Pull the battery and check the voltage. That will provide a starting point.
PSA; batteries self discharge over time at a fairly predictable rate for the particular chemistry. They should be checked periodically. A Li-On battery, which the ST-16 employs, can be stored fully charged. The aircraft batteries, being Li-Poly, cannot.
I've had the same problem with an ST16 in storage. After leaving it on the charger overnight, it did charge ok. Just takes a long time.Thanks that's great, found a spare and seems the battery was the problem.
I didn't think to maintain battery charge.
Silly me