Black n decker lithium battry charger keeps flashing red

There could be several reasons why the Black and Decker 18V battery charger's light stays red and the battery doesn't charge. Some possible causes include:

1. The battery is not properly seated in the charger. Make sure the battery is securely inserted into the charger's battery compartment.
2. The battery is damaged and cannot be charged. Try using a different battery to see if it charges properly.
3. The charger is not functioning properly. Try using a different charger to see if it charges the battery.
4. The outlet or power source you are using is not providing enough power. Try plugging the charger into a different outlet or using a power strip with surge protection.
5. The charger's fuse is blown or the charging contacts are corroded or dirty. Clean the contacts and check for a blown fuse.

If none of these solutions work, it may be best to contact Black and Decker's customer service for further assistance or to get a replacement.

This could be faulty battery

This worked for me .. Put battery in charger , unplug and plug in charger repeatedly until it charges

Try inserting battery quickly 5 or 6 times to see if it is just a connection problem, sometimes this works in cleaning the terminals.
press the helpful button.
I think the red blinking light is a fault, normally should be a green flashing light while charging.

Did you check if the battery holds the charge? Also, you should verify the charger's output with a voltimeter.

What I do know is that some NiCd 12V Black decker batteries that look similar to the dewalt ones they are charged by the dewalt charger.
But there are 12V Nicd black and decker batteries that they can not be charged due to an extra plastic edge that does not allow to be charged by the dewalt charger.
Now if you do have the dewalt lithium 36V charger and if the black and decker battery is a 36V lithium one and can not be charged due to a plastic edge then I would recommend to risk to destroy the plastic edge in order the black and decker battery to be charged.

could be either one. Have you tried another battery? If not I would recommend taking BOTH in for inspection. They have a scanner that can detect if the battery has a problem. If you need to find a service center close to you go to dewaltservicenet.com and they have a locator. Let me know if you need more assistance.

Is it the charger or the battery? Have you tried other batteries? If the battery is bad (they do go bad over time) then the charger wont work. But chargers also go bad too and can't be fixed. I've had four Dewalt charges go bad over the years (same parent company)

Porter Cable has not offered a replacement for their poorly designed 19.2V chargers. I had the same problem and happen to like my drill and circular saw. I found a universal charger called a model number CHUN-2420. Google it--I got mine for $27 plus shipping. All you need to do to make this work is splice 3 wires.

My charger is a PC model 8624, so the instructions might vary slightly between the chargers. Remove the six screws on the bottom of your PC charger. Remove the bottom section. Remove the screw(s) that attach the circuit card to the plastic housing. Disconnect the 3 pin white charging connector from the circuit card, and cutoff the connector. Discard the circuit card and power cord. Cut the 3 conductor (red/yellow/black) cable coming out of the Chun-2420 approximately 2' from the charger. Splice these three wires to the three wires in the charger housing that connect to your battery. In my situation, Red/Red, Yellow/Yellow, White/Black. To verify, red=positive, black/white=negative, yellow=temperature sensor. Wrap the red/yellow/black cable with electrical tape where it exits the PC charger through the power cord hole. Re-attach the bottom.