I have an old Sunpack 611 potato masher that has a rechargable battery pack using 4 cells that are slightly smaller than the regular C cell. If I don't use the battery pack I can load that same compartment with 4 regular C cells. Neither of these battery options has ever given me a full days of service, now that the battery pack is older it hardly even holds a charge for 10 minutes. THese packs are expensive n hard to find.My solution... get rid of the standard batteries n replace it with an external DIY lead acid remote battery.
I figured out the circuit of the battery pack using my meter to see which contacts are needed to power the flash n polarity. I removed the cells of the battery pack and replaced them with wooden dowels the same size as the cells. The dowels that fell on the needed power contacts; I drilled them n ran my pos n neg wires to a washer glued to the bottom that will sit on the proper contacts. I made a slot in the lid so when I put the lid on the batttery compartment, my leads will clear.
My flash is now powered by a small sealed lead acid 6V battery I hang off my shoulder in a small fanny pack. The charge on the battery will last days of continuous use compared to the few hundred flashes I would get off a set of batteries both rechargeable or regular C.
My lead acid battery is easily charged using a small car trickle charger meant for a 6V battery. These lead acid batteries come in different voltages so you can taylor your DIY remote battery to your flash very easily.