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?
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.)
