Why remove the alternator....
The alternator can be tested just a s well in the car as it can out of the car. Actually it is better to test on the car, so that it can be tested under actual conditions. If the battery is tested good, then the alternator is doing its job by keeping it charged. If the movement of the needle is the only thing that concerns you, it may be caused by the age of the alternator, the condition of the grounds, different temperature related resistance values of wiring, or any number of things. I would have the alternator/battery/charging system tested in its in car state by a reputable battery/alternator/electrical shop and let them make a recommendation. I personally run the best battery/alternator combination that I can afford on all of my vehicles. I usually run Optima Yellow Top Deep Cycle batteries, and Motorcraft Alternators that I take to a local Alternator shop and have rebuilt to High Output Police Package Specifications. My reasons for using a deep cycle battery are direct from an Optima site.
1. Delivers higher peak amps faster than conventional batteries
2. Provides up to twice the life of conventional batteries
3. More consistent voltage across the discharge curve
4. Superior cold and hot weather performance vs. conventional batteries
5. Available in models with SAE posts, stainless threaded studs, or both
6. Spill-proof, even if the case is physically ruptured
7. Will not off-gas in normal operation, eliminating fumes, odor & corrosion
8. Over 14x more shock and vibration resistant than conventional batteries
9. Provides more power from a smaller package
10.True zero maintenance
11.Holds charge better during periods of nonuse
12.Operates in any position without spilling