Did you recycle the Alternator? I ask, as some of the pilots that fly our Aircraft are unaware of this, I realise you probably had your hands full, but selecting the Alternator off for a brief period of time, ( about 20 seconds) then turning it back on resets the Voltage regulator and brings the alternator back online. This may or may not have worked in your circumstances, or indeed if there was a fault elsewhere, but its worth knowing if you are unaware....... The Battery does sound a bit suspect......
Just as well it wasn't a DA 42, as prior to the Airworthiness Directives that are currenty whistling through, you would have ended up flying a twin engined glider.