I have an APC UPS 750G unit which is now giving me red warnings about the battery. The unit is more than 5 years old. A new APC unit - it's a different model - is about 2 1/2 times the price of just buying a "replacement battery cartridge" for my unit? Is it better to buy the new unit? It doesn't have as many outlets as the UPS unit I have. The sales rep kept telling me it's worth it because of the warranty (3 years). Buying just the battery cartridge is just a one year warranty. I would appreciate any advice.