How to Check a Cordless Battery for Excessive Internal Resistance

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A cordless battery can show a full charge and still struggle under load. One possible cause is excessive internal resistance: as current flows, the battery loses more voltage inside the pack and produces extra heat. You can check for this with a multimeter and a controlled load, but the result is a practical screening test—not a substitute for the manufacturer’s battery diagnostics or a repair-shop test.

What internal resistance means

Every battery has some internal resistance. When a tool draws current, that resistance causes voltage to sag. The relationship is approximately voltage drop = current × resistance. For example, a 0.10-ohm pack carrying 20 amps would lose about 2 volts internally and turn some of that energy into heat.

Resistance tends to rise as cells age, become damaged, or operate at low temperature. A pack with high resistance may run a tool weakly, trigger its low-voltage cutoff early, or heat up during demanding work. The same symptoms can also come from dirty contacts, a faulty tool, a poor charger connection, or a pack that is simply cold or nearly empty.

Safety and tools

Use a digital multimeter rated for the voltage involved, plus a load that draws a known, steady current. A power-tool battery tester designed for your battery type is the safer, simpler purchase for routine checks. A multimeter’s current input is not a load: never connect its current terminals directly across a battery. That creates a short circuit and can cause burns, fire, or pack damage.

Do not open a battery pack or probe its cell connections. Stop if the case is cracked, swollen, leaking, unusually hot, or smells sweet or solvent-like. Keep the test away from flammable material, wear eye protection, and follow the battery and tester manufacturers’ instructions. If you are not confident wiring a suitable load, use a purpose-built tester or have a service center check it.

Prepare the pack

Inspect the pack contacts and clean off dust or grime with a dry cloth. Do not scrape or bridge the terminals with metal. Charge the battery fully with its intended charger, then let it rest at room temperature for at least 30 minutes. Cold packs can show extra voltage sag, while a just-charged pack can give a misleadingly optimistic reading.

Check the pack’s nominal voltage and confirm the tester or load is rated for it. Use the same state of charge and similar temperature when comparing two packs. A healthy spare from the same platform is a useful reference; comparing unrelated pack designs is less meaningful because their cell arrangement and protection electronics differ.

Measure voltage sag under load

First measure the battery’s open-circuit voltage at its accessible output terminals, if the manufacturer permits it. Record the reading. Connect the rated load according to its instructions, apply it briefly, and measure voltage while the load is active. Do not hold a high-current load on longer than the tester specifies; a few seconds is often enough for a short screening test.

Record the loaded voltage and the load current. Estimate resistance with R = (open-circuit voltage − loaded voltage) ÷ current. If a pack reads 20.0 volts at rest and 18.8 volts under a steady 10-amp load, the estimated resistance is 0.12 ohm. This is an approximate whole-pack figure: battery protection electronics, contacts, wiring, and changing current can affect it.

Do not treat one resistance number as a universal pass-or-fail limit. Manufacturers rarely publish comparable limits, and packs with different capacities and designs can behave differently. A sudden voltage collapse, an early cutoff, or much greater sag than a similar healthy pack under the same load is more useful evidence. Repeat the test once after the pack has cooled, checking that connections are secure.

Choosing a test method

Interpret the result

If the pack sags sharply, shuts down quickly, or gets notably hotter than a comparable pack, stop using it for heavy work. A tired pack may still run a low-draw light or small driver, but avoid relying on it for saws, grinders, or other high-current tools. Heat is not a precise resistance measurement, and a hot pack should be allowed to cool before handling or charging.

Before blaming the battery, repeat the test with clean contacts and a second compatible tool or known-good pack. If both packs struggle in one tool, the tool may be at fault. If one pack fails across several tools, the pack is the stronger suspect. A battery that cuts out only near empty may simply be reaching its normal low-voltage limit; compare it at a similar charge level.

What to buy, and when replacement makes sense

For an occasional check, a basic multimeter can be enough if you already own one and can use a properly rated load safely. A cordless tool battery tester is the more straightforward choice if you want repeatable results without building a test setup. Check that it supports your battery platform and voltage before buying.

For frequent diagnostics, a tester that reports voltage under load and current is more useful than a simple voltage display. A rated DC electronic load tester offers adjustable loads, but requires careful attention to voltage, current, heat dissipation, and connections. A cheaper multimeter is fine for general electrical checks; it cannot safely replace a load tester.

Replace a pack that is damaged, repeatedly overheats, or cannot complete the work you need after a full charge. Use a genuine replacement or a reputable compatible pack with clear protection and warranty information. Avoid opening or rebuilding lithium-ion packs unless you are qualified: individual cells can deliver dangerous current, and a bad repair can create a fire risk. Dispose of failed packs through an appropriate battery recycling program, not household trash.