1999 996 C4 battery/check engine light

Vehicle Information: Model: 996 C4; Year: 1999; Body Type: Coupe; Total Mileage: 13,997;

My car was serviced at the end of March. The battery was replaced and I was told that the "check engine" light remaining on was a function of low voltage from the old battery. The car was idle for several weeks while we were away in April and the "check engine" light is again on full time. My mechanic says this is related to low voltage and that we should leave the trickle charger on even if the car will be idle for several days. The engine seems to be fine.I don't understand why this voltage issue never surfaced during the first five years I owned the car (bought new). Could there be something else draining the voltage? Does the low voltage triggers check engine light make sense?