Does the charger work with the other batteries?
If so, then you can say it's the battery itself.
Every now and then I have had a problem with a battery that flashes red on the charger. Sometimes you can recover it, sometimes not. For my waterproof camera, I have three batteries for it that I rotate through. One time when I hadn't used the camera for a while, I dug it out and started charging the batteries. One of them wouldn't charge...started flashing red, no matter what I did. So I'd pop one of the others in the charger and when it was done, try the "bad" one again. No dice. Stuck the third one in and charged it...went back to the "bad" one, and it started charging.
I had a very similar thing happen at a co-worker's house I was watching while he and his wife were out of town. They have a Kobalt lawnmower with two batteries. When the first battery went dead, I grabbed the second one to finish the lawn...and it was also dead. Went inside to put in on the charger...flashed red. Tried several times...same thing. Oh, well...put the other one on the charger for a while then finished the job. After charging the good one when I was done, I tried the "bad" one again...and it started charging.
Sometimes I think a battery may be too far discharged and the charger may think it's bad. So keep trying. It might be that the brief time it tries to charge the battery will get it up over the edge and let the charger go to work.
If not...oh well, time for a new battery.
