Power tool charger not working — Makita, Bosch, DeWalt diagnosis

You drop a battery onto the charger and nothing happens — or the indicator starts flashing fast and charging never begins. Before you throw out the charger or the battery, the decisive question is which of the two is actually faulty. At the bench we settle this with one move, and you can run the same move at home.

Chargers and batteries are interchangeable: one charger serves several batteries and the other way round. That is exactly why they can be checked against each other — the cross test is the foundation of all this diagnosis. The core principle is simple. If the problem follows one battery, the battery is at fault. If the problem stays with the charger no matter which battery you fit, the charger is at fault. Let us walk through each symptom in turn.

No light at all — charger or socket

Total silence — no indicator lit, no fan, no reaction to a fitted battery — almost always means the charger is getting no power at all, or its internal power supply is dead. This is not a battery symptom: even a fully flat battery makes the charger at least light an indicator and start an attempt.

Before you think about repair, rule out the simplest causes:

1. Check the socket. Plug something else into it — a phone charger, a lamp. In older Riga apartment blocks with dated wiring, individual sockets can be half-dead or wired through a switched-off switch.
2. Check the charger cord and plug. Wiggle the cord at the inlet — if the indicator flickers for a moment, the cord is broken internally. This is a common fault on tools that live in a case where the cord folds at a sharp angle.
3. Clean the contacts. Rarely, the contacts oxidise so deeply that the charger does not see the battery at all.

If the socket works and the cord is intact, yet the charger still shows nothing, the problem is inside it — most often in the power supply (transformer or switch-mode supply). That is bench work, not a home repair.

Flashing an error — usually the battery, not the charger

Here is the key misunderstanding. When the charger lights up or flashes but refuses to charge the fitted battery, people assume the charger is faulty. In practice it is the reverse: a steady error flash with a battery fitted almost always points to a battery-side problem — a damaged cell or a tripped BMS protection. The charger is doing exactly its job: it finds an unfit battery and refuses to charge it for safety.

Read the indicators:

• Makita (DC18RC, DC18RD): fast red flash = battery fault detected; red-green alternating = temperature protection (battery too hot or too cold).
• Bosch (Professional 18V): LEDs flashing erratically or alternating = battery rejected; all flashing evenly = temperature.
• DeWalt (20V MAX): fast flash = fault, usually a hot or cold battery, or a damaged cell.

The cross test to run at home: fit a different, known-good battery onto the same charger.

• The second battery charges normally → the first battery is faulty, the charger is fine.
• The second battery gives the same error → now suspicion falls on the charger.

Before concluding a damaged cell, let the battery sit at room temperature for an hour — temperature protection clears on its own. This matters most in winter, when you bring the battery in from a cold car or from outside: a cold Li-ion cell will not charge until it warms up.

Charges, then stops too early

The charger begins charging normally, but after a minute or a few minutes it stops and switches to an error or "complete". Three possible causes, and the cross test sorts them again:

1. Battery thermistor. Every battery has a temperature sensor that the charger reads. If the sensor or its contact is faulty, the charger "sees" a false overheat and cuts charging for safety.
2. Charger temperature reading or a clogged fan. If the charger's own cooling is clogged with dust, it overheats within a couple of minutes and shuts itself down.
3. Damaged cell with high internal resistance. The cell shoots up to full voltage, the charger counts that as complete, but the real capacity is negligible.

Cross test: a different battery on the same charger reaches full → the first battery is at fault (thermistor or cell). The same battery reaches full on a different charger → the first charger is at fault (cooling or temperature reading).

A sign that points specifically to cell degradation rather than the charger: the battery "charges" in a couple of minutes, but the tool runs with it for only a couple of minutes. That is solved not by a charger repair but by a battery rebuild.

Running hot or the fan is silent

Most tool chargers have a cooling fan that runs during charging — you can hear it and feel the draught from the vents. If the charger becomes noticeably hot during charging and you do not hear the fan, stop. This is a real fault, not a minor one:

• A clogged or failed fan means the charger cannot shed heat. A hot charging process right next to Li-ion cells is not something to play with.
• Silence can also mean the fan has burned out or its connection has come loose.

What to do straight away: remove the battery, unplug the charger from the socket, and let it cool. Do not use it again until the cause is known. Baltic humidity and the dry, dusty air of the heating season together clog cooling vents fast — so clogged fans are a routine find in Riga. Cleaning the fan, replacing it, and checking the temperature control are bench work.

Charges only some batteries

A deceptive scenario: one charger charges two of your batteries but refuses the third — or the reverse, one battery charges on one charger but not on another. This is no coincidence, and the cross test resolves it quickly.

• The charger rejects only one specific battery and charges the rest → that one battery is at fault. Its cells or BMS are damaged, and a healthy charger correctly rejects it.
• One battery will not charge on any charger while the others charge → again that battery is at fault.
• All batteries refuse to charge on one specific charger but charge on another → that one charger is at fault (most often weak power or a damaged contact group).

Write down what works with what and what does not — this simple table of two chargers and two batteries almost always puts a finger on the culprit before you even open the case.

Charger or battery — how to tell and what to do

Let us pull it all into one table. The left column is what you see; on the right is what it most likely means and what the next step is.

A clear boundary: DIY is checking the socket and cord, cleaning contacts, warming or cooling the battery, and the cross test with a known-good battery — all without tools and without opening the case. The service centre begins where the charger has to be opened: power supply, fan, temperature-control circuit, and any battery-side cell and BMS diagnosis. Inside a charger there are high-voltage capacitors that hold a charge after unplugging — those are not opened at home.

If the cross test shows the real problem is in the tool itself rather than the charging chain, the article on when a power tool will not turn on will help. For more on what we do at the bench, see the page on power tool repair.