Hello, I have a 15" Aluminum PB G4, and I'm looking to replace my battery, I bought a NewerTechnology NuPower AL15 55.5 Watt-Hour high capacity battery, and had it for about 1-2 years, now the charge will last for about 15-20 min. on a full charge. I read the battery guide, and I think it's time to buy a new one, my question is, what would be the best affordable battery to get??? Any help would be appreciated. Thanks. Cris.