The drill stopped working after having driven six screws, even though it was completely charged an hour ago. The battery is either wearing out or it's giving you a different message than 'old'. The entire point is to work out which situation you're in.
A battery that is dying manifests itself in two very different ways. The gradual decline is due to normal ageing and involves a shorter running time and a need for more charging; the dangerous symptoms are swelling, leaking, odd odours, and actual heat, all of which mean that you should stop using it today, not next week.
The guide looks at the signs that a power tool battery is failing, dividing them into two categories, and then provides you with a 5-minute test along with a decision table to tell you whether you should replace the battery or carry on using it, together with the correct method of getting rid of the old battery. For information on how batteries work, see our electric tool battery guide.
The dangerous signs: stop right now
This is not just the result of ageing; it is damage, and the Power Tool Institute is clear in its safety advice as to what this means.
- Swelling or bulging. The casing puffs because gas is building up inside a failing cell. A swollen pack that won't click into the tool is the clearest sign there is.
- Leaking fluid. Any liquid from the pack, any time.
- A chemical or sweet smell. Venting cells smell wrong, and "wrong" is the alarm.
- Real heat. Warm during use is normal. Uncomfortable to hold, or hot long after the work stopped, is not.
- Hissing, popping, or smoke. The last stage of the same failure.
You could do the following: remove the pack from both the tool and the charger, and if possible place it in a fireproof location, then do neither charging, opening nor shipping it. The Power Tool Institute has stated that a damaged pack can catch fire weeks after the damage has occurred, not me.
The slow fade: aging you can schedule
Most batteries don’t fail suddenly; instead they wear out, and this wearing out follows a certain pattern.
- Less run time per charge. The same jobs die sooner.
- More trips to the charger. Two charges a day where one used to cover it.
- Self-discharge. A pack left for a week is empty when you come back.
- Full, then nothing. The indicator says full, and the tool cuts out under load anyway. That's one weak cell tripping the protection, not a metering glitch.
- A jumpy indicator. The level meter bounces around because the cells don't agree with each other anymore.
The 'full-then-nothing' type is the one for which we receive the most emails, since it suggests that the tool is lying. It isn't; on the contrary, it is the healthiest action a dying pack can take.
Some brands offer a health check as part of their product package. For example, Bosch batteries have a defect indicator; all you need to do is press and hold the button which shows the charge level for about three seconds and the lights will then display the level of risk, there being one light for a high risk of defect and five lights meaning the battery is in good condition. Therefore, it's advisable to check with the manufacturer to see whether it has a feature similar to this before you buy the multimeter.
The 5-minute test
If the signs are ambiguous, three checks settle it.
- The visual once-over. Swollen, leaking, or singed? You're in the dangerous category, and the verdict is already made.
- The multimeter check, if you own one. A healthy 18V lithium pack fresh off the charger reads around 20 to 21 volts at rest. Below roughly 17.5 volts, the pack is heavily aged. That's a shop rule of thumb, not a manufacturer spec, but it's a fast triage.
- The load test. Fully charge the pack, then run a job you know by heart. If it dies in half the usual time, the capacity is gone. If it reads full and cuts out the moment you load it, a cell is done.
If the pack won't charge at all, that's a different branch of the flowchart, and our power tool battery not charging guide has the seven fixes.
The verdict: replace or tolerate
Time to sentence the patient.
| Symptom | Verdict | Action |
|---|---|---|
| Swollen, leaking, hot, or smelly | Dangerous | Stop now, recycle this week |
| Dies in half the usual time | Capacity gone | Replace when it annoys you, it won't recover |
| Reads full, cuts out under load | Weak cell | Replace |
| Charges and runs like it always did | Not the battery | Look at the tool or charger |
| Slightly shorter runtime, still does the job | Tolerate | Keep using it, start budgeting |
The age guideline also applies: the support pages for Black & Decker state that a normal lithium battery lasts for two to three years or for 300 to 500 full charge cycles, whichever occurs first. If a battery that is within that range fails badly it's worth having warranty coverage looked into; but if one that is well beyond that range fails then it's just the passage of time.
For the deeper numbers on lifespan, how long power tool batteries last has them, and when you do replace, the OEM vs aftermarket power tool batteries comparison covers which kind to buy. You can also browse replacement batteries by brand across the twelve platforms we stock.
What to do with the old pack
The last step is the one people skip, and it's the one that matters for safety.
- Tape the terminals. Electrical tape over the contacts so nothing can short them.
- Drop it at a battery recycling point. The Battery Network, the nonprofit behind the Call2Recycle program, lists over 20,000 drop-off locations in North America, including major hardware stores.
- Never in the trash. Lithium in household garbage starts truck and landfill fires, and the tape in step one is what keeps it safe on the way.
An hour of use a week isn't worth a fire. Three steps, two minutes, done.
Common questions about dying batteries
How do I know if my power tool battery is bad?
Split it into the two kinds. Dangerous signs, swelling, leaking, odd smells, real heat, mean stop immediately. Slow-fade signs, shorter run time, more charging, sudden cutouts under load, mean aging, and the 5-minute test above tells you how far along it is.
Why does my drill battery die so fast?
Usually capacity loss. Lithium packs lose capacity as they age, permanently, and heavy use, heat, and deep discharges speed it up. If the pack is years old or has hundreds of cycles on it, dying fast is the calendar talking, not a fault you can fix.
Is a swollen battery dangerous?
Yes, and it's the clearest warning sign a pack can give. The swelling is gas building inside a failing cell. Stop using it, don't charge it, and take it to a battery drop-off with the terminals taped.
Can you repair a swollen power tool battery?
No. A swollen cell can't be fixed, only removed, and opening a lithium pack to try is how fires start. The repair for a swollen battery is a replacement pack and a trip to the recycling point.
How long do power tool batteries last?
Most lithium packs run about two to three years, or 300 to 500 full charge cycles, whichever comes first. Those are manufacturer ratings. Heavy use, heat, and running packs to zero shorten the real number, and the aging is irreversible.
A battery that is dying will indicate what category it belongs to. The dangerous signs are obvious: swelling, leaking, heat, and odours. The gradual deterioration is subtle: each month it lasts a bit less until finally you notice. Identify the category, carry out the five-minute test, read the verdict, and then the old battery leaves the house in a safe manner, wrapped in tape and recycled before being replaced.
References & Sources
- Power Tool Institute battery safety guidance (Power Tool Institute, the power tool industry's safety association)
- Battery Network (the nonprofit that operates the Call2Recycle battery recycling program)
- Black & Decker batteries lifespan support article (Black & Decker support)
- Bosch support (battery defect risk indicator guidance)
Need a replacement?
If the verdict indicates that the batteries should be replaced, then look at the replacement batteries by brand. Since we carry compatible battery packs for twelve platforms, the new pack will have the same model number and any questions can be sent to a real person via our contact page.