Non VFR electrical question

switching out the battery on my wife's car and don't want to lose all the memory stuff -- again. So I was wondering, either I have a really clever idea or a really stupid one - I suck at electrical stuff, so I am not sure. I was thinking, what if I hooked up my Battery Tender to the cables before I unhook the battery, that way while I run to costco to swap out the battery, would that:
a) fry the car's electrical system
b) brilliant idea, why didn't I think of that
c) wouldn't blow the car up, but wouldn't work either

 

Back when I had an Audi TT, there was a guy on Audiworld (Jeff Bipes) who created a battery backup for stereo codes etc, that simply plugged into the 12v socket. Worked a treat for battery changes.

I have no clue what happens to a car/bike when you power the 12v system with no battery in place, though.

 

I have a tool that plugs into the cigarette lighter that does just that very thing. A battery tender most likely wont cut the mustard though if you open a door and activate the courtesy/dome lights as that will be too much for it. If your careful not to turn anything on it should work.

 

A mechanic used a 9v transistor radio (remember those?) battery plugged into the accessory jack when I had a battery replaced. I agree with T.C. that I wouldn't open a door or anything that would turn on lights.

I wouldn't worry about frying anything. Your car battery can provide in excess of 600amps, a battery charger 6-10. At work our mechanics have a starter booster that puts out 26volts for jumping. The only thing they ever fry is the 2-way radios that I maintain.

 

A quick search on Google suggests that most cars draw less than 50mA when the car is turned off, but a Battery Tender won't work; it needs to sense a pre-existing voltage before it will produce an output. A "normal" battery charger might produce too high of a voltage without the battery as a buffer/load, and the only way to check that is with a voltmeter.

 

Hook up another battery in parallel. Take the neg off first then pos. replace same order. Or get a lighter/outlet adapter and hook it up to a 12v sq flashlight batter or whatever and then plug into your outlet as others have said.

 

I had one of those, well still have it in my tool box, but it is no longer of any use as on all my cars for a while, the power outlets have all been switched. Meaning if the car isn't running, the juice doesn't flow. Mine was store bought, like $5 or so, used a little 9v battery.

I actually went by a auto parts store on my way home, and stopped in, and I was glad I did. Even thought they din't have it on their website, they had something that works in a similar way, except instead of the cigarette lighter, you plug it in to the OBD port and it provides power to the elec system. $12, but sooo much easier than having to do the radio stations again, the Nav system settings, seat/mirror memory, etc. Also doesn't run off a 9V battery, but instead uses the cigarette lighter outlet on a booster pack. Weird, but it worked.

 

THANKS All!