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Everything below applies to 12 volt lead-acid batteries — flooded, AGM or gel. For a 6 volt battery, halve the voltage figures. For lithium of any kind, none of this applies; see when to replace instead.
What you need
- A digital multimeter. The single most useful tool here. An inexpensive one is fine.
- A hydrometer — only usable on flooded batteries with removable caps. Not usable on sealed AGM or gel.
- A battery charger, to bring the battery to a known state before testing.
- Optional: a load tester, or a shop that will run a load test for you, often free.
Step 1 — Look at it before you measure anything
A visual check settles a surprising number of cases immediately. Stop here and scrap the battery if you find a cracked or split case, a swollen or bulging case, leaking electrolyte, or a melted or badly corroded terminal post. None of those are recoverable and a damaged case is a hazard.
Heavy white or blue-green crust on the terminals is different — that is external corrosion, it is normal, and it cleans off. Clean the terminals before testing, because corrosion alone can cause bad readings and starting problems that have nothing to do with the battery's health.
Step 2 — Measure resting voltage
Resting voltage means the battery has been disconnected and left alone. If it has just been charged or just been in a running vehicle, it holds a surface charge that reads artificially high. Let it sit for at least a few hours — overnight is better — or briefly switch on the headlights for a minute to bleed the surface charge off.
Set the multimeter to DC volts, red probe to positive, black to negative, and read across the terminals.
| Resting voltage (12V battery) | Roughly what it means |
|---|---|
| 12.6 V and above | Fully charged |
| 12.4 V | About 75% charged |
| 12.2 V | About 50% charged |
| 12.0 V | About 25% charged — sulfation is likely starting if it sits here |
| 11.8 V and below | Deeply discharged; extended time in this state causes hard sulfation |
| Around 10.5 V after a full charge attempt | Strongly suggests one shorted cell — see step 4 |
These figures assume roughly room temperature. A cold battery reads slightly lower without being in worse condition, so don't judge a battery harshly on a freezing morning.
Step 3 — Charge it, then see whether it holds
Put the battery on a charger and bring it up as far as it will go. Then disconnect it and leave it sitting, unconnected to anything, for 24 hours. Measure again.
A healthy battery sits at or near 12.6 V. A battery that climbs to 12.6 V on the charger and then sags to 12.1 V or lower overnight, with nothing connected to it, is failing to hold charge internally. That pattern — charges up fine, will not stay there — is the classic sulfation signature, and it is the case where reconditioning is at least worth attempting.
A battery that will not rise above roughly 12 V no matter how long it charges is a different and much worse situation.
Step 4 — Check for a shorted cell
A 12 volt battery is six 2.1 volt cells in series. If one cell shorts internally, you lose roughly 2.1 volts off the top and the battery tops out near 10.5 V no matter how long you charge it.
On a flooded battery you can confirm this directly with a hydrometer. Draw electrolyte from each cell in turn and read the specific gravity. On a fully charged battery expect readings around 1.265, and around 1.120 when discharged. What matters most is consistency: if five cells read close together and one reads far lower, that cell is the problem. A spread of more than about 0.05 between the highest and lowest cell points to a bad cell.
A shorted cell is not a sulfation problem and will not be fixed by reconditioning. That battery is finished.
Step 5 — Load test it
Voltage alone tells you what the battery holds at rest, not whether it can deliver current. A load test is what separates a battery that looks fine on a meter from one that actually still works.
The standard test applies a load of about half the battery's cold cranking amps for 15 seconds while watching the voltage. A battery in serviceable condition stays above roughly 9.6 V throughout at normal temperature. One that collapses well below that under load has lost real capacity, even if its resting voltage looked respectable.
If you do not own a load tester, most auto parts shops will run this test for free, and it is worth the trip before you invest time or money in recovery.
Reading the results together
| What you found | What it points to |
|---|---|
| Charges fully, then self-discharges over days; even cell readings; no physical damage | Reasonable reconditioning candidate |
| Sat discharged for months, now has reduced capacity but no other faults | Reasonable reconditioning candidate |
| Tops out near 10.5 V; one cell far lower than the rest | Shorted cell — replace |
| Cracked, swollen or leaking case | Replace, and handle carefully |
| Fails a load test badly despite a full charge | Capacity is gone — usually replace |
| Well beyond its normal service life | Recovery unlikely to last even if it partly works |
If your battery lands in one of the candidate rows, the when to replace page covers the remaining judgement calls, including the cases where recovery is technically possible but not worth your time.
These are general guidelines for typical lead-acid batteries, not a substitute for your battery manufacturer's own specifications. Follow the manufacturer's documentation where it differs.