We've discovered a truth in life. When you have four or more vehicles in your fleet, your biggest problems become empty gas tanks and dead batteries. Gas is an easy one; you can siphon it directly from the lawn mower or the neighbor's boat. But what about the battery? Why does it die, and what kills it? Knowing the answers to these questions will help you understand and hopefully prevent a belly-up battery.
The BatteryIn simple terms, a battery consists of a lead-coated electrode called the anode and a lead-oxide coated electrode called the cathode that combine to form a cell. There are six of these cells in a 12-volt battery, each contributing about 2.1 volts. They are immersed in a solution of sulfuric acid and distilled water called an electrolyte and connected through a system of grids and plates in a series that ends at the positive and negative terminals. As the acid eats away at the metals, the cathode releases positively charged ions into the electrolyte solution. Since it retains the electrons, it become negatively charged. Similarly, the anode reacts to the positively charged electrolyte and releases electrons and becomes positively charged. This movement of electrons creates a polar difference between the oppositely charged plates in each cell and creates a difference or voltage between the two terminals. When you hook up your battery cables, it creates a circuit and allows electrical current to flow. Simple enough.
Battery DeathWhen the French dude Andr Ampre gave us ways to measure this electric current flowing through a wire, the amount of storage capacity in a battery soon became rated in terms of the ampere hour (A·H) using a common scale, such as the 20-hour rate of discharge. For example, a 100 A·H rated battery will discharge below a useable level (10.5 volts) in 20 hours with a load of 5 A (5 A x 20 hours = 100 A·H). Obviously, most batteries are rated below this, but we used an easy number for illustration purposes. If that 100 A·H battery had a 2.5A draw (like a dome light) it would drop below 10.5 volts very quickly (2.5 A x 40 hours = 100 A·H).
To perform diagnostics, you'll need more than a voltmeter. You need to observe the movement of the amp flow, and that requires an amp gauge. The rule of thumb here is to draw the line at the amp load you're willing to accept while the car is sitting. You likely won't notice a draw of about 0.15-0.17 A on a car that's driven every day, but if it is a street machine that sits for weeks at a time, you should get the total draw down to 0.01 A. According to the Battery Council International, anything above that point indicates a problem that should be fixed.
Buy A MeterYou'll need an amp meter that can measure at least 0-10A DC to perform these tests. Most multimeters can do this, and you can spend as much or as little as you want to get one. We checked Harbor Freight and found an AC/DC meter made by Cen Tec that can measure up to 20 A of DC current with overload protection for $19.99. We also found a Wavetek for $169.95 and a Fluke meter for $209 that does the same thing but has a lifetime warranty. If we had to buy a new one, we'd go with as much meter as we could afford-the new models have temperature probes and other goodies. The Fluke we used in this story was purchased in 1989 for $85 and it still works great, so you get what you pay for.