[ecomodded]

First off replace the battery because some cars will up the idle to charge a depleted battery.

I use a battery tender junior for all my batteries.
I have permanent connectors on the jetski and the car.
the junior is only .750 amp and works very well.
2 amps is plenty but the 3/4 of a amp worked for all my past vehicles batteries fine, a overnight + affair.
I would suggest the full size Battery Tender, 2 amps is more reasonable, my battery tender junior came free with a motorcycle a few years back. I also have a full size 10 amp 3 stage charger i use for real charging not just topping up.

Broken sensors can cause a high idle, you should read , on the engine control sensors and modules, often they will explain what a bad sensor will cause and how to check.

I am curious how the scanquageII could drain off a battery, something small and simple (circuit board/display) like that should not be a major drain.
Off hand i would assume 0.050 amp at rest ( it may not be).
Possible it is keeping the cars computer awake or some such thing , i don't think it could be seriously effecting a battery in even a week of sitting, but i have no first hand experience, unlike some users..