12-Volt Battery
A chemical store of electricity. Its real jobs are cranking the starter, keeping modules alive while the car is parked, and steadying system voltage — it is not what powers the vehicle while the engine is running.
- Typical cost
- $180–$500
- Wear item
- Yes
- Service life
- A consumable. Three to five years is the common working life in a temperate climate. Sustained heat shortens it considerably, and hot-climate vehicles often see less. Short trips that never fully recharge it, and every episode of being run completely flat, take permanent capacity away — a battery that has been deeply discharged several times is never the same again. Check the date code rather than guessing from mileage.
The battery does three things. It delivers a very large burst of current for a second or two to crank the engine. It keeps the alarm, keyless entry, clock, and module memory alive while the car sits. And while the engine runs, it acts as a buffer, absorbing voltage spikes and filling in dips when a big load switches on. That last job is why a weak battery causes a bizarre spread of unrelated-looking electrical faults on a modern car: control modules misbehave, throw communication codes, and set warning lights when voltage is unstable. A great many mystery electrical complaints are a tired battery.
Six cells in series, each producing about 2.1 volts, giving roughly 12.6 volts fully charged. Each cell has lead dioxide positive plates, sponge lead negative plates, and sulfuric acid electrolyte. Discharging converts both plate surfaces toward lead sulfate and dilutes the acid; charging reverses the reaction. Because the acid's concentration tracks the state of charge, resting voltage is a usable proxy for how full the battery is. Standard rested figures for a 12-volt flooded battery, measured a few hours after charging or driving: about 12.6 volts full, 12.4 around three-quarters, 12.2 around half, 12.0 around a quarter. AGM batteries read roughly a tenth or two higher. The ratings on the label mean specific things. Cold cranking amps is the current the battery can deliver for 30 seconds at 0°F while holding above 7.2 volts. Reserve capacity is how many minutes it can supply 25 amps before falling to 10.5 volts. Amp-hours is total capacity. Chemistries differ: conventional flooded, enhanced flooded (EFB), and AGM, which handles deep cycling and high accessory loads and is required on most stop/start vehicles. Fitting a flooded battery where AGM was specified generally shortens its life sharply. Hybrids and EVs still carry a 12-volt battery to power the control systems that wake the high-voltage system up.
Sulfation from sitting discharged hardens the lead sulfate on the plates and permanently reduces capacity — this is what happens to a battery left flat over a winter. Heat drives grid corrosion and plate shedding, which is why hot climates kill batteries and cold weather merely exposes what heat already did. A shorted cell leaves resting voltage stuck near 10.5 volts, refuses to accept a charge, and drags the alternator to full output trying to fill it. An open cell gives a dead or intermittently dead battery. Overcharging boils off electrolyte and warps plates. Internal resistance rises steadily with age, so an old battery can read a perfect 12.6 volts and still collapse under cranking load — voltage alone never proves a battery is good. Terminal corrosion and loose clamps produce identical symptoms to a dead battery and cost nothing to fix. A loose hold-down lets the case vibrate and crack internal connections. And a battery that keeps dying may itself be healthy: a parasitic draw, a charging fault, or a pattern of short trips that never fully recharges it are all real causes.
- Slow, labored cranking, noticeably worse when cold
- Rapid clicking with the dash lights collapsing when you turn the key
- Fine one day, completely dead the next morning
- Stop/start quits working — often the earliest warning
- Unrelated warning lights, odd module behavior, radio or window malfunctions
- White or blue-green crust on the terminals
- Case bulged or cracked, or a rotten-egg smell near the battery
- Needs a jump every few days
These symptoms overlap with several other faults. They are a reason to test this component, not evidence that it has failed.
Order matters, and so does distinguishing this from the alternator. 1. Inspect. Terminals clean and tight, clamps not moving when you twist them, case not bulged, hold-down secure. Check the date code — age is more predictive than mileage. 2. Rested voltage. Let it sit several hours, or turn the headlights on for 30 seconds first to remove surface charge. Read across the posts. Use the figures above. Anything below about 12.4 volts is not fully charged, and testing an undercharged battery tells you nothing — charge it first. 3. Load or conductance test, at full charge only. A traditional carbon-pile load test applies about half the rated CCA for 15 seconds; voltage should stay above about 9.6 volts at roughly 70°F, and the acceptable threshold falls as temperature drops. Electronic conductance testers do this without the heat, but you must enter the CCA rating printed on the battery or the result is meaningless. 4. Cranking voltage. Watch at the posts while someone cranks. A good fully charged battery generally holds above about 9.6 volts. 5. Charging voltage, engine running. Conventional systems land in the 13.5 to 14.7 volt band. Smart-charging systems vary the target on purpose, so compare your measurement against the scan tool's commanded voltage rather than assuming a low reading is a fault. 6. If the battery is good and the charging system is good but it still goes flat, do a parasitic draw test. Wait for every module to sleep — this can take 20 to 60 minutes on a modern vehicle — then measure current with a meter in series with a disconnected cable or with a proper low-current clamp. Under roughly 50 milliamps is a common target, but the allowable draw is vehicle-specific. Then pull fuses one at a time to isolate the circuit. Battery versus alternator, the pattern that decides it: - Jump starts, runs fine all day, dead again after sitting overnight → battery or parasitic draw. - Jump starts, then dies within minutes of removing the cables, or dies while driving with the charging light on → alternator or charging circuit. - Lights dim as you add electrical load and brighten when you rev → charging system output is low. - Cranks slowly but charges to 14 volts and runs indefinitely → battery. - Rested voltage stuck near 10.5 volts and will not accept a charge → shorted cell, battery. And the trap that catches people: a failed alternator flattens a healthy battery, and a shorted battery destroys a healthy alternator. Test both before replacing either. Registration: many vehicles with smart charging require the new battery to be registered or coded so the charging strategy matches its type and age. Skipping it can shorten the new battery's life. Whether it is required, and how, varies by manufacturer. Safety: batteries vent hydrogen, which is explosive — no sparks, no flames, no smoking. The electrolyte is sulfuric acid; wear eye protection. Disconnect the negative cable first and reconnect it last. Never jump or charge a battery that is frozen, bulged, or leaking.
Specification values differ between manufacturers and model years. Where a figure is not genuinely standard across OBD-II vehicles, check it against service information for your specific vehicle rather than a generic number.
Most commonly in the engine bay, but plenty of vehicles put it in the trunk, under a rear seat, under the floor, or in a wheel well, with a remote jump-start post under the hood. Group size, terminal orientation, hold-down design, and vent tube routing are all vehicle-specific, and using the wrong size or an unvented battery in a cabin location is genuinely unsafe.
Typically $180–$500 at a US independent shop.
US independent shop, parts and installation, for a conventional flooded or AGM battery in an accessible location. AGM and larger group sizes sit at the upper end. Batteries under a seat or in the trunk, and vehicles requiring battery registration or module relearns, add labor. A free test at a parts counter is a screening check, not a diagnosis — it will not find a parasitic draw or a charging control fault.
Ranges are wide because access varies enormously between vehicles — the same part can be a twenty-minute job on one engine and a half-day on another.