Just wanted to share a 'failure mode' I finally diagnosed and found a simple solution for, using Ike DS-50's (I'm sure it's the same on the DS-51's). The strobe uses 4 AAs, two 'face up' and two 'face down' in a diagonal arrangement, and the inside of the battery cover has the connection metallization to complete the circuit and make a 6V stack out of the four 1.5 V batteries. The inner side has spring contacts which supposedly help make sure the contact is made.
I'd had problems where the strobes would either never reach full charge (red LED) of the cap, or would reach it, but never fire and/or have really slow recycle times, even with completely fresh (charged just a couple hours before, and allowed to cool) equivalent maH-rated AA batteries inside. I used to do a lot of battery swapping to find a combination that worked, and thought maybe I had bad batteries (underspec voltage or whatever), although they'd all give me the green light in my charger. Even matched sets of batteries from the same vendor. I was even screwing down the cover to the point it was deflecting a little -- which I did NOT think was a good idea for a simple glued-on, face-sealing o-ring -- and I could see little indentations in the negative side flat faces of some of the batteries, for the two that were minus-side toward the door. But those indentations weren't equal depth, which gave me a clue!
Turns out, the length of the little positive side 'nipple' on the batteries matters! If you're like me and buy the bulk-packs of 20 - 40 rechargeable AA batteries at places like Fry's or Best Buy, take a look at a bunch of them side by side and you'll see that the height of the positive side nipple isn't always the same. If you get one that stands up a lot prouder than the others, even with the spring loading in the battery compartment, you risk the connection not being completed as the 'tall' battery holds the door up a bit (the metallization contact on the door inner surface isn't terribly compliant). So with the door all the way 'down' and the springs more or less in full compression too, if one battery is ending up holding the door up a bit, the other might only be making intermittent contact along the same 'row' or 'column'.
Once I came up with this supposition, I found on this last trip that as long as I made sure those nipples were (visually - no calipers required) about the same height on all 4 batteries being inserted, I stopped having problems, and the battery covers just had to be hand-tight. The difference was night and day.
Maybe this is a no-brainer to some of you, but since I struggled for a while I thought I'd pass it on.
