OK, so in part 1 I wrote about the advantages of using a portable, independent power supply as an alternative to the power grid. Now I'm going to show you how I turned that into a working device.
What are the two items needed to create a battery pack?
- 12V battery
- Power inverter that turns 12VDC into 220VAC (or 110VAC depending on your equipment).
The Battery:
I've chose to use a 7AH Sealed lead Acid battery (aka SLA battery) that is also the type they use for most battery packs. These batteries are available over the internet and in shops that sell electric supplies. It's rather cheap; the 7 amps batteries I use cost me less then $20 each. Those batteries are available in various sizes and capacities so you can build yourself small packs as well as monster ones with only few bucks difference, but take my advice, don't buy a battery that's way over your needs if you want to maintain the advantage of portability and low price. My experience with the Impact on events has taught me that a 7AH allows me to operate it over a whole evening on which I shoot around 800 photos with the flash on minimum power setting.
The Inverter.
I've chose the type that is sold for cars and camping, that inverts 12V (typically coming from the car's battery) into 110V/220V AC we need for our flash. Inverters vary from another in wattage and in wave form output. Those inverters can be bought for as low as $20-30 on ebay and I have a positive experience with them. They are built in metal fan cooled boxes, which is essential for battery pack applications.
Happily, I found out that my flash operates smoothly on "modified sine wave" (square wave) that's produced by the more common, inexpensive inverters. Some electric appliances require "true sine wave" to operate that is only produced by much more expensive inverters.
I used a 500watts inverter for my battery pack. The reason I chose a 500W inverter for the task of charging a 100ws monolight is that flashes typically draw a large amount of power when charging. I wouldn't want the Inverter to work too hard (or too slow) to provide a fast recycle for my flash over a series of hundreds of fires, so I figured I'd better choose one that is not just strong enough, but also stronger then my needs.
Integration:
Putting the pack together is simple and easy to perform. You need the battery, the inverter and thick, strong, low resistance wires. You connect the inverter to the battery, making sure polarity is as should be and that's all. You should also think about the way you're going to connect your charger to your battery obviously.
My implementation:
I chose to solder the wires to the battery for a constant, durable connection and then covered the terminals with gaffer tape to avoid short circuits.
I used heavy duty ring tubular terminals (electric supply store) to perform the connection to the inverter terminals. I did the same for my charger, so I can connect it to my battery even when it's connected to the turned off inverter by threading the chargers terminals together with the battery ones.
C'est tout! The battery pack is ready to rock your flash.
Is that realy all?
Almost, I mean electric-wise the battery pack is complete but you should also figure how to attach it to your light stand. There is no single right way of doing that. You can come up with whatever setup you want. I came up with the simple carriage you see in the photos, combining the battery and the inverter together. At first I thought of keeping them apart from one another for insulating the heat, but then I realized that the fan cooled inverter stays cool throughout the event and can be fastened to the battery safely. The carriage is made of aluminum rail which I bent to the desired shape, and in its middle I formed a hook hanger that quickly hangs on the base of the light stand.
Placing the battery pack at the bottom of the stand also adds weight and stability to the stand. Securing of the battery and inverter to the carriage is done with two Bungee balls; a very useful item. I always carry a bunch of those in my bag.
Performance:
When I use the Impact monolight on low output (the typical setting inside a wedding venue or outside after sunset) the 7 amps battery lasts several hours, and about 800 fires or more. That is usually enough to last the whole event without changing batteries. For reference, I use the flash bounced of an umbrella (or shot through) with a typical ISO range of 800-1600 and apertures ~ 2.8-6.3. If you want to squeeze more out of your battery pack you can use a higher capacity battery (60 amps car battery anyone?
). Overall I'm very satisfied with the performance and reliability of the pack. It does the job smoothly and the flash charges as fast as when connected to the outlet, over hundreds of shots. Since I started using it for event photography, I can move my lights freely across the venue and be more creative with my setups; Priceless for group photos, terrific for areal fill light.
Summary:
The DIY car-inverter-based power pack is good for:
- Event photographers looking for a cheap alternative to the expensive brand-named battery power packs to operate low power monolights.
- Food, product and portrait photographers who shoot on location with relatively weak flashes and need a portable power supply.
Next time you go camping be sure to take your DIY power pack with you. You won't believe how useful and cool this item is...
be sure to read the following related articles:
