Compaq Presario Laptop lost power

Question

Model 1525 US working with battery and AC Power Supply Adaptor suddenly displayed message that battery was low and I should save work.The external power supply was connected at the time.The Power icon disappeared and the low battery icon appeared. I exited nicely. System shut down and will not turn on again without battery.External Power Supply is good, reading nominal 18.5.I have disassembled top cover palm rest and am looking at the little battery charger board in front of me. Could that be the problem or is there another circuit on board which would cause me to lose power with the supply connected. System is 5 years old.Thanks in advance,John K.