What a dead cell is
A typical 12-volt car battery is built from six cells connected in series, each producing roughly 2.1 volts when fully charged. A cell can fail because of internal damage, a short between plates, or buildup of sulfate crystals after a long period of being discharged. When one cell fails, the battery can no longer deliver its full voltage, and it often cannot supply the heavy current the starter needs.
Symptoms of a dead cell
- Slow or labored cranking, or a clicking sound instead of the engine turning over.
- A battery that seems to charge but will not hold the charge, so the car works after a jump-start or charge and then fails again soon after.
- Low resting voltage even after charging. A healthy, fully charged battery typically reads around 12.6 volts at rest. A battery with a failed cell often reads noticeably lower, commonly somewhere around 10.5 volts, though readings vary.
- Dim lights and electrical issues, which are also common signs of a dying battery.
- Heat, swelling, or a rotten-egg smell in some cases of internal shorting. These deserve caution; see leaking car battery.
Can a battery with a dead cell still start a car?
Occasionally, especially with a jump-start or in mild weather, but it is unreliable. The remaining cells may not provide enough power under load, and the car may fail to start the next time. A jump-start only borrows power from another source, so it can hide the underlying battery problem for a short while.
How to check
A multimeter reading is a starting point, but a load test is more conclusive. Many auto parts stores and repair shops will test the battery for free or a small fee. It is also worth testing the alternator, since a charging fault can mimic or contribute to battery failure.
If you check voltage yourself, make sure the car has been off for a while so the surface charge does not mislead you, and keep tools and jewelry away from the terminals to avoid shorting them.
Can a dead cell be fixed or recharged?
In practice, not reliably. A charger may raise the voltage of the good cells and make the battery appear healthier for a while, but a truly failed cell generally does not recover. Online “repair” methods are not dependable and can be hazardous because of the acid and the hydrogen gas batteries can release. Recharging is worth trying if the battery was simply drained, but if it will not hold a charge or tests poorly afterward, replacement is the safer path.
What to do next
- Have the battery load tested.
- If it fails, replace it with the correct type and size for your vehicle.
- Check the alternator and look for anything draining the battery.
- Recycle the old battery. Most retailers that sell batteries will take it back.
For a sense of when a battery is likely to reach this stage, see how long car batteries last.
Causes and prevention
Age is the most common cause. Deep discharges, extreme heat, vibration, and leaving a battery unused for long stretches can all speed the problem along. Keeping the battery charged, secure, and clean helps, as covered in car battery maintenance tips.