Automotive power supply with battery backup?

I'm getting close to putting my first Arduino project where it belongs - in the car.

The biggest hurdle for me at this point is power - I plan to primarily use power from the car (12-14v), but I'm also intending to use a key-on switched line - making it a lot more difficult for me to drain the car battery by leaving the Arduino powered on. But, since that's the case, I'd like to have a battery backup - in case I want to turn the car off, etc. Don't need to recharge the battery from the car power, just a simple 9v will do fine.

With a little bit of research, this is what I've been able to come up with. Any glaring problems?

The circuit should work fine but the 9V battery won't last very long.

The Quick Shield: breakout all 28 pins to quick-connect terminals

the 9V battery won't last very long.

seconds, minutes, hours? Normal use case would be 2-10 hours of continuous use, with < 1 hour of that spread throughout in 5-15 minute chunks.

A typical 9V alkaline battery has a capacity of about 565 mAh and an Arduino doing nothing draws about 20mA so as a first cut you're looking at 565/20 = 28 hours theoretical, as little as half that practical depending on how fresh the battery is, temperature, whether the 20mA number is accurate (it'll be higher if the Arduino is actually sourcing/sinking current by blinking LED's, transmitting wirelessly, etc.)

So I'm going to go with "hours, but not days".

Beat707: MIDI drum machine / sequencer / groove-box for Arduino

thanks for the explanation, sounds like it should suit my needs just fine.

although a small, rechargable system would be pretty nifty... ]:slight_smile: