A little battery math of the math-challenged: Sears Platinum AGM group 34 has 68 Amp*hours when discharged at 20A--that is, it can supply 20Amps for 3.4 hours. Lets say it gets a tad better at low discharge rates (like when the car is turned off and locked) 68 AH becomes 72 AH. At 1A idle current the battery will be flat as a fart in 3 days (72AH/1A/(24H/Day)) At 100mA idle current, the battery will be flat at 30 days I don't know anyone who can get 30 days off a F355 even with a brand new battery. I can get 21 days and still be able to start my F355, but in order to do this, I had a relay installed to kill the aftermarket stereo amplifier and CD changer when the key is off. Still, as the battery ages, this 21 days becomes 18 then 12 then by the time you only get 7 days per charge, it is really time for a new battery. Thus, you should be looking for an idle current in the 100-200 mA range with the key off.
Mitch's post brings up a good question: has anyone measured the current draw of a 355 with the key off? It's an easy measurement to do, other than getting access to the battery. You just set your multimeter to current mode (usually involves moving the leads to different plugs), detach the ground cable from your battery, and then use the leads from the multimeter to complete the circuit between the ground cable and the battery.
a 12 Volt battery is considered discharged at 10.5 volts useable power is about 40%-50% of rated reserve capacity ( a battery with a rated reserve capacity of 100 AH can approximately deliver only 40AH -50AH ) The usable power in a battery is NOT like using all the fuel in a gas tank to empty, only the initial portion of the rated spec is useable. The voltage changes as the power is used up, the remaining power in a battery is unuseable as it goes out of spec and electronics start shutting down CCA - cold cranking amperes, is a high demand rating measured in seconds ( as in starting a car ) it has little or nothing to do with reserve capacity which is the ability a battery has to deliver power over an extended period of time
I always use the kill switch since I rarely drive my 355, maybe once every two weeks. Never had a problem with it. Unfortunately I don't have a power outlet in my garage otherwise I'd use the charger instead of the kill switch. Had the same problem with my Aston Martin V8 Vantage. Car alarm drains the battery fast.
I have this cool solar trickle charger. It plugs into the cig lighter receptacle. There are numerous types out there. My car is in a garage. I just run the wire under a door. In order to put the sensors in the sun. Works great. Just thought I would put it out there. If you Google search for auto solar chargers there's a bunch to choose from. Boaters use them a lot. Free power.
But I can get my machine (1999 F355 Spider) to start after sitting for more than 30 days. I've done it. I'm getting really curious. I think I'm going to find out just what the current drain is on my my car. I'll post the results.
