I thought the stock m248 only had a plug for the charger not for external batteries - the battery configuration of the m248 is similar to the 148 in that is a number of 12v sla's run in series and parallel in order to power the op-amps and also provide 48V phantom - something like 4-6 of these 12v batteries. The charging circuit is only used to charge the batteries and cannot be used to power the unit.
Doug did this to remove the need for DC-DC converters - is my understanding.
So to add external power you would need to have someone replace these batteries with a DC-DC converter circuit.
I'm wanting to repair my charger. It needs a new plug.
MK5, MK8, MK41, KM184D, CK77, B3 ---CMD 2U XT, KC5, KCY, AKI---KCY Tinybox, Ugly BB---AETA 4MinX, PMD661 MKII, R-26, M-10, MR-1