Alan

Ernie,
This can work but there are a couple of hurdles:

1) If you do have a significant drain this won't help too much because it will drain a 9v battery very fast. An option would be to supply your ECU's via a big diode and put the battery (maybe even a rechargeable) after this... Should work but...?

2) The 9v battery will be killed by the drain of the interior lights - so you need some way to disconnect the car battery without having thre interior lights go on - maybe a switch in series with the rear hatch light switch so you can diasble the hatch tigger...

You are right that the plug in lighter socket type won't work - you need a direct connect method

Alan

ew928

I was thinking just powering up the LH with the rechargeable power source.
A dedicated source just for the important stuff.
Isn't the 4 square zener diode loop one of those electron 1-way filter usuable in this case to just feed power to the engine brain? Or am I too optimistic on this.
Don't care for feeding the electric gremlins in the car.

So many wires. So little time.

Alan

All you need is a single diode but it needs to be a really big one - you can cheaply get say a 40-50A diode - I would go overkill here because you don't want the diode acting as a fuse. This would only be on the battery (30) line. I'd assume the drain on this line is no more than 15A when running but its hard to say... maybe John Speake knows, when off it is miniscule.

I have thought about similar things including a backup battery for the alarm (though it has to be quite a bit bigger).

Alan