I got what you were asking. I also
upgraded my stock Parallax converter/charger and plan on re-purposing the old one as a shop charger and DC power source. I just haven't done it yet. Let us know how it goes.
For proper charging, it's important to know your battery manufacturer's recommended charging voltage (varies between 14.4 and 14.6 volts). You may have to go to their website to find it. The old converter/charger just provides a constant 13.8 volts which may take forever to fully charge your batteries, and then overcharges it when the battery is sitting fully charged instead of floating at around 13.2V.