If I leave the car without starting for 15 days, will that drain it? And if I do remove the + and - cables, will the default settings upon reattachment screw things up that may have been changed by the mechanic when he put in a "new" computer? All the default settings should be the same for the 850, right?

 

Hmmm, that depends on your battery. My guess is 15 days will take it out. If you plan on it sitting for two weeks and are worried about your battery...disconnect it.

 

Yes, it's .025. My only solution would seem to be a recording volt/amp meter/computer. Seemingly something has to change, in terms of drain, while it's sitting. If I could review a chart or graph of a week, I suspect I would see a current drain spike. I'll try Amazon and/or Google.

For LuluMike, don't worry about running on default. It is set up for running fine on default. In a few minutes it's back to normal.

 

OK, thanks for that.

 

It's a new battery (replaced in November). Maybe I'll just disconnect it if the default settings on the computer won't cause problems.

Thanks

 

Back of envelope calculations (meaning am ignoring minor variables that could modify the result, but below are the major components and will put us in the ballpark..).

Assumptions:

1. Battery is in good/decent condition and good state of charge at beginning.
2. Battery is rated at min of 40 Ah (amps per hour)
3. and at least 600 CCA (cold cranking amps for 30 seconds)
4. Cranking a cold engine can take 200 to 300 amps for up to 30 seconds to get it started.
5. Parasitic current draw is 25 mA (or 0.025 amps) and is constant.

Assumptions #2 and #3 are pretty conservative if we're assuming 850/S70 body style car. So assumptions #3 and #4 say we need the battery at least at half its capacity (say 300 CCA) if we want a chance to start the engine. Meaning need about 20 Ah (half of 40Ah) of the charge left in the battery; therefore:

(20 Ah)/(.025A) = 800 hours = 33 days, or about one month of drawing 25mA of juice from the battery before its too low to start the car.

Things to keep in mind: even with the battery cables disconnected, the battery will eventually discharge too, just at a slower rate. Extreme cold makes it all worse; diminishes battery capacity and starter will draw more current to crank a very cold engine.

 

Thanks much gdog. That gives me the data (half of which I don't understand but enough) to help me make my decision.

 

Lulumike, Your really dragging this on. You didnt need all this info. If your worried about your battery going dead, disconnect it.

 

Any chance you can just put a trickle charger on it and be done with it? That way the computer and radio don't lose memory and you aren't messing with the cable terminal. They even make some that are water resistant and can be mounted under the hood permanently. Just make sure it's a small (1-2 amp) charger that "floats" once it senses your battery is fully charged. That way it doesn't boil the water out of it.

BTW you only need to disconnect one terminal to isolate the battery.

 

I believe what GDog says. I have left my car for up to 6 weeks without starting it and it cranked right up. The down side of not starting a car for 6 weeks is the gas gets old and the motor has almost NO oil on it from sitting. If you're not out of town, drive your car at least once per week at least 20 miles. Other than being out of town or storing a car for a season, keep the battery connected.

One think I recently read was that a cold battery has more cranking amps. At normal cold temp the cranking amps on the 850 battery is 520 A, when below 32 degrees it says 750 A. Not sure why it says that on a lot of batteries.