So, a little bit of a sidetrack, but not really as it is a rechargeable battery device
I have a few of these around:
So, what exactly is this?
This was made by the now defunct German company Metz, which was known for making(mostly) high quality/high end photographic flash products. I don't think that was their only business venture, but if you mention Metz to people who have been around photography for a while they'll likely think of the big 45CT and 60CT series handle-mount flash guns.
The above device is a "Power Pack P76". It's a rechargeable NiMH battery pack that can be connected to a variety of Metz flash units. It's meant to be carried from a shoulder strap or attached to your belt, and actually feeds high voltage(I think ~350V or so) directly into the flash capacitors of the devices to which it is attached. This is a big and heavy unit, and it would be used for one of two reasons(or possibly both): it last a LOT longer(gives more flashes per charge) than the batteries in the flash unit, and possibly of even more significance is that it shortens the "recycle time", or time it takes the capacitor to refill and be ready to use again after firing the flash. Testing with a Metz 58 AF-2, for example, it will recycle from a full discharge(which you'd almost never do) in about 1.5 seconds, where 4x AA Alkalines take about 8 seconds.
This was not an inexpensive device new. I believe they were around $500 when they were discontinued in 2015 or so.
Also, the battery was most certainly not intended to be user replaceable. Metz would do it for you, although I have no idea what they charged.
Of course Metz is now gone, and I had a totally dead one so took the "battery is not user replaceable" as a challenge!
Here's the result of that challenge
(I still need to "pretty it up" for final reassembly).
Basically these contain 8 sub-C NiMH cells. To replace it, you first need to unsolder the old pack, which is actually soldered at 6 places. You cut open the shrink wrap and CAREFULLY extract the temperature sensor, which I found in my testing was actually extremely important. There are a couple of different ways of charging/sensing full charge of NiMH cells, some of which involve watching charging voltage and/or current, but arguably the best way(especially for fast charging) is sensing the rate of temperature increase.
In any case, to put it all back together, you need a battery spot welder and nickel ribbon to do the basic assembly/electrical connections. There's also a thermal fuse that's actually critical to operation that has to be soldered in as well. Once it's all spot welded, the batteries need to be physically glued together(I actually used wood glue-not sure what was originally used), the temperature sensor glued in place, the whole thing shrink wrapped, and then soldered in place. I should say that's what all I'll do in one go the next time I do one, but this one went through QUITE a few iterations(including me stupidly installing NiCd cells at one point, since the originals were cardboard wrapped and I paid more attention to the cardboard wrapping than the chemistry...and it didn't work since the temperature characteristics during charging are too different). This one still isn't complete as I need to redo and "pretty up" the shrink wrap as well as put some self adhesive foam rubber on it to keep it from rattling around.
Even though I was able to replace the battery, I doubt anyone would actually consider this user replaceable.
BTW, even if something has a replaceable battery, those do go bad. I've rebuilt my fair share of them especially for some obscure camera applications. Sometimes you open get lucky to find something like AA sized cells, but at other times you'll need something like 2/3A or Sub-C or something else like that. You can often find those cells with solder tabs on them(soldering directly to a battery is usually considered a bad idea, plus the terminals themselves often aren't overly solder friendly) but the inexpensive battery spot welder I bought was a worthwhile investment for this kind of stuff as it lets me configure things however I need.
I also had fun years ago rebuilding the battery pack for my Macintosh Portable with 3x E size Enersys Cyclon cells. I've come to love the Cyclon cells and actually need to order some size D and size DT for a different project(a different Metz flash that was meant to use a lead acid gel cell battery that as best as I can tell is no longer made with the exact physical dimensions required). The Cyclon cells are sealed lead acid/AGM cylindrical cells(nominal voltage 2V) with form factors similar to other common cylindrical cells.
I try then sometimes re-read things too late to edit *LOL*
and the IEC standard it meets?
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