A cordless tool battery that will not charge is often being rejected for a simple reason: poor contact, an out-of-range battery temperature, a charger problem, or a pack condition the charger is designed not to override. Start with the matching charger, its indicator-light instructions, clean accessible contacts, and a cooled battery. Do not open the pack, jump-start it, bridge terminals, or attempt to bypass its protection circuitry.

If the battery appears fully charged but will not run the tool, test the system with another known-good battery of the same approved platform if one is available. A good battery that runs the tool points toward the original pack; a good battery that also fails points toward the tool, its contacts, or a need for authorized service.

Start with the charger light pattern

A battery charger flashing light does not have one universal meaning. Solid red, solid green, alternating lights, and flashing lights can mean different things among tool systems and charger generations. Use the status-light chart printed on the charger or in the current manual for the exact model before deciding that a pack has failed.

For example, a Milwaukee service bulletin for a specific 12-volt charger lists solid red as charging and solid green as charge complete, while simultaneous flashing red and green calls for checking battery seating and, if needed, resetting the charger by unplugging it. That procedure applies to the models named in that bulletin, not automatically to every charger. Milwaukee’s bulletin illustrates why the model-specific instructions matter.

Check the power source and charger first

Make sure the charger is firmly plugged into a working outlet and that its cord, plug, housing, and battery bay are undamaged. If the charger has no normal power indication when empty, do not assume the battery is the problem.

If you have a second known-good battery approved for that charger, use it as a comparison:

  • If the known-good battery charges normally, the original battery is likely the issue.
  • If neither battery is accepted, suspect the outlet, charger, or its power connection.
  • If a different approved charger accepts the battery, stop using the questionable charger and arrange service or replacement as appropriate.

Do not use a charger from a different battery platform merely because the battery seems to fit. Verify compatibility in the current manufacturer documentation.

Reseat and inspect the battery contacts

Remove the battery from the charger. Look at the exposed metal terminals on both the battery and charger for sawdust, grit, oil, oxidation, bent parts, melted plastic, or signs of overheating.

Clean loose dirt from accessible areas with a clean, dry cloth. Do not scrape terminals aggressively, spray liquid into the charger, insert metal tools into the battery bay, or use solvents unless the manufacturer specifically permits them. Reinstall the pack fully and squarely; it should seat as intended without force.

A battery that is not fully seated may produce a flashing or fault indication. On the charger covered by Milwaukee’s bulletin, checking that the battery is firmly seated is the first response to its specified flashing-light condition. See the manufacturer bulletin.

Let a hot or cold battery return to a normal temperature

A battery used hard in a drill, saw, grinder, or outdoor tool can be too warm for charging immediately afterward. A pack left in a cold vehicle or unheated shop can also be outside the charging range. Set it indoors in a dry location away from direct heat, sunlight, and flammable materials, then allow it to return to normal room conditions before trying again.

Do not put a battery on a radiator, use a heat gun, leave it in direct sun, or try to cool it in a freezer. Those shortcuts can damage the pack or create a safety hazard.

Black+Decker’s support guidance specifically advises allowing a battery to cool before reinserting it in the charger and contacting support if it still will not charge. Read the support guidance.

If the charger flashes or rejects the battery

After checking the exact light code, use this safe sequence:

  1. Remove the battery and inspect the contacts and case.
  2. Let the battery cool or warm naturally to room conditions.
  3. Reinsert it fully in the correct charger.
  4. If the manual permits a charger reset, unplug the charger, wait as directed, reconnect it, and try again.
  5. Compare with a known-good approved battery if available.
  6. Stop and seek manufacturer or authorized service if the fault returns.

Discontinue use immediately if the battery is swollen, cracked, leaking, unusually hot, has a burnt odor, has melted terminals, or was badly dropped, crushed, or exposed to water. Do not charge a visibly damaged pack.

Why you should not revive or bypass a lithium-ion pack

Online videos may show people opening battery cases, applying power directly to terminals, transferring cells, or “jump-starting” a pack that the charger will not recognize. Do not use those methods on a cordless-tool lithium-ion battery.

The charger and battery pack work together as a system. If the pack refuses to charge, that may be a deliberate protective response rather than a minor inconvenience. Bypassing that response can conceal a damaged pack, cause overheating, damage the charger or tool, and increase fire risk.

Only an authorized service provider should evaluate an internal battery fault. A user should not open a sealed pack, probe internal cells, alter the wiring, or attempt to charge it with an improvised power source. For lithium-ion tool battery safety, follow the battery, charger, and tool manuals for your exact model.

Battery says charged but the tool will not run

A green light or full fuel-gauge display does not prove that the battery can deliver power under use. First, remove and reinstall the battery in the tool and inspect the tool-side contacts for dirt or physical damage.

Then make a controlled comparison if you have another approved battery:

  • The second battery runs the tool: The original pack likely needs replacement or authorized evaluation.
  • Neither battery runs the tool: The tool, its battery interface, or its switch may be at fault.
  • The battery runs one tool but not another: Confirm both tools are approved for that exact battery platform, then have the nonworking tool checked.

If the tool starts and immediately stops under load, avoid repeatedly cycling it until the problem is identified. The cause may be related to the tool, the battery, the work being performed, or a protective shutdown. For related symptoms, see why a cordless drill starts then stops.

When replacement is the practical answer

Replace the battery or use authorized service when it remains unrecognized after the safe checks above, when a known-good charger rejects only that pack, or when there is any physical damage or overheating concern. A pack that will charge but has very short runtime may also be nearing the end of its useful service life.

Use a replacement battery and charger that the tool manufacturer identifies as compatible with your system. Do not assume that voltage markings, connector shape, or a claimed universal fit establish safe compatibility. For more on platform limits, see universal power tool batteries.

Prevent another charging problem

Keep batteries and chargers dry, reasonably clean, and protected from extreme heat and cold. Remove heavy dust from the battery bay before charging, avoid dropping packs, and store batteries where terminals cannot contact loose metal objects. Use the specified charger and follow the storage and charging directions in the current product manual.

The safest outcome is not always getting an old battery to accept a charge. When the charger continues to reject a pack after basic inspection, temperature recovery, and comparison testing, replacement or authorized service is the appropriate next step.

References

  1. How To Fix a Lithium Ion Battery That WON’T Charge. (n.d.). https://www.facebook.com/Primitivemillworks/videos/how-to-fix-a-lithium-ion-battery-that-wont-charge-tipsandtricks-woodworkingcommu/467145729431648
  2. Tool Battery Wont Charge? Try This EASY FIX! - How To Revive Dead Cordless Tool Batteries. (n.d.). https://www.youtube.com/watch?v=yIH2sExaL9E
  3. Power Tool Battery Not Charging? 6-Step Fix Guide – BattGlobal Power Hub. (n.d.). https://battglobal.com/blogs/blog-1/power-tool-battery-not-charging-heres-how-to-diagnose-and-fix-it
  4. Five common battery issues and possible solutions. (n.d.). https://www.klium.com/en/blog/battery-problems
  5. SOLVED: Why won’t my battery charge? - DeWalt DC970. (n.d.). https://www.ifixit.com/Answers/View/303602/Why+won't+my+battery+charge
  6. Battery shows fully charged but does not work. Tried jump …. (n.d.). https://www.reddit.com/r/Dewalt/comments/kdx03v/battery_shows_fully_charged_but_does_not_work
  7. How To Fix Common Issues With Cordless Drills And Batteries - Blog - News - GreatStar. (n.d.). https://www.workprotool.com/news/how-to-fix-common-issues-with-cordless-drills-83312638.html
  8. If my battery is not charging , what can I try?. (n.d.). https://support.blackanddecker.com/hc/en-us/articles/360012702998-If-my-battery-is-not-charging-what-can-I-try
  9. TO INFORM YOU. (n.d.). https://documents.milwaukeetool.com/TIY476.pdf
  10. Lithium Battery Not Charging? Troubleshooting & Fixes by ACE Battery. (n.d.). https://www.acebattery.com/blogs/how-to-fix-a-lithium-battery-that-won-t-charge