DIY Battery pack revival.

Ages ago I did an article on making your own DIY battery pack. I'll reprise the article here, but also have additional comments and thoughts to add at the end.
I've been getting a little fed up with slow recharge and short lifespans on AA batteries in my flash. I've a few sets from 1800-2500mAh, which in theory should give long life. Seeing as I'm using the flash on a stand, I figured I would look into external power for my flash. If you've researched this yourself, you will know that units such as made by Quantum are extremely expensive. I did a little research and found out that a person I'm in contact with had a tutorial on doing it for his 430ex. I'm using a 550ex and a Sigma 500, so this is based on those, but applicable to any flash. The person is David Burren, who is well known online for his Infrared work.

He recommended that you make fake wooden batteries that attach to a 6V SLA (Sealed Lead Acid) battery. I went shopping in Maplin and got all the bits there.

Parts list: 6v 4.2Ah SLA battery, SLA charger, 2 14mm dowels, 2 female, 1 male power connector. Soldering iron, tape, screws, dremel/drill. Parts cost: under ?35.

1.) Here's the charger: Fortunately it came with a very long cable with spade connectors. I immediately decide that it would be best to cut near the charger and add a set of male female connecters there:

2) Here's the charger with the connectors added. These had wire on them already so I stripped them back and the soldered the wires together. I went red to red and black to black to keep it all consistent. To seal I used insulating tape. I would've preferred crimp connectors, but this was what they had.

3) Here's the cable connected to the battery: You can see the pins on the male connector. I had to choose where the male went. I choose this end because I thought it was better this end than on the charger. The pins are recessed, which helps prevent shorting the battery. You may prefer it the other way. I figured that I'd rather not be able to short the charger.

4) To get power into the flash, I created some wooden batteries using 14mm dowel, courtesy of my instrument making friend, Paul Doyle. He also cut a V along the edge to allow the cable to sit mostly flush.

5) Here's the completed fake batteries that get power into the flash. You can see the solder on the + battery which allows it to make proper contact internally. I actually made the + one shorter because both cables pass over it on the way out, requiring more room than over by the - battery.

6) The batteries correctly placed inside the battery compartment of my 550ex.

7) Here's the lead coming out, showing where I dremeled a cutout on the battery compartment door, to allow the lead to sit along the flash.

8) The battery powering the flash.

9) Firing using my DIY sync socket.

10) Just showing the sync socket.

11) The setup. Loads of foamcore and a Sigma 500 into a white brolly at 1/8. Triggered with the GI trigger.

12) The whole shebang. Triggered the 550ex with the GI trigger. The Sigma triggered optically.

A quick note on the Sigma. It will only trigger with the wireless triggers in ETTL or Slave mode. In C0 Slave mode, it will trigger either via the wireless or optically.

With this setup I get the following recharge times:

Manual power:
1/1 (full power)= 6 seconds.
1/2 Power=2 seconds
1/4 and below=instant

I'm able to shoot ISO 200, 1/100 sec f6.3 into an umbrella at 1/4 power giving instant recharge. You must be careful with this speed and power because the tiny strobe will burnout if not allowed to cool sufficiently after a burst of images. It is actually possible to fire on Continous and have the flash fire every time at 1/4 power, so BE CAREFUL and don't blow your flash. Consider yourself WARNED!
And finally, apologies for the haphazard foamcore studio. The perspex on my actual product table went AWOL, so I'm currently just improvising!
I did add a case to beautify this up, and added a strap to put it on my shoulder, ot hang it from a stand.AddendumSince building this and using it quite effectively, I came across further recommendations about it. First is that I should add a fuse, like a 4 or 5 amps fast blow, in case of issues with it. Some also say you should charge with the battery well ventilated.I will be adding a fuse holder shortly (panel mounted) and I do charge with the top off to be safe, but I might drill vents instead.The Horror StoryOne thing about external packs is that there is sufficient power to destroy your flash. And here's my story.Being pleased with the performance of these packs, I'd been considering the benefits of higher power batterys. 4.2 Ah is not quite double the 2700 mAh AA batteries you can get, so when I saw 2 7.2 Ah batteries on sale in Maplin, I bought them as the would use the same charger. Messing about showing my buddy Mike Patterson the new battery, I managed to fry my 550ex. How? Well I only a cable for plastic fake batteries for the new set and mangled the connections, so I may have powered them incorrectly. Whatever I did the 550ex didn't smell so good and I had to dump it. Since then I've been reasonably cautious about the pack. I also chided myself for not putting a fuse in. That's not the end of the story.And todayI replaced the 550ex with a 580ex MkII, But I also purchased a 2nd hand 540ez. Normally the 540ez doesn't work with Digital bodies (automatically at least), but it can be run in Manual mode. I added a sync socket to it for triggering. Today I rebuilt the power cable for the original battery pack and ran a series of tests with the 540ez and everything is working perfect. As the 540ez was a cheap deal, I wasn't worried about it failing. I'd rather have had the back prove unsafe by losing it, than finding out with a more expensive flash.