Sunday, March 16, 2003

Flash power pack

Sick of waiting for your flash to recharge after firing? Fed up with your batteries dying too quickly? Maybe a discussion of batteries would be useful before we talk about changing your flash...

Internal or external?

Most on-camera flashes use 4x AA batteries which together supply 5-6V, which the flash internally scales up to the high voltages used to power the flash recharge circuitry (the 6V from the batteries is typically also used to power the switching electronics and the interface to the camera).
Some also have a socket for an external power back to supply the high voltage power directly to the flash circuitry (one example is the Canon 550EX). Typically these flashes still need the internal AA batteries to power the switching electronics, only using the high-voltage input to recharge the flash itself. The external rechargable battery packs for these units (such as those from Canon, Quantum, and are quite expensive, but allow the flash to recharge very quickly and last a fairly long time between recharges.

Internal AAs

AA (LR6) alkaline
Many people use alkaline batteries (such as Energisers or Duracells) in their flash units. Often this is because they don't know about other available battery types. While alkaline cells don't lose power when they're not used (much: see below) and are good at supplying power for a long time in devices with low to medium power requirements (for example radios and torches) they are not the best choice in applications where you need a lot of power quickly.

For quicker flash recharging and longer life, better choices can be nickel-metal-hydride (NiMH) or nickel-cadmium (NiCd) batteries. Both these types are rechargeable, which can obviously save costs. They both work better than alkaline cells in devices like flashes, providing quite reasonable recharge times. They each provide approximately 1.2V instead of the 1.5V supplied by alkalines so a set of 4 cells provides 4.8V instead of 6V, but the flashes cope with no problems. Note that the voltage of an alkaline cell drops off as the cell fades, so a 1.2V alkaline cell is usually OK even though it's obviously not fresh.

Of course no solution is perfect: they each have their own drawbacks. NiMH batteries are easier to recharge at any point than NiCds without incurring the side-effects commonly referred to as "the memory effect". NiMH AA cells are available in higher capacities (e.g. 2000 mAh) than NiCds (e.g. 1000 mAh) and thus last for longer (on the whole, but it depends on the power consumption profile of each device). Both NiMH and NiCd cells will gradually "self-discharge" at similar rates: a few percent per day (compared with a few percent per year with alkalines). If your device is sitting on the shelf for too long, you might pick it up and have flat batteries! Recharging them regularly is a good idea, as is using a good charger. Overcharging batteries will reduce their life, as will undercharging.

All this applies not just to flashes, but also to other devices with high power requirements (such as digital cameras). Most people using such cameras (that take AAs) use NiMH batteries.


I use NiMH AAs in most of my equipment when in the field (including GPS, digital "P&S" cameras, UHF CB radios, and camera flashes) although I do swap out the NiMH cells in the GPS when I get home and replace them with alkalines so that I can pick up the GPS at any point and know it's going to have power (I swap NiMH cells back in when heading out on trips longer than about 1/2 a day).

I have quite a few sets of NiMH AAs, in capacities ranging from 1300 mAh to 1650 mAh (they get bigger each time I buy a new set). When putting cells into a device such as a flash I make sure all the cells are of the same capacity (also the same manufacturer, but maybe I'm just paranoid).

Some (most) NiMH rechargers will charge batteries in groups of 2 or 4, but unfortunately when one battery starts to fail (which they will eventually do) it can cause the rest of the group to also fail. The charger is just charging them as if they were one big battery. Because of this (and because some of my devices use 3 AAs at a time) I use a charger that will recharge up to 4 cells at a time, but charges each one individually. That way I can charge 1-4 cells at a time and can tell when one starts to fail (I've had 2 fail in the past 3 years). I can also charge cells of different capacities at the same time.

Other battery types

Lithium batteries are another type of AA cell that's recently become available. They last for a long time, but they have two characteristics that count against them: they're expensive, and they're not rechargeable. Actually, there are three basic types: Lithium Alkaline, Lithium Manganese, and Lithium Thorium. I'm not sure of the specifics, except that batteries such as the Energizer Lithium Photo batteries are actually alkalines with some additional lithium in the chemistry.
Many devices today such as recent digital cameras use Lithium-Ion (LiIon) batteries that ARE rechargeable. These are excellent power sources, but unfortunately are not available as AAs. Actually this is probably a good thing: with the bursts of high discharge involved in a flash they would probably overheat and be at risk of exploding!

Back to questions of flash power...

In my Canon 420EX flash I have used NiMH AAs for a long time. On a fresh set of alkaline cells the flash will recharge in 5.9 seconds (timed with a stopwatch, so my reaction time has to be factored in) but as the cells lose power the recharge slows down fairly quickly. On a fresh set of 1650 mAh NiMH cells the recharge time was only 4.3 seconds. You might ask "so what?" about the 1.5s improvement, but when you're taking a photograph of action (people, wildlife, whatever) 1.5 seconds can seem like an eternity - even 4.3s is a long time. I'm amazed when people wait 15 seconds for their flash to recharge (e.g. on a non-fresh set of alkalines) and think it's normal! Depending on what I'm photographing, when my NiMH cells drop to 6-8s I swap in a fresh set. Also, at the end of a day's photography if I've been using the flash at all I will typically put in a fresh set of batteries just to be sure.

Note that these timings are for full-power discharges. When the camera uses TTL flash it decides how much flash to use, and in many cases will not use a full discharge (e.g. if the subject was fairly close to the flash). Also, when using fill-flash where the flash is not the main source of light the flash will typically only use a full discharge if the subject is a long way away. If the flash only uses half its power, obviously the recharge time will be shorter.

Sometimes you want more "reach" from your flash. Usually the solution is to use a bigger flash (such as moving from a 420EX to a 550EX). Better power sources cannot increase the power of the flash - all they can do is reduce the recharge time and last for more flashes. If you're using a lens of 300mm or longer then you can use a "flash extender" such as the Better Beamer which concentrates the flash beam. This is often used by wildlife photographers using fill-flash (including myself).

External power - longer life & quicker recharge?

dummy batteries
I've often been frustrated by the recharge time of my 420EX, even using NiMH batteries. But for a long time I put up with it: the 420EX officially has no provision for an external power pack.
However it is possible to use external power sources with a flash such as the 420EX. If you have a power source that supplies 6V, you can connect this to the flash by using a set of "fake batteries" which have a cord that comes out to the external power source. I made a set of these using 12.5mm wooden dowel. The fake batteries connect into the bottom two bays of the 420EX's battery cage, and I put one more piece of dowel in there to keep everything snug. The fourth cell is left empty to allow the cable room to escape.
dummy cells in 420EX
NOTE: It is extremely important that you do NOT put the fake batteries in the top bays of the 420EX, as those are connected together inside the flash. If you do this and connect power you will short-circuit your power source!

You can cut a groove in the door covering the flash's battery compartment so that the cable can escape with the door closed, but because of the sliding design of the door on the 420EX, it's not necessary. Just put a rubber band (or gaffer tape if you prefer) around the flash to hold the door closed. If you want to be able to slide the door up so it latches and holds itself closed you will have to cut a groove, but no 420EX's were harmed in the making of this article....

cable exiting 420EX
As it turns out, some commercial external flash batteries such as the Quantum Bantam and QB1 units use the same means of getting power to the flash (but their "fake battery" is made of plastic instead of wood). The prices for these external batteries start at around US$200 and go upwards fairly quickly. If you're prepared to create your own device you can get away with spending a miniscule fraction of that. That is, if you're prepared to do without features such as battery level indicators and protection from deep-discharge.

In this home-grown design the other end of the cable has a connector on it that allows you to easily attach and detach an external battery. You can use pretty much any power source you like as long as it supplies approximately 6V. You're no longer bound by the physical size limitation of 4x AA cells.

By supplying higher power to the flash's 6V supply than available with AA batteries (which the flash is internallly converting to high voltage for the flash recharge) we can improve the recharge speed. We may not be able to match the speed achievable with external sources that feed high voltage power directly into the flash recharge circuitry of flashes such as the 550EX - the flash's internal voltage conversion circuitry may become the next bottleneck. But we should be able to reach reasonable recharge times without having to replace the flash with a unit that has a high-voltage input.

Quicker recharge from a 420EX

6V 5AH SLA cell
You can use a single sealed lead-acid (SLA) 6V battery (as found in portable spotlights and burglar alarms). It doesn't have the fancy features of the Quantum units such as LED battery level indicators, but it's small, uses standard batteries, and is CHEAP.

SLA batteries are different again from the alkaline, NiCd, and NiMH batteries we can get as AAs. They can supply lots of current (which is good because it means your flash can recharge quickly). However they don't like to be discharged fully (this actually damages them) so recharging them regularly to top them up is required (they don't noticeably self-discharge either: only 5%/month). The major drawback of SLAs is their weight. They are heavy when compared with NiMH cells, but they're available in larger capacities. A useful introduction to SLAs can be found in a PDF file from Jaycar Electronics.

A standard "small" 6V SLA has a capacity of 4Ah or 4.5Ah. When I went shopping I managed to pick up a 5Ah cell in the same size (which was on special and cost only AU$10). A 240V SLA recharger cost AU$23. Attach a short cord to the battery with a connector to match that on the cable from the flash, change the connector on the charger cord to suit and you're set. The connectors are "sexed" so that the flash and charger cannot be connected together, only to batteries.

The SLA battery is much heavier than the 4x AAs it replaces, so it's best to put it in a case that you can clip on your belt, hang over your shoulder, or attach to your tripod. I already had a selection of old Inca and Tenba cases which suited perfectly. Obviously the cord that comes out of the fake batteries in the flash should be made long enough!

The 500,000 candlepower rechargeable spotlight that lives in my 4WD uses a 6V 4.5Ah SLA cell, and I've changed the cabling inside it to use the same connectors as the flash rig. This means I can easily swap batteries (if I'm near the spotlight of course). The spotlight has a 12V charging connection, so I can use this as an SLA charger when on the road.

Connector clips

It's important to use connectors which are keyed so you can not accidentally connect the battery in reverse. The connectors should also provide a secure enough fit that they will not pull apart easily (the round "DC" connectors used on many digicams are guilty of this). One final safety tip is to orient the connectors so that it's hard to accidentally short out the battery pack (which could catch fire or explode as a result). Some earlier DIY projects on the web used connectors from Molex which are keyed and provide a reasonably firm fit, but for some reason put the "female" connector on the battery and the male on the charger. This seems like an accident waiting to happen.

I had a few of those Molex connectors, but instead used these other connectors I found at Dick Smith Electronics. I think they provide better protection of the contacts, and have a convenient hook to stop them separating when you're not expecting them to. You may be able to see I've used two sizes of heat-shrink tubing to provide a better fit than I could with just one. The difference in size between the cable and the connector was just too large otherwise.

The final result

The product of all this is a belt-mounted battery that easily recharges my 420EX flash in about 2s from a full discharge. And all for less than AU$50 (plus an afternoon of DIY labour). That's a reasonable cost for me, unlike the commercial battery packs.

Do I use it all the time? Not quite: the flash with 4 NiMH AAs is a smaller and more convenient assembly. But it's easy to switch between the two power sources, and having the ability to shoot high-speed sequences with fill-flash is amazingly useful.

Black-browed Albatross (Diomedea melanophris) impavida race
EOS 10D, 100-400mm lens, 420EX flash with Better Beamer
This was part of a sequence I took at 3 fps: the flash had no trouble keeping up.


Each time you fire the flash it puts out a lot of power. Some of that is in the form of heat, and the flash head needs time to cool down between shots. If you fire the flash too quickly for too long it will overheat and fail: the bulb can burn out, the plastic lens can melt, etc. This problem is not unique to this SLA battery pack: it's a fundamental property of most flashes.
In the heat of action photography my cameras will fire at up to 3 frames per second. If the flash doesn't have to provide full power, the SLA battery can often keep up for extended bursts. But I need to be careful to give the flash time to cool down before firing another burst...
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