HOW-TO GUIDE
Why Does a Power Tool Keep Stopping? How to Check Its Battery and Electrical Connections
By the OhmSavvy Research Team ·
Keep clear of the tool's moving parts while testing under load
- Keep hands, meter leads, and clothing clear of any bit, blade, or moving part while the tool is running during a load test.
- Remove or secure any attachment that isn't needed for the test.
- Disconnect the battery before inspecting or cleaning contacts.
1Check the battery's resting voltage
With the battery removed from the tool, measure voltage across its main contacts. Compare the reading to what the manufacturer specifies for a charged pack at that voltage rating — this sets your baseline before testing under real load.
2Check voltage while the tool is working
With the battery installed, connect the meter across its terminals (back-probing if needed) and run the tool under a realistic load while watching for the lowest voltage reached, ideally using min/max capture since the dip can be brief.
| Observation | Likely cause |
|---|---|
| Voltage holds steady under load | Battery is healthy — look at contacts or the switch |
| Voltage sags sharply and recovers when load is removed | Weak or aging battery cells |
| Voltage drops to zero momentarily, then recovers | Intermittent contact — check the battery/tool interface |
| Voltage stays normal but the tool still cuts out | Likely the trigger switch or internal tool wiring |
3Check the battery and tool contacts
With the battery removed, inspect both the pack's contacts and the tool's receiving contacts for corrosion, discoloration, or bent/worn spring tension. A contact that doesn't seat firmly can produce exactly the kind of intermittent cutout described above.
Swap a known-good battery to isolate pack vs. tool
4Check the trigger switch
If the battery tests fine under load and the contacts look clean, check the trigger switch's continuity with the tool disconnected and disassembled enough to access it — a worn or dirty trigger switch can lose contact intermittently under vibration, mimicking a battery fault.
Which meter to use
A meter with min/max capture makes catching a brief voltage sag much easier. See the full best multimeters roundup.
Meters in our reviews
Hands-on tested. Min/max recording catches a brief voltage sag under load.
Hands-on tested. Same 550 g body, True RMS and CAT III 600V as the 117, without non-contact voltage detection.
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Frequently asked questions
What's a normal resting voltage for a cordless tool battery?
It depends on the pack's voltage rating and cell chemistry — check the label for the nominal voltage (e.g. 18V, 20V) and compare your resting reading against what the manufacturer specifies as a charged state for that specific pack, since chemistries and cell counts vary between brands and tool platforms.
Why would the battery read full voltage at rest but the tool still cuts out?
Battery cells can hold a reasonable resting voltage while being unable to sustain the tool's real current draw, especially as packs age — the voltage simply collapses once significant current is demanded, then recovers once the load is removed. This is exactly the same under-load-vs-resting-voltage distinction used when testing a car battery.
Could dirty or corroded contacts cause intermittent cutting out rather than a total failure?
Yes — a contact that's making a poor connection can work fine under light load, then lose contact momentarily under vibration or heavier current draw, producing exactly the intermittent stop-and-restart symptom many people describe. Clean contacts with an appropriate contact cleaner and inspect for pitting or corrosion.
Is it the battery or the tool if swapping batteries doesn't fix the problem?
If a known-good battery from another pack of the same platform still produces the same cutting-out symptom, the fault has moved with the tool rather than the battery, pointing toward the tool's own trigger switch, internal wiring, or motor rather than the battery pack.
Can extreme temperature cause a battery pack to cut out protectively?
Yes — many lithium-ion tool packs include thermal protection that deliberately cuts power if the pack gets too hot or too cold, which can appear as the same symptom as a failing pack. If cutting out correlates with heavy, sustained use or very hot/cold conditions, let the pack return to a moderate temperature and retest.
