Why is my cordless drill battery not charging on its docking charger?

I had the same thing after my drill sat a couple months and the charger just blinked; I cleaned the contacts, let the battery warm up indoors, then briefly woke it by touching it in parallel to a good pack for a few seconds and it took a normal charge again. If it still will not recover, switching to DCB102 dual charger fixed it for me because it recognized a low voltage pack and charges both 12V and 20V packs while handling two batteries at once, though it is a bit bulky.

TOTALLY get it.

Been there. Most of the time the charger blinks because the pack is either too cold or too discharged to be accepted. Let it sit inside for a few hours, clean the contacts with a pencil eraser, then click it in firmly and leave it for a while. Some chargers will eventually wake a low pack after a few tries.

If you can borrow a compatible charger, that is an easy test to rule out a bad base. If a second charger will not take it either, the pack is the culprit and you can stop chasing ghosts.

If a lithium pack sat too long and the charger won't start, try brief on off charge cycles to wake it and make sure the pack is at a normal temperature. If it still blinks, the pack is likely failing and should be retired and recycled.

Mine did that after sitting all winter and the charger blinked at me. Warming it and cleaning pins did nothing. What finally worked was a gentle precharge from a current limited supply to lift it above the cutoff then the dock took over. If you go this route keep the current low, watch voltage, and stop if the pack warms. Works great.

Important bit is safety. If you are not comfortable with a bench supply and a meter skip it. A risky quick bump from another pack can end badly if you get polarity wrong or the cells are damaged, so I would not suggest it unless you know what you are doing.

That blink is usually the charger saying the pack is too low or out of temp range after sitting. Start simple. Bring the battery to room temp, wipe the contacts on both the pack and charger with a clean cloth or a little alcohol, then make sure it is fully seated. Unplug the charger for a minute and try again.

If you have a meter, check pack voltage. If it reads near zero the internal protection may have opened. If it reads well below what the charger expects, sometimes a few insert remove cycles will coax it to start. Try a different outlet too.

If none of that wakes it, the cells may be too deeply discharged to recover safely. At that point do not force it. Recycle the pack or see if the maker will warranty it if it is still within coverage.