Automobile Repair/Battery Checking

The 12-volt battery starts the engine and keeps the electronics alive when the engine is off. Most batteries fail gradually, over weeks, and a quick check every few months (and before winter) catches the decline before the first cold morning does. Three checks are useful, in increasing order of effort: a look at the battery, a voltmeter reading, and a load test. Visual check Terminals and clamps: a white or blue-green crust is corrosion, which raises the resistance of the connection. Disconnect (negative first), clean the posts and clamps with a wire brush or a terminal tool, refit (positive first) and tighten so the clamp cannot be turned by hand. A thin coat of petroleum jelly or terminal spray slows the return of the crust. Case: a bulged or cracked case means the battery has been overcharged or frozen and must be replaced. Hold-down: a battery that can move in its tray will suffer from vibration and its terminals will work loose. Electrolyte level (batteries with removable caps only): the plates must be covered. Top up with distilled water, never acid, and never above the mark. Date: a code on the case or a sticker gives the month and year of manufacture. Most batteries last four to six years; in hot climates less. Voltage check A digital multimeter set to DC volts, red probe on the positive post and black on the negative, gives a reasonable picture of the state of charge provided the battery has rested for several hours with nothing switched on (an overnight stand is ideal; a reading taken just after driving is misleadingly high). Resting voltage Approximate state of charge 12.6 V or more fully charged 12.4 V about 75 per cent 12.2 V about 50 per cent 12.0 V about 25 per cent below 11.9 V discharged; recharge and retest A battery that reads 12.6 V after charging but drops to 12.2 V or below after a night's rest is losing charge internally and is near the end of its life. With the engine running, the same measurement shows the charging system: about 13.5 to 14.8 V is normal. A reading at or below the resting voltage means the alternator is not charging; a reading above about 15 V means the regulator has failed and is overcharging. Load test Voltage alone cannot show a battery that holds 12.6 V but collapses when asked for the starter's 150 to 400 amps. Two ways to test under load: Cranking test with the multimeter still on the posts: have a helper crank the engine. A healthy battery stays above about 9.6 V during cranking (at normal temperatures); a battery that drops toward 8 V or lower is weak even if its resting voltage looked fine. Electronic tester (conductance tester): the hand-held testers used by workshops and parts stores measure the battery's internal resistance and report its remaining capacity against the rating printed on the label (cold cranking amps). Many parts stores will test a battery free of charge. A printout that says replace, good, recharge or charge and retest means what it says: recharge first with a proper charger and test again before condemning the battery, since a discharged battery can test as faulty. What a weak battery looks like in use Slow cranking, especially on a cold morning. The dashboard lights dim or reset while cranking. The car starts after a jump start but not on its own the next day. Warning lights for unrelated systems (ABS, power steering, driver-assistance) appearing together, which is a common symptom of low system voltage on modern cars. If the battery is new and keeps going flat, the problem is elsewhere: a charging fault, or a parasitic drain from something that stays on with the ignition off. A drain is found by measuring current in the negative lead with everything off and the car asleep; a few tens of milliamps is normal on a modern car, several hundred is not. Replacing the battery Fit a battery of the specified size, terminal layout and technology (an engine with stop-start needs an AGM or EFB battery, not a conventional one). On many cars built since about 2010 the replacement must be registered with a scan tool so the charging system adjusts to it, and some cars lose settings (radio, windows, steering angle) when the battery is disconnected; the owner's manual lists what needs resetting. See also Jump start.