In our This Guy's Garage department this month, Tom Owens says that when he
Simple Battery Test
Electrical problems are often the bane of many car crafters. If your car sits for long periods of time even the minor electrical drain of a digital clock can be enough to drain a battery. For example, a typical stereo unswitched connection may pull only 100 milliamps (0.001 amp), but it's enough to radically discharge even a strong battery over a period of several months. That is why a small battery charge maintainer is a great idea to keep a charge in the battery, which also dramatically extends its life span.
While major automotive shops can perform a battery load test to indicate battery condition, you can perform a similar test with nothing more than a digital multi-meter. The simplest form of this open circuit test is to merely connect the positive and negative leads from the multimeter to the battery with no load being applied. You can gain a relatively good sense of a battery's condition by its at-rest voltage. A slightly more sophisticated test is to put a 25-amp load across the battery for two minutes, such as a single large electric fan. Measure the voltage at the battery after the two-minute current draw and compare the voltage with the accompanying chart. If the voltage takes a serious tumble from its at-rest voltage after a two-minute current draw, you have an idea that the battery is nearing the end of its useful life. You'll note that there are slightly different voltage numbers for an absorbed glass matte battery like the Optima. Because these batteries offer less internal resistance, the voltage numbers are a little higher.
|BATTERY STATE OF CHARGE
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