There's nothing but a loud buzzing noise coming from under the hood when you try to start your car. Jumper cables get you up and scurrying to work--but you need another jump to get home. Time to pull some maintenance on the battery.
Automotive batteries have a finite life span. Undercharging, overcharging and heat all team up to degrade your battery. Poor electrical connections make it more difficult for even a good battery to do its job.
ALL CHARGED UP
Start any battery maintenance program by checking open-cell voltage, either with a dedicated battery tester or a voltmeter. With the battery fully charged and all electrical drains-dome light, warning buzzer, etc.-shut down, the voltage across the terminals should be 12.5 to 12.6 volts. If the battery is not completely charged, but still adequate to turn over the motor, you may see a voltage closer to 12.0 volts.
If the battery shows less voltage or won't take a charge, it's time to get out the hydrometer. This device checks the proportions of sulfuric acid and water in the electrolyte, which is a precise indication of the level of charge. Pull up the battery fill caps--if you can. Add distilled water to any cells in which the level of electrolyte isn't touching the bottom of the fill port. Use only distilled water. The minerals in tap water will eventually reduce a battery's capacity.
If your battery is one of the so-called low-maintenance varieties, you may not see any filler caps. These batteries claim to never need water added. While it's true that they have a slightly different chemistry that consumes less water, and the level of electrolyte in the cells is deeper over the top of the plates, eventually these batteries run out of water and die. Look carefully and see if there is a way to pull up a set of filler caps. They may be hidden under a sticker that can be slit open. Others are permanently sealed shut.
If the level of electrolyte is very low, suspect a charging system that's generating too high a voltage. The maximum voltage you should see across the battery terminals with a fully charged battery and an engine running well above idle is about 14.6 volts. Another cause of low electrolyte is excessive heat. Sun Belt drivers should top up battery levels regularly because underhood temps will climb high enough to drive water out of the battery very rapidly, even if the charging system is doing its job correctly. If your vehicle came with an insulator around the battery, be sure it's in place and in good shape.
Time-honored baking soda paste is still the best way to neutralize and remove the acid on the battery top.
Back to the hydrometer: With your electrolyte level correct, suck up enough battery electrolyte to cover the hydrometer float. Leave the bottom of the hydrometer inside the port to keep any drips contained. Don't spill any electrolyte on yourself or the car--it will eat holes in your clothes and remove paint. Wear safety glasses. If you do spill any, immediately rinse with lots of water. Tap lightly to make any bubbles clinging to the float rise to the surface. Read the specific gravity on the hydrometer's scale at the bottom of the meniscus. A fully charged battery should have a specific gravity of at least 1.265. More important, all six cells should show very similar readings. One cell lower than the other five by 0.05 or more? Start shopping for a new battery.
If the battery is discharged, you can't do any further diagnosis. Use a battery charger until the open-circuit voltage is 12.6 volts. Avoid quick-charging, as the high current can warp the plates. If the battery is deeply discharged, don't use the engine's alternator to charge it by jump-starting and running the vehicle--the alternator is not designed to produce that amount of current for that long and may be damaged. Use a battery charger. If the battery is very deeply discharged, its internal resistance will be high and it may not accept any charge. Just be sure the battery charger is putting out at least 13.5 volts, and wait. The battery's internal resistance will fall, and the charging current will rise to normal levels. Be sure to not overcharge and boil off the electrolyte. And, never let a fully discharged battery remain discharged for long--this promotes sulfation of the lead plates.
Don't leave a discharged battery in below-freezing conditions. Its electrolyte is basically just water, and it will freeze and damage the plates inside. Bring a discharged battery into a warm environment as soon as possible. Then charge it immediately, even if this means removing the battery from the vehicle or towing the vehicle to someplace where it can be brought inside.
SO NEAR AND YET SO FAR
If your battery has passed these tests and your car still won't start reliably, you may have some issues with the wiring. We'll leave starter circuit problems for another Saturday Mechanic. Now it's time for a general overhaul of the clamps and cables.
Start with a gentle cleaning of the battery clamps and the general area with a handful of baking soda, some cold water and an old toothbrush. Rinse thoroughly. Now you've neutralized most of the sulfuric acid that's collected near your battery so it won't get on your hands or clothes or in your eyes.
Remove the battery clamps by loosening the pinch bolt. Won't move? It's common for a clamp to become almost permanently attached to the post. Don't bang on it or try to pry it off--the post is lead, soft as a stick of margarine, and alarmingly easy to break off flush with the top of the battery case, requiring the battery to be scrapped. Drop a few bucks on a battery-clamp removal tool.
Now clean the clamps and posts with more baking soda and water. Follow that with a cleaning of the entire battery top using a few drops of detergent and plenty of water. Any dirt that accumulates on top of the battery will attract moisture. The moist film of dirt will conduct a small amount of current from one post to another, constantly discharging the battery even when the vehicle is turned off. Keep your battery clean and dry.
Follow up with a post-cleaning tool. This double-ended wire brush should be used to get bright, shiny metal showing on both the post and the inside surface of the clamp. Reinstall the clamp; avoid overtightening.
If your battery has side terminals, the procedure is much the same, although you won't need a clamp removal tool. Clean and brighten the terminals and reinstall. There's a reason the bolt used on side-terminal batteries is smaller than a tax collector's heart. It's to prevent you from overtightening the bolt and destroying the threads in the soft post. Use a small wrench and tighten only to about 10 lb.-in.
UNIVERSAL CLAMPS AREN'T
If the clamp or wiring looks cheesy, replace it. When the starter is engaged, these components conduct several hundred amperes--more than some arc welders. Avoid "universal" replacement clamps, except as an emergency repair. Use proper molded-on clamps and heavy-gauge replacement cables, which you can get in any auto parts department for less than 10 bucks. Positive and negative posts are slightly different in size--don't substitute the clamps or they won't fit properly.