a non oem solution
I run a metz 60ct2 in an umbrella sometimes, and I know what you mean. The 6V 4.2aH dryfit can get low, and for me that means about 2 days to juice it back up to full charge.
I have taken the old original battery, now sulfated and useless stored power wise, and cut the guts out of it to give me the right sized form package to feed power into the flash power pack from an external source. Kind of like a quantum battery approach as seen in old vivitar 283's. I first ran it from a 6V larger UPS battery to verify the concept.
Then I fed the thing from a surpls PC power supply when the rest of the computer was tossed as obsolete. The PC power supply puts out 5V at 20A all day long if you wnat it to. The lead acid battery fades to lower then 5V prettry quickly, so I don't see any trouble using it. It recylces from a full manual dump in under 6 seconds. I don't want to cyce my metz head any faster for fear of melting the plastic around the flash tube; these things were never built to cycle like a real studio flash.