Electronic – Need help designing a custom box

batteriesbattery-chargingdischargepowerswitches

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?

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Best Answer

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.

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