Car Battery Maintenance: What Actually Kills Batteries and How to Extend Their Life
Car batteries fail for a small number of predictable reasons, most of which are accelerated by behaviors that are avoidable once understood. The lead-acid battery that sits in the engine bay of most conventional vehicles is a mature technology with well-documented failure modes and a service life that ranges from two years, in the worst conditions, to six or seven years with appropriate care. The difference is not accidental.
The primary enemy of battery longevity is deep discharge — allowing the battery to drain significantly below its nominal charge before recharging. Lead-acid batteries are not designed for repeated deep discharges. Each significant discharge-recharge cycle causes some sulfation of the lead plates — the buildup of lead sulfate crystals that reduce the plate area available for chemical reaction and permanently reduce the battery’s capacity. A battery that has been deeply discharged multiple times is a battery with reduced capacity that will struggle to start the engine in cold weather and will fail sooner than one that has been maintained at proper charge.
The Common Deep Discharge Scenarios
Lights left on overnight are the most common cause of deep discharge. A single complete discharge is not a death sentence for a modern battery — a quality unit that is recharged slowly and fully after a deep discharge will recover most of its capacity — but repeated occurrences accumulate damage. The solution is not the jump start that follows the dead battery; it is the slow recharge that should follow the jump start. Jump starting and immediately driving the car to recharge the battery through the alternator is an incomplete solution — the alternator charges the battery during driving but does not provide the slow, complete charge that a dead battery needs to recover fully.
Infrequent use is a related and underappreciated cause of battery failure in collector cars and seasonal vehicles. A battery that sits at low state of charge for extended periods self-discharges — lead-acid batteries lose approximately 4 to 6 percent of their charge per month in storage — and sulfates at a rate that the subsequent usage cannot fully reverse. Cars that are stored for more than a month benefit from a battery tender — a smart charger that maintains the battery at full charge without overcharging — connected during the storage period.
Temperature Effects
Cold weather reduces battery capacity — a battery that delivers 100 percent of its rated capacity at 70 degrees Fahrenheit delivers approximately 40 percent of that capacity at 0 degrees Fahrenheit. Starting an engine in cold weather simultaneously demands more current from the battery (because the engine oil is thick and the engine turns over slowly) and receives less capacity from the battery. A battery that starts the engine marginally in cool weather will not start it in a cold snap.
Heat is the other temperature extreme that accelerates battery degradation. High under-hood temperatures accelerate the evaporation of electrolyte — the sulfuric acid and water mixture inside the battery — and the corrosion of internal components. Batteries in hot climates or in engine bays with poor cooling have shorter service lives than identical batteries in temperate climates, independent of usage patterns.
Testing Before Failure
A battery that is tested under load — a load test that draws specified current from the battery and measures the voltage drop — can reveal reduced capacity before the battery fails completely. Most auto parts stores perform free battery load tests. The test takes five minutes and provides accurate information about the battery’s remaining capacity relative to its specification.
Testing annually, or when the battery is three years old and showing any symptoms of reduced performance — slow cranking in cold weather, longer crank times than normal, dimming lights during starting — provides the information needed to replace the battery before it leaves the driver stranded. A battery that tests at 70 percent of rated capacity is an overdue replacement, not a serviceable unit. The failure will arrive; the only question is its timing.
Terminal Maintenance
The corrosion that forms on battery terminals — the white or blue-green deposits at the cable connections — increases resistance in the circuit and reduces the current available to the starter. Cleaning terminals with a wire brush and a bicarbonate of soda solution, then applying a terminal protection spray or grease, is a ten-minute maintenance task that costs less than two dollars and prevents the connection problems that can mimic battery failure.