar panel -- I bought this

  4. add option to charge batteries when there is no power available, but solar panel is giving power [need help] I purchase this, not sure if it going to help

  5. add option that prevents from draining the battery when car is not running [need help]

Problem: I do not have access to car ignition; and there is no way for me to do anything about it! Therefore only 2 wires are available: red and black. No access to harness or any other ignition-controlled wire.

How this solution should work:

When the can is on:

          `1. charge the 8x AA batteries`

          `2. continuously supply power to my electric device`

When the car is off:

          `1. cut the power from the car battery`

          `2. charge 8x AA batteries if power is available from the solar panel`

          `3. continuously supply power to my electric device`

When the car is not connected:

          `1. charge 8x AA batteries if power is available from the solar panel`

          `2. continuously supply power to my electric device`

Could you please advise which components I am missing and where to buy them to make my solution work?

enter image description here

  • \$\begingroup\$ The project seems simple enough, but this would be much easier with access to the ignition - you say there is nothing you can do about it, but how about for example; the car radio wiring? There should be an ignition wire connection there to tell the radio the ignition is on. Or anything that is activated when the car is on and you can tap a signal from - the car wiring diagram (car handbook) would come in handy here. \$\endgroup\$
    – Oli Glaser
    Commented Jan 31, 2013 at 1:47
  • \$\begingroup\$ No access; this is a specific external unit. I described the problem and I am at this moment looking to solution to outlined issues! thanks \$\endgroup\$
    – Andrew
    Commented Jan 31, 2013 at 1:48
  • \$\begingroup\$ Okay, I see - Anyway, I'm not sure that you need to worry about 8xAA batteries draining the car battery, even if charging from the battery is always active - as long as the charging terminates. The difference between the relative capacities is large, so the AA's should only present a problem if the car battery is at the end of it's life. For example, compare a typical 40Ah 12V = 480Wh battery with 8xAA 1.2V * 1.5Ah batteries, which equals 14.4Wh. \$\endgroup\$
    – Oli Glaser
    Commented Jan 31, 2013 at 1:50
  • \$\begingroup\$ Of course, it's your project, so if you are fixed with your specifications, we'll see what we can do. I'm just throwing ideas out there. One other question - where do the red/black wires come from? Directly from the battery or e.g. a lamp or similar? \$\endgroup\$
    – Oli Glaser
    Commented Jan 31, 2013 at 2:00
  • \$\begingroup\$ @OliGlaser, I added a picture! Does it make any sense? Thank you for looking into it! PS- right now, after I added a picture, I am not sure how to charge the 8xbatteries and supply the power the 12v device at the same time! \$\endgroup\$
    – Andrew
    Commented Jan 31, 2013 at 2:02

2 Answers 2


The general strategy I would use would be to always directly power your device from the internal batteries. The only question then would be when to use car power to charge the batteries, and likewise for the solar.

For the car power, you need a way of detecting when it is present and the alternator is running, and then to switch in the charger. In this case, I would use a comparator circuit to detect whether the external car voltage is above or below 13 V. Most car voltage regulators are set to around 13.6 - 13.8 volts when the alternator is running, which is more than a 12 V lead-acid battery will produce on its own. Even if you do erroneously take some power from the battery if it happened to exceed 13 V, the battery would have to be very full for this to happen and you would stop automatically as soon as it got discharged a little.

As for the car power charging circuit, you can probably get away with a Schottky diode, P channel FET and resistor in series. Size the resistor so that the batteries are not charged too aggresively, and then have something watching their state of charge. When your internal batteries get full, this system disconnects the charger until they get a little discharged again.

The solar panel is a little easier. You can size it so that even in full sun it can't supply enough current into a 12 V load to charge the batteries too fast. You can therefore just connect it to the batteries with a FET as a switch and a Schottky diode. You can leave the FET on whenever the batteries aren't fully charged.


Discharge protection / detecting when the car is running: your problem here is not having access to the normal signals for this. There is a way it can be done: detecting whether the alternator is running (and charging the car battery) by looking at the voltages. The voltage should be about 14V while charging and 12-13V while not charging.

Depending on what your application is, you might be better off with another 12V battery and a solar charger designed for use in RVs; this is a common system design for powering e.g. small fridges.

  • \$\begingroup\$ Your answer is very general and vague; which device could help me to achieve this? Do you have a model number? Seems like this issue could be ignored, since device won't be able to discharge the car battery. My main issue is: which device/pcb can I buy that I can charge my 8x batteries and provide power to my 12V device when car is on, OR supply the power to my 12V device from 8x batteries when the CAR power is not available (car is disconnected). \$\endgroup\$
    – Andrew
    Commented Feb 1, 2013 at 19:33
  • \$\begingroup\$ @andrew this site is not for just giving you places to buy things and is instead about explaining how to design a system/how to find the parts you need. ie. We dont give you a fish generally, except if we happen to catch one for you while we are teaching you to fish. \$\endgroup\$
    – Kortuk
    Commented Mar 3, 2013 at 14:08

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