Accessory battery
 

Sort of on-topic, sort of off-topic.

I'm a geek, so I like my neat toys in the stereo. I upgraded to a head unit that will play music off a USB thumb drive or control an iPod, and it runs pretty nice. The only problem - sometimes when I start the engine, and power is briefly applied and then removed (as it passes through the "run" position on the key), the head unit does a light reset and forgets where it was, and starts over at the beginning of my music collection. I've been just reactivating the random mode and skipping forward, but this is kind of a hassle when I want to stick in an album or audiobook.

The simple option would be to tie the switched power in line to something that stays on with the starter. This would also mean that the radio would be fighting for power with the starter motor, which already sounds a little suspect (one of the reasons I EOC but don't kill the engine sitting at lights - I trust a bump start more than the starter). So my second thought was provide a little 12V battery for the radio. I've got some old UPS batteries that are in relatively good condition, maybe 7Ah rated.

My plan goes something like this:

Key off: main and acc batteries connected to each other via the always-on wire
Key on run: main and acc batteries connected to each other via the always-on switch, switched power coming from the acc battery via a relay
Key start: relay between the main and acc battery disconnected, but relay between acc battery and both lines of the radio stay connected.

Is having the batteries always paired together like that going to be too big of a risk? I might try to wire it up so the ACC battery gets disconnected when the car isn't running, although then I have to be careful that I don't ever disconnect the main power line for the radio, so I don't lose my stations and other config.

I take it you don't have a manual?

You could always put in a switch to run it through constant power when you want to EOC and back to normal other times. I wouldn't worry too much about fighting with the starter, but then I've always bumped started so I don't have much personal experience with that.