Cordless drill loses torque or cuts out under load — how to tell why

You drive a screw, the tool sinks a few, then the moment it meets resistance the power drops away, the motor groans, and it stops — but the clutch (the numbered ring around the chuck) is set correctly and isn't slipping. When a cordless drill-driver loses torque or cuts out under load, the cause is almost always in one of four places: a tired battery, worn carbon brushes or a failing motor, protection electronics that cut the power themselves, or overheating. This is an honest read from the bench — how to tell them apart after a few minutes of work, what you can safely check yourself, and where the service centre begins.

When a cordless drill loses torque or cuts out under load — how to tell the causes apart

Most people blame the battery the instant the power drops and rush to buy a new one. Often that's right — but not always, and a fresh battery on an unfixed motor or a tripping protection circuit is money thrown away. Before you change anything, you need to work out which of the four systems is actually backing out.

The whole diagnosis rests on one question: exactly how does the tool lose power under load? Three typical patterns point to three different causes.

The first step is free and anyone can do it: take a second full battery (from the same tool, or a friend's same-model pack) and repeat the exact same job. If the power comes back with the second battery, the fault is in the first battery — read on about battery fatigue. If the tool still "collapses" with the second battery, the fault is in the tool itself: the motor, the brushes, or the board.

The second thing to rule out straight away is mechanics. If the chuck turns but the bit doesn't — or the bit spins free but stalls at the first resistance — that may be a clutch or gearbox question, not a power one. We cover that in detail in Cordless drill won't drive. Here we assume the clutch is fine and the mechanics turn — this is specifically about loss of power.

Battery fatigue: drives a few screws and cuts out

This is the most common cause, and it has a very recognisable behaviour. Charged, the tool reads full and drives normally at first, but after a few screws — or a few seconds of drilling under load — the power drops away sharply and the tool shuts off. Leave it alone for a minute and the battery "recovers" enough for a couple more screws, then quits again.

The reason is rising internal resistance in the lithium cells (usually a pack of 18650 or 21700 elements). With age and charge cycles, cells lose the ability to deliver high current. At rest the voltage looks fine, but the moment you draw current the cell voltage drops below the threshold, and the BMS (the battery's protection board) shuts the whole pack down for safety. From the outside it looks exactly as if the tool had "run flat".

Often the fault isn't the whole pack but one or two weaker cells. The rest are good, but the weakest one drops first and drags the whole pack down with it. That's exactly why lithium packs are worth a cell replacement in the pack — swap the tired cells for new ones and rebalance, rather than throwing out the whole tool.

There's a close relative to this: a battery that reads full after charging but discharges far too fast, even without heavy load. If your case is more about quick discharge than "collapsing" under load, read Tool battery shows full but dies fast — cell fatigue is laid out more fully there.

What to check yourself:

1. Charge the battery fully and let it cool down (a pack warm from the charger gives a false picture).
2. Do the same job with a second full battery. If the problem follows one specific battery and not the tool, the fault is in that pack.
3. Look at the battery foot: oxidised or burnt contacts, a bulged casing, swelling. Do not use and do not charge a swollen lithium pack — take it to a service centre.

Motor and carbon brushes: power fade plus a burning smell

If the power drops even with a known-good battery, look at the motor. In older and cheaper drivers the motor has carbon brushes (a brushed/commutator motor): two small graphite blocks press against the spinning commutator and feed it current. The brushes wear down over time — and worn brushes give you exactly this loss of power under load.

Typical signs:

• Power doesn't drop suddenly but gradually over months — the tool "gets weaker".
• Sparking around the motor end (through the vents you can see a bluish flicker).
• A smell of scorched insulation or graphite after a few minutes of work.
• Sometimes the motor "sticks" and only starts after a light tap — that's a symptom of worn brushes or a dirty commutator.

Carbon brushes are a replaceable, easily accessible part and one of the most rewarding repairs — swapping them often restores the tool's power completely. But if the brushes are left too long, worn down to the holder, they start to scratch and damage the commutator — and then it's the armature or the whole motor that has to go.

Newer brand tools (Makita LXT, DeWalt XR, Bosch Professional, Milwaukee Fuel) often have a brushless motor — no brushes at all. In that case a loss of power with a good battery usually points not at the motor but at the control board (next section) or a winding that has started to short. Checking a brushless motor or its Hall sensors needs the bench — you can't safely judge it by hand.

A burning smell is a red line. If the tool smells scorched — release the trigger, pull the battery, and don't use it until it's been looked at. It may simply be worn brushes, but it can also be a winding that has started to short, and to keep driving means finishing the motor off for good.

Protection electronics cutting out under load

This is the trickiest pattern, and it's often confused with the battery. The tool runs fine with no load, but the moment you give it real work — a thick screw, hard wood, the bit jams — the power drops instantly or the tool clicks and shuts off completely. Release the trigger and press again, and it all runs once more, until the next heavy load.

This isn't random — it's protection tripping by design. The reasons, from most common:

• Overcurrent protection on the control board. In brushless tools the board measures motor current and cuts it if the current exceeds a limit (the tool jams, the motor stalls). If a MOSFET or a sensing circuit starts misbehaving, the board "sees" overcurrent earlier than it should and cuts the power at a normal load.
• A poor contact between battery and tool. Oxidised or worn battery terminals create resistance. At no load it doesn't matter, but under load the voltage drops across them and the tool "collapses" exactly like a flat battery. A clean alcohol wipe across the terminals sometimes solves the whole problem.
• The trigger switch (variable-speed). The switch itself contains the electronics that control the speed. Burnt contacts or a faulty switch module produce surges in power and cutouts under load.
• The BMS threshold in the battery. As above — a weak pack makes the BMS cut out under load, and that looks identical to protection tripping on the board. That's exactly why the second-battery test matters so much: it separates the battery's BMS from the tool's board.

The self-check here is short and safe: clean the battery and tool contacts, check the battery clicks fully home, and try a second battery. If after that the tool still clicks and shuts off at a normal load with a good battery — the fault is in the board or the switch, and the service centre opens it. Don't solder the board yourself: a wrongly fitted MOSFET or reversed polarity destroys the whole control module.

Overheating: why the tool shuts off after a few minutes

A separate case: the tool drives normally at first, but after several minutes of continuous work it shuts off, and the casing or battery is noticeably hot. After it cools, it all runs again. That's thermal protection, stopping the tool for safety before something melts.

Where it can trip:

• In the battery. The BMS monitors cell temperature. Heavy load heats weak cells, and the pack shuts off. A tired battery heats faster — this often goes hand in hand with the battery fatigue above.
• In the motor. Worn brushes, blocked ventilation (packed dust and wood shavings around the cooling vents), or too heavy a sustained load overheat the windings.
• In the electronics. The control board's power transistors run hot; if their cooling deteriorates, the board protects itself and shuts off.

Self-check: clear the vents of dust and shavings, let the tool cool between heavy jobs, and don't run it continuously — a drill-driver isn't built for non-stop load for hours. If the tool overheats and shuts off after a few minutes of light load, when it should be pulling along calmly, that's no longer normal — a hot or weak battery, or the start of motor damage, and it's assessed at inspection.

Diagnostics in Riga: what to check yourself

Before bringing the tool to a service centre, these steps separate most of the causes and sometimes solve the problem on the spot. They're all safe — battery out, no breaking the casing open.

1. Second-battery test. The same job with a second full, cooled battery. Solves it → battery. Doesn't → the tool.
2. Contact cleaning. Clean the battery and tool terminals (a dry alcohol wipe), make sure the battery clicks home.
3. Ruling out the clutch and mechanics. Turn the clutch ring to the drill symbol (max torque). If it only slips at low settings, that's the clutch, not power.
4. Sound, smell, sparks. Listen to the motor at no load and under load; smell for anything scorched; look through the vents for sparks.
5. Temperature. Does the battery or casing get hot before the cutout? That points to thermal protection.

Where to stop clearly and turn to the service centre: a burning smell, a swollen or deformed lithium battery, a tool that clicks and shuts off at a normal load with a good battery, or checking a brushless motor or control board. The inside of a live lithium pack and the wrong intervention in it is a fire risk — don't open it yourself.