I shoot roller derby, and during tournaments when there are six bouts in a day, constantly switching out and recharging AA batteries for speed lights is a real pain. Even with high quality NiMH rechargeables, I would barely get one bout on a set, and sometimes I’d lose my flashes during a game. There are commercial external battery packs available, but they cost a fortune. I like to do stuff cheap and I like to make things, so I searched for DIY flash battery pack and found several designs using Sealed Lead-Acid (SLA) batteries and little dummy batteries to hook them into the flash, usually made out of wooden dowels. You have to cut a little notch in the flash’s battery door to accommodate the wire, but otherwise the flash stays stock and you can still use regular AAs in it as well.
A lot of the designs I saw used a 4.5 Ah battery, but that doesn’t give you a whole lot more than a good set of AAs would. Since size and weight isn’t much of an issue, I decided to go with a 12 Ah battery. They’re also cheaper per amp hour than 4.5 Ah rated batteries.
This is just my version of a system that lots of other people have already made. I’m not taking credit for it – just showing the version I made. My one innovation, and I use that term loosely, was the addition of a voltmeter.
With lead-acid batteries, you need to be careful not to discharge them too much. If you take them down past 50% capacity, they can become permanently damaged and their capacity and performance will suffer greatly. I fitted my DIY packs with a little DC digital voltmeter so I can check the charge of the battery. Once it gets down to about 5.5 volts (5.25 is the absolute minimum) it’s time for a recharge. There are cheap 6 volt chargers on Amazon in the $8-$10 range, but I chose a $20 model which is much smarter. It’s automatic and once the battery is charged, it switches to float mode to top it off and keep it full and happy. SLA batteries shouldn’t be stored discharged, and if not used for a long time they’ll very slowly lose their charge, so you should make sure to fully recharge them every few months. Another benefit of this charger is it comes with a few different connections and you can use these connector plugs in the battery pack. There’s one hookup coming out of the pack and you use this to charge it, as well as to hook up the dummy batteries in the flash.
It’s a very simple circuit, as shown below. Connections can be made with crimp connectors or soldered with electrical tape or heat shrink tubing. The fuse is very important for safety. These batteries can put out a lot of amperage, so if there’s a short, it can be a fire hazard.
Important: MAKE SURE THE COVERED PRONG ON THE CABLE GOES TO POSITIVE WITH THE FUSE IN LINE. That’s the way it needs to be to directly connect to the charger. Make sure this side goes to the positive in the flash too.
I’ve seen people use project enclosures from Radio Shack (about $8) for the case, and some even use a cheap fanny pack from Wal-Mart. I found a $0.97 pencil case from Wal-Mart was just the right size. Not terribly durable, but the price is right! I spray painted the inside black, drilled and dremeled holes for the cord, button (to turn on the voltmeter and check the voltage) and voltmeter display. Put it all in there and made the connections, stuffed some packing foam around the battery so it won’t slide around inside, and taped around the lid to keep it closed.
In my Yongnuo 560iii flashes, I use the bottom two battery slots, with positive on the left (toward the lcd screen of the flash). I cannot tell you which slots you should use if you have a different model. Do some googling and try to find the answer. Be careful – you can fry your flash if you get it wrong.
Testing & Performance
First test was very positive. I had one flash using a freshly charged set of Eneloops, and another using my battery pack. I did full-power flashes over about an hour period. The flash with the AAs ran out of juice at 197 flashes, going from a starting voltage of 5.72 down to 4.64, and the batteries got extremely hot. The flash with the battery pack was still going strong with zero degradation in performance from the first flash to the 200th. At the point the AA flash quit, the battery pack had only dropped from 6.44 volts to 6.32. Plenty of room left until we hit the 5.5 – 5.25 limit where we don’t want to discharge any further. This is as far as I went with testing, but I’ll have a better idea of the expected longevity when I finally shoot a tournament. If that discharge rate were to continue, I would expect to get maybe 1600-1700 full power flashes out of it.
At the end of the test, the AA flash’s battery door was 104 degrees, whereas the SLA flash’s battery door was only 87 degrees. The actual AA batteries were 140 degrees, whereas the SLA battery was still room temperature. Even better, the SLA took about 2.5 hours to fully recharge, whereas the AAs took 7 hours. Also, at full charge, the SLA-powered flash recycled about 15% faster than the AA-powered flash, and the SLA’s recycle time stayed consistent while the AA’s gradually slowed down.
Here are the parts I used and the best prices I could find on them. The ebay items can be had extremely cheap if they ship from China, though that takes a few weeks.
- Battery - 6 Volt 12 Ah – $17
- Charger - $20
- Voltmeter - $3
- Momentary Switch - $2
- Pencil Case from Wal-Mart or Project Enclosure from Radio Shack – $0.97
From hardware and/or automotive store:
- Inline Fuse Holder – $1.50
- 15A Fuse – $1.50
- 16 Ga Wire (if longer than 8’, go with 14 gauge) – $4
- 1/4 Female Spade Connectors – $1.50
- 3/8” Pex – $2
- Power cord connectors (if not using ones that came with the charger) – can find in the trailer lights section of a hardware or farm store.
My build, with the charger, was $53.50 for one, and averaged across the three I made, $47.50 a piece, including a dedicated charger for each. Since the smaller items come in packs of several, you have enough to build a few batteries. You could use one charger for multiple batteries, but I wanted to be able to charge them all at once.
The battery pack can be built for about the price of 8 Eneloop AAs, but will last several times longer on a charge, and no having to go to the flashes and change the batteries all the time!
UPDATE: I shot a double header yesterday. Approx 1500 photos, with the flashes set between 1/4 and 1/2. Started with the batteries at 6.4 volts, and ended up with them around 6.25 volts! I’m pretty thrilled with that result.
UPDATE 2: Shot at champs, and even on the 6 game day, they stayed above 6 volts! I could probably have gotten by with a 9 amp hour battery, or even a 7, but I’m happy having 12 so my voltage stays higher, which makes for faster recycle times. The heavy batteries make pretty good light stand weights too. It was VERY nice not having to change batteries at all during the day!