Buy a Meter
You'll need an amp meter that can measure at least 0-10A DC to performthese tests. Most multimeters can do this, and you can spend as much oras little as you want to get one. We checked Harbor Freight and found anAC/DC meter made by Cen Tec that can measure up to 20 A of DC currentwith overload protection for $19.99. We also found a Wavetek for $169.95and a Fluke meter for $209 that does the same thing but has a lifetimewarranty. If we had to buy a new one, we'd go with as much meter as wecould afford--the new models have temperature probes and other goodies.The Fluke we used in this story was purchased in 1989 for $85 and itstill works great, so you get what you pay for.
We connected the amp meter and opened the car door to turn on the domelight and got a 2.37A reading just to see if the amp gauge was workingcorrectly. If you turn on the headlights, you'll smoke the fuse in themeter. | 
When we closed the door, we found that there was a 0.43A drain occurringsomewhere in the system--but only for about 30 seconds. We suspected itwas the dome light switch, but we wanted to go through the steps to findthis draw and give the car a clean bill of health. |
The meat of this job was pulling fuses one at a time until the amp meterread 0.01 A. We pulled fuses that are live with the key off like thecigarette lighter, brake lights, and the clock. Later cars will have atrunk and underhood light, so snuff those before you start. After eachstep we opened and closed the door to see if the temporary amp draw hadgone away. | |
Amp Hour vs. Reserve Capacity
Even though Amp-Hour (A.H) and Reserve Capacity (RC) are very similar inmathematics and general theory, they have subtle differences. For thepurpose of this article, we used A.H rating to show the rate a batterywill discharge with a small amp draw for a fixed period of time, likewhen your street machine is in storage. The RC rating is the number ofminutes a battery will last if your charging system fails while you'redriving your car. Temperature is a factor, but generally, the RC ratingassumes a 25-amp draw to run essential accessories and the A.H ratinggives you a formula to illustrate what would happen with a smaller ampdraw over a longer time period. Since RC is a more practicalapplication, it is more commonly seen as part of the battery rating.
This car has a newer stereo with a detachable face and a memory so wewere going to call it guilty. This kind of thing drawing amperage isnormal for a late-model car with a CPU and several different forms ofmemory. Surprisingly, this stereo was not drawing any current so wemoved on. | 
Eventually we found that the interior lights were still drawing0.047-0.050 A for a small period of time after the door was closed. Withthe interior lights disconnected, we were left with 0.01 A on the meter,so we had found the cause. The temporary nature of this drain shouldn'tcause a problem on this car. If the battery fails, we'll have to look atthe charging system or the battery itself. |