How to Tell If a Power Tool Battery Is Failing

A failing lithium tool battery gives itself away long before it dies completely. The usual giveaways are runtime that has collapsed to a fraction of what it used to be, a pack that charges in a suspiciously short time, heat that builds fast under light work, and a tool that cuts out mid job even though the fuel gauge still shows charge. If two or more of those show up together, the cells are on the way out.

Runtime has dropped off a cliff

Every lithium pack loses capacity gradually. That is normal and you barely notice it, because the decline over the first couple of years is slow and steady. What is not normal is a sudden step change: last month the pack ran a full afternoon of screwdriving, this month it needs charging after twenty screws.

A sharp drop like that usually means one cell in the pack has degraded much faster than the others. The battery management system shuts the whole pack down when any single cell hits its low voltage limit, so one weak cell drags the entire battery down with it. There is no way to fix that from the outside, and it only gets worse.

It charges far too quickly

This one catches people out because a fast charge feels like good news. It is not. A healthy 5.0Ah pack that has been run flat takes a meaningful amount of time to fill. If a pack that used to take an hour now shows a full green light in fifteen minutes, it is because there is far less capacity left to fill.

Pair this with the runtime test and you get a very clear picture. Short charge plus short runtime means the usable capacity has genuinely shrunk. Short charge plus normal runtime usually means the charger or the gauge is misreading, which is a different problem.

It runs hot when it should not

Batteries warm up under heavy load. Driving 100mm screws into joists or running a circular saw through wet timber will heat a pack, and that is expected. What tells you something is wrong is heat during light duty work, or a pack that comes off the charger hot rather than mildly warm.

Internal resistance rises as cells age, and higher resistance means more of the energy turns into heat instead of work. A pack that is uncomfortably warm after a few minutes of drilling pilot holes has cells that are no longer moving current efficiently.

The tool cuts out with charge still showing

You squeeze the trigger, the tool bogs down or stops dead, and the fuel gauge still shows two or three bars. Wait a minute and it works again briefly before doing the same thing.

That is the battery management system tripping on voltage sag. Under load, a tired pack's voltage drops below the safe cut off almost immediately, then recovers when the load comes off. The gauge reads resting voltage, which is why it looks like there is charge left. There isn't, at least not usable charge.

The charger refuses to accept it

Most chargers flash a fault pattern rather than charging when they detect a pack outside safe limits. Check the manual for what the flash sequence means, because the causes are not all terminal:

• Too hot or too cold. Bring the pack to room temperature and try again. This one is common in winter garages and after heavy use.
• Deeply discharged. A pack left flat for months can drop below the voltage the charger will wake up. Some chargers have a recovery mode, many do not.
• Cell imbalance or a dead cell. This is the terminal one. The charger sees one cell out of line with the rest and refuses to continue.

Before writing the pack off, clean the terminals on both battery and charger with a dry cloth, and try the pack in a second charger if you have one. Dirty or corroded contacts create resistance that a charger can misread as a fault.

Physical warning signs you should not ignore

Put the pack on a flat surface. If it rocks, or the casing has visibly bowed, or the battery no longer clicks properly into the tool because the housing has grown, stop using it. Swelling means a cell is generating gas internally, which is a genuine safety issue rather than a performance one.

Same goes for a cracked case, a burning or sweet chemical smell, any leakage, or scorching around the terminals. None of those are recoverable. Take the pack out of service and get it to a proper battery recycling point rather than leaving it in a warm garage or a general waste bin.

A simple check you can do at home

If you have a multimeter, measure across the main positive and negative terminals on a fully charged pack. An 18V lithium pack has five cells in series and should read somewhere around 20 to 21 volts when properly full. A reading well under 18V on a pack the charger claims is full points at cells that can no longer hold their voltage.

Then run the pack down under normal work and measure again immediately after the tool cuts out. A healthy pack lands close to 15V. A failing one will have collapsed much lower, or will have cut out early while still reading high, which is the voltage sag problem described above.

Is it worth replacing?

Buy a genuine replacement pack if the tool is a brand you already own several batteries for, the tool itself is in good order, and the pack is one of a matched set. Buy a whole new kit if this is the last surviving battery, the tool is a decade old, or genuine packs cost more than half the price of a current bare tool plus battery bundle.

Be wary of very cheap third party packs. The cells inside are frequently lower grade than the label suggests, protection circuitry is inconsistent, and a bad pack can damage the tool it is fitted to. If you go aftermarket, buy from a seller with a real returns policy.