I recently purchased this Battery Charger and when I hook it up to a battery, the needle fluxuates from chaging to fully charge. The fluxuations are one right after another about 1/2 second apart. Seems like it worked ok when I first got it but now I'm having this problem. Not sure if it's the battery or what. I also have another battery charger that does the same thing on the same batteries. Any ideas?