My business designs and manufactures battery monitors for the specialist vehicles and defence applications - we own the IP for the worlds most accurate state of charge and state of health technology. Unless you have a state of health monitor there is no way to check if that battery is flat or f****d. Many batteries are damaged (permanently) with excessive discharge - a jump start being an obvious symptom of excessive discharge. If the DA42 systems are indeed as described here, there’s no way you should attempt to reuse a flattened battery. Accident waiting to happen. What a stupid system - why didn’t they employ a proper electrical engineer to design a sensible fail-safe system?
Most modern batteries are pure lead AGM types - they’ll accept as much current as you can throw at them as long as the voltage is properly regulated.
The only way to accurately monitor battery state of health is to monitor the charge and recharge trend and compare that against the stated capacity of the battery over time. Battery testers (even expensive ohmic ones) don’t work (Enersys the worlds largest battery manufacturer states that battery testers cannot and will not work with their batteries).
FWIW Id only ever fit the Concorde AGM or EnerSys Pure Lead battery in my aircraft..