1998 Infiniti Q45 Battery will not stay charged

Several warning lights (airbag, TCS off, brake, battery) came on as I was driving down the road. Next my brakes started shutting down. When I parked my car it would not start after turning it off. I had it towed and took the battery to a local auto parts store. They told me it had a dead cell so I bought a new one and replaced it. It turned over beautifully and ran for two days. Today the lights came on again and it would not restart after turning it off. I used a volt meter and it is reading only 9.8 instead of 12.