How to Test 18650 Lithium Batteries (And When to Stop)

By Rahib Ali · Founder, DIYTech Labs

Old laptop battery packs are full of 18650 lithium cells, and many of them are still usable for torches, power banks and DIY projects. But "repairing" a lithium cell is the wrong word: you cannot fix a damaged cell. What you can do is test cells properly and reuse only the healthy ones.

Safety first: lithium cells can catch fire if shorted, punctured, overheated or charged incorrectly. Work on a non-flammable surface, never leave charging cells unattended, and recycle any swollen, leaking, rusty or hot cell immediately. If you are not confident, take the pack to a battery shop instead.

Step 1: Extract Cells Carefully

Open the pack slowly, cutting plastic — never the cells. Avoid tearing the nickel strips across both terminals at once (a short). Note the original configuration.

Step 2: Measure Voltage

3.0V – 4.2V: normal range, proceed to charging with a proper 18650 charger.
2.5V – 3.0V: deeply discharged; some smart chargers can recover these gently — monitor closely.
Below 2.5V: retire and recycle the cell. Reviving cells below this level is not safe, whatever old forum posts claim.

Step 3: Charge and Capacity-Test

Charge fully, rest the cell for a few hours, then run a capacity test with a charger/analyser that discharges and measures mAh. Label each cell with its real capacity. Cells above roughly 80% of their rated capacity are worth keeping for low-drain projects.

Step 4: Match and Store

Use cells of similar capacity together, never mix strong and weak cells in one pack, and store them at partial charge in a proper case — never loose in a drawer with metal objects.

Do and Don't

Do	Don't
Wear eye protection when opening a pack; springs and strips can flick out	Pry at a cell itself with a metal tool — punctured lithium cells can ignite
Work on a clear, non-flammable surface and keep cells in a case	Leave cells loose in a pocket or drawer with keys and coins
Use a charger made for 18650 lithium cells and stay nearby while charging	Charge with a random adapter, or leave charging cells unattended overnight
Label every tested cell with its measured capacity and date	Mix strong and weak cells together in one pack or device
Recycle swollen, dented, rusty or leaking cells at a proper collection point	Throw lithium cells in household rubbish, or try to "repair" a damaged cell
Keep a suspect cell somewhere cool, dry and away from flammables	Carry on using a cell that gets hot during normal charging or use

A classic DIY Tech Pro electronics tutorial, maintained by DIYTech Labs.