I know this thread hasn't been touched for a while, but it is the first thing that Google throws up when you search for the Braun Hobby 300 - so this seems a good place to post additional info about the thing.
Anyway. I refurbed one of these units about 8 or so years ago now. Mine was fitted with the original lead-acid "dryfit" cells when it came into my possession. Needless to say, the cells were all shot and rotten. Mine had a completely solid battery door, but I believe there was an earlier version which took wet lead-acid cells and the battery door had slots in it through which could be seen some coloured float balls indicating the state of charge of the batteries. Phew! Sorry about that over long sentence.
So, cell technology has moved on, and I didn't see any reason to stick with the old lead-acid technology. I found that 6 C size Nicad cells would easily fit into the space left by the original batteries. I wired two sets of 3 tag-ended C sized NiCads in series and attached the original wired plugs from the old batteries. I was a bit fearless and didn't do any pre-testing; just turned the old thing on. A reassuring whine followed and eventually the neon indicator came on. The test button was pressed and an impressive burst of light shone forth.
To cut a long story short the NiCd cells worked a treat and the unit behaved as if there was no difference in voltage - even though the NiCds theoretically feed it a slight undervoltage. The built in charging worked as it should as well. The 2 sets of 3 C cells were taped together to make a jam fit into the battery compartment and the cover held them in place nicely. No hole or taped-on box needed! (with apologies to Mike, the OP)
As I say, this was 8 or 9 years ago, and a recent investigation has revealed that the NiCd cells have all grown nice furry beards in the meantime. I think I'll try Nickel Metal Hydride cells this time around. They're cheaply obtainable in the common C size with a capacity of 4 Ah or more. If all goes well I'll post back.
BTW, thanks to the OP for pointing out that the voltage of the unit is 500v. I was going to use it to power an old 6 inch diameter "Minicam" brand flash head, but I think the head was designed for only around 350 volts or so. It might be time to get fearless again!
Last edited by Nick Con; 05-09-2011 at 06:53 AM. Click to view previous post history.