OT-Need help with electrical math

This means the battey can provide 25 amperes for 90 minutes and still have (just) enough energy to start the car.

The storage capacity of the battey is thus 37.5 amp-hours at a 25 amp discharge rate. It will be somewhat higher at lower currents due to the non-linearity of lead acid batteries; call it 40 amp-hours at most. Batteries lose capacity with age, and too many cycles of deep discharge / rapid recharge (as you have evidently been experiencing) will accelerate aging.

If the vehicle draws 210 milli-amps, then we can expect the battery to be drained in 40/.210 hours amp-hours /.210 amps, or 190 hours, or about 8 days. If the battery is original, or more than 4 years old, I would not be surprised to see only 4 to 5 days standby before the car won't start, especially in cold weather.

The normal guideline for acceptable parasitic battery drain (key off) is 125 ma max. Your 210 ma is way too high. 50 ma is more like what I would expect. Radios, clocks, and fuel injection computer parameter memory are the usual expected source of parasitic drain.

A common source of unexpected power drain is aftermarket accessories like alarm systems or poorly installed amplifiers. Also look for stuck relays -- have you disabled any accessories (like the horn) because they wouldn't shut off? A typical 12 volt relay draws 50 to 100 ma through the relay coil.

If you got the parasitic draw down to 53 ma, then you shoul d expect a new battery to start the car after about 40/.053 or 754 hours of idle time. Call it one month.

A couple of years ago, we had a car in the shop that was draining the battery much like yours. Pulling the radio fuse would fix it. We discovered there was a security system tied in with the radio, that was causing the draw. (The car was imported... Honda or Nissan, maybe.)

The radio and security system seemed to work as they were supposed to, but still drained too much power.

The owner was a high school kid, without a lot of money, and this was a factory radio, and since it was an older car, and this is not a high-crime area, his Dad suggested we put a little toggle switch in the radio's feed wire from the fuse, and he would shut it off when he parked the car. That solved the problem. I'm not sure if replacing the radio would have fixed it, but there was no money for that, and the switch got him by.