Great news!
I once found my airplane battery would not hold a charge and it was only a year old.
I bought a new battery and it died within 6 months. The charging system tested fine. I disgustedly bought a new battery. It died within the month.
....long story....(with expletives deleted)....
Bottom line: Bad connections at the Ammeter.
Knowlege gained: ALL current passes through those Ammeter connections. Keep 'em shiney/tightey.
Add the following to your "Post-Start Checklist":
After engine start-CONFIRM high charge-rate. (Charging system is working.) Add the following to your "Pre-T/O Checklist":
Battery charge rate almost completely diminished. (Battery is re-charged)
Keep in mind that the REAL REASON for a battery is NOT to start your engine. Starting your engine is only a GOOD TEST of your battery capacity.
After engine start...you battery should have a need for a healthy RE-CHARGE...and that should show up as a high-amperage charge rate. (something on the order of 20A or MORE....for a minute or two.)
The REAL REASON for a battery on an airplane is for EMERGENCY ELECTRICAL POWER in the event of failed generated power. Therefore...MAKE CERTAIN YOUR BATTERY IS essentiallY RE-CHARGED BEFORE FLIGHT. This will be indicated by that ammeter greatly reducing the charge rate having re-charged the battery from it's losses incurred during the start.
These checks are almost as important as checking for oil-pressure after engine-start, so ADD the checks to your check-list.
Keep in mind that landing/taxi lights are not a continuously operating item. The above comments are made without the use of land/taxi lights which will obscure/confuse the ammeter readings. In other words, use the ammeter readings to measure your battery health without the land/taxi lights interference.
Hope this helps.