Here's a situation you've probably hit: you slap a battery on the charger, the light blinks red instead of green, and you assume the pack is dead. Before you go dropping $80 on a replacement, do this first — check the terminals. More often than you'd think, a "dead" battery is just a dirty one. Crud on the contacts blocks the connection, the charger can't read the pack, and it throws a fault. This is the fix I reach for first on every truck I run, and it's saved more batteries than any other trick in the book.
What You'll Need
- A dry rag or microfiber cloth
- Isopropyl alcohol (90% if you can find it)
- Cotton swabs
- A pencil eraser — the white vinyl kind
- A small stiff brush (an old toothbrush works)
- Compressed air (optional, for the charger cradle)
That's the whole kit. Nothing you don't already have rolling around in the van.
Step 1 — Pull the battery and look before you touch
Take the pack off the tool and off the charger. Hold it up under good light and actually look at the metal contacts — the little blades or pins on the battery and the matching ones inside the charger cradle. You're hunting for three things: green or white crust (that's corrosion), black carbon buildup, or plain job-site grease and dust. If the terminals look dull instead of bright metal, that's your problem right there.
Step 2 — Dry brush first
Take your stiff brush and knock off the loose dust and grit. Do this over a trash can, not your lap. Remove the surface gunk before you add any liquid — otherwise you're just smearing dirt into a paste.
Step 3 — Hit the corrosion with the eraser
This is the old-timer trick that works better than it has any right to. Rub the metal contacts with the pencil eraser like you're erasing a mistake. The rubber grinds off light corrosion and carbon without scratching the metal the way sandpaper would. A few firm passes and the terminals start to shine again. Wipe the eraser crumbs away with your dry rag.
Step 4 — Alcohol for the stubborn stuff
If there's still green crust or greasy residue, dip a cotton swab in isopropyl alcohol and work the contacts. Don't soak the battery — a damp swab is plenty. The alcohol cuts the oil and dries fast, so it won't leave moisture sitting on the terminals. Give the charger cradle contacts the same treatment while you're at it.
Step 5 — Blow it out and let it dry
If you've got compressed air, give the battery terminals and the charger cradle a quick blast to clear leftover dust and swab fibers. Then let everything air-dry for a couple minutes. Alcohol evaporates fast, but give it the time anyway — you do not want to plug a damp battery into a live charger.
Step 6 — Re-seat and test
Put the battery back in the charger and watch the light. If it goes solid red and starts climbing to green, you just fixed it for free. If it still blinks the fault code, the terminals weren't the whole story — but at least you've now ruled out the cheapest fix before spending money.
Common mistakes that make it worse (don't do these)
- Don't use sandpaper or a file. It scratches through the plating and gives corrosion a better place to start next time. The eraser does the same job gentler.
- Don't use water. It spreads corrosion into the pack and takes forever to dry. Alcohol or nothing.
- Don't clean terminals on a battery that's hot from charging. Let it cool first — heat plus fumes is a bad combo.
- Don't forget the charger side. Half the time the crud is in the cradle, not on the battery. Clean both.
When cleaning isn't enough
If the terminals are clean and the charger still faults, you're looking at something deeper — a dead cell, a blown internal fuse, or a charger that's gone bad. That's when you start considering a replacement. Here's a full guide on how to tell if your battery is failing, and if you do need a new pack, we stock Dewalt 20V replacement batteries that drop right in.
If the terminals are clean and the pack still will not take a charge, the fault is usually inside the pack or the charger. A replacement charger is the cheaper test, and a DeWalt 12V-20V fast charger covers both voltage classes.
FAQ
How often should I clean battery terminals?
Every couple of months, or any time the charger starts acting flaky. It takes two minutes and it's cheap insurance.
Can I use WD-40 on battery contacts?
Skip it. WD-40 leaves an oily film that attracts more dust. Isopropyl alcohol is the right tool — it cleans and evaporates clean.
Is green crust on battery terminals dangerous?
It's corrosion (usually copper oxidation), not a leak from the cells themselves. Not dangerous to clean, but wear gloves if your hands are on it a while, and don't breathe the dust.
Will cleaning terminals revive a completely dead battery?
No. If the pack won't hold any charge at all, cleaning won't bring back a dead cell. But if it charges intermittently or throws a fault, dirty contacts are a real candidate — clean first, buy later.
Do I need to clean new batteries?
Not usually, but it's worth a quick look. Some packs sit in warehouses and pick up a light film before they ever reach you.