Self Propelled (LM2102SP) motor not working at 75% battery level

I have had the LM2102SP (with 7.5 battery) for a little over a year. The self propelled motor works at full charge then once the battery hits about 75% (guesstimate), the self propelled motor stops working but the mower cuts fine and until the battery fully dies. Anybody have similar issue or an idea why this would be happening?