Hey, my battery when it's hooked up to my car will drain really fast. I can watch it drop on my multimeter. It can have a full charge on it, and any time it sits for an extended period, it goes dead, extended as in over-night. So I know it's got a parasitic drain, but how do I isolate it?

 

Parasitic drain is simply the current draw when the engine is off. It is typically 80-150mA (0.08 - 0.15 amperes). Genereally, anything over a couple hundred milliamperes (200-300mA or 0.2 - 0.3 amperes) is considered too high.

To isolate it, connect an ampmeter to the battery and start disconnecting fuses, one at a time to determine where the source of the current draw is.

If you've removed all the fuses and it's still too high, start removing things permanently connected to the battery like the wires to the alternator, the radio memory, ECM, alarm, etc.

EDIT:
I was curious to see exactly how much the parasitic load was on my '91 Z28 so I measured it to give you an idea of a typical example.

27mA. That's 0.027 amperes, and is what the ECM and alarm draw with the engine off. This figure will vary depending upon what you have connected to the car that draws current with it off, like the alarm, radio (memory), clock, etc.

The 27mA value does not include my radio, which is connected to a rear mounted Optima battery, isolated from the front (by a relay). I woud imagine that it (memory) doesn't draw more than a milliampere or two - maybe even less (nano-amperes?).

[This message has been edited by Stuart Moss (edited August 09, 2001).]