Introduction: A Portable Power Box for Photographers
Many photographers have built Vagabond style portable power systems to use with their strobes. This is an attempt to incorporate several design ideas and safety features. The power box can also be used with laptops or other delicate electronic equipment.
Metal tool box (18'x8'x9' or larger)
300 watt Samlex pure sine wave inverter (Frys.com $130)
(I have been told that Samlex's new 300 watt inverter with usb will not work with a power box. Based on Samlex's new design, a 600 watt inverter is needed to run just one 300 watt strobe. There must be other pure sine inverter alternatives out there, that people can recommend)
Scooter/wheelchair battery 26AH-35AH ($40-$100)
40 amp Square D Breaker QO (single pole AC/DC breaker $14 electrical supply store)
8 AWG Braided Electrical Cable (SOOW 8GA $12)
Bolts and lock washers
(2) Brass washers
(2) 2 1/2' and 3' angle brackets
Ground Fault Circuit Interrupter (GFCI) and Box Housing ($16+$5 Home Depot)
Old scrap extension cord with plug
Large nail or spike
Flat piece of hard plastic
Copper ground wire stripped from an electrical cable
SAFETY: Disconnect the battery when working on the circuit. Wear gloves when handling the battery.
Step 1: Angle Brackets
Use a metal tool box to house the power supply. It will serve as the common ground for all the components. The box should be tall enough so that when closed, the lid doesn't touch the battery terminals. Place the battery as close to the centre as possible for even weight distribution. Do not use a car battery for this project as it quickly will become damaged from being repeatedly discharged.
Mark and drill holes for the angle brackets. Remove the paint from under the left 3" bracket. This bracket will attach to the ground spike. 2 1/2 " brackets were used on one side because the box was not wide enough for 3 inch.
Mark and drill holes for the inverter and the inverter ground bolt. Remove any paint from around the holes so there is a good electrical contact.
Step 2: The Inverter
If you don't use a pure sine wave inverter, you will risk damaging your strobes. Some modified sine wave inverters are deliberately mislabelled and sold as pure sine wave inverters.
Attach the inverter with bolts and lock washers. Whatever surface you mount the inverter on, it must be horizontal or the internal fan will not work properly. Use the stripped electrical grounding wire to attach the inverter ground connector to the tool box ground bolt. Ideally the grounding wire should be 6 gauge, but it's difficult to work with anything that large in a small space.
I've heard rumours that Samlex has significantly changed the design of their inverters since I built the power box. I have not tested these new inverters, but it would be interesting to know how well they work. Update: The new 300 watt Samlex inverters are unusable. They now automatically shut down under stress.
The inverter company Samlex has stated that when building power boxes the inverter's surge rating should be four time the watt rating of the strobe. A 300 watt strobe would need a surge rating of 1200 watts, which matches the surge rating of a 600 watt Samlex inverter.
Step 3: Spacing
Place the battery in the box to check spacing. If I did it again, I would mount the inverter facing the other direction on the left side of the box, so that the power switch is at the top and easily accessible.
Step 4: Wiring
The Samlex manual indicates that the inverter needs a 40 amp fuse. It's a common electrical practice for a 40 amp circuit to use 8 gauge cable. With a large cable, it is easier for the inverter to draw the amps needed to run the strobes. Standard household 12 gauge wire is too small and will cause the inverter to beep and switch to a modified sine wave.
Use braided 8 AWG wire instead of stranded. It is much more flexible and will put less force on all the connections. SOOW 8GA cable has several braided wires inside and is relatively inexpensive.
Tin the ends of the cables and solder/torch on the battery connectors. Use full sized battery connectors. Smaller sized connectors can limit the maximum load by over 400 watts. I was unable to find the appropriate inverter connectors, so I tinned and shaped the wire with pliers so they would fit inside the inverter power sockets.
If the inverter will not power over a 1000 watts of strobes without beeping, unsolder the battery clamps and try directly attaching the wires to the battery terminals with bolts and washers. Use two washers on each terminal to help prevent the bolt from being pulled through. Cutting the ends off a 2 1/2" corner brace makes very strong square washers. In the future I might try using the wire from a pair of old jumper cables to see if there are any amps to be gained by having a larger conductor.
Step 5: Breaker
The 40 amp Square D QO breaker is designed for use in DC circuits. The breaker will help to protect the inverter and is easier to use than a 40 amp fuse.
The circuit for the power box is:
battery positive --) breaker--) inverter positive
battery negative --) inverter negative
Drill bolt holes and zip tie holes in the flat piece of plastic. Solder on the positive battery cable to the breaker with a 100w soldering iron. Attach the breaker output cable to the inverter. Bolt and zip tie the breaker to the side of the box.
Step 6: Ground Fault Circuit Interrupter
A GFCI is designed to instantly cut power to the outlet if there is a short. When shooting outside you will get rained on, so a GFCI is essential.
Assemble the GFCI in a housing according to the instructions. Attach the end of an old extension cord to the GFCI and test it to see if the reset button is working properly.
Make sure there is enough space in the tool box to easily plug cords into to the GFCI after it is bolted into place. Remove paint around bolt holes for a good ground. Plug the GFCI into the inverter.
Step 7: Ground
A ground spike may not be necessary/usable under all circumstances, but it adds an additional layer of protection. In the event of a short, it will hopefully channel the current harmlessly into the earth.
Tin the ends of a 8 gauge cable. Solder one end to a brass washer and hose clamp the other to a large nail or spike. Electrical tape the hose clamp connection and bolt the washer to the left 3" angle bracket.
Step 8: Finish
Portable Power Box Setup
1. Push ground spike into the earth (If possible)
2. Attach negative cable to battery
3. Turn on breaker
4. Attach positive cable to battery
5. Turn on inverter
6. Turn on strobes
Always connect the positive terminal last, so that any arching occurs on the cable that is protected by the breaker. When the positive cable is disconnected, do not allow it to touch the metal box. Turn the breaker off first. Some people have reported that the capacitors in the inverter can still hold a charge and can feed voltage back through the battery cables.
Turn off all the strobes before turning off the inverter. Do not store the power box with the battery connected. The portable power box with a 33AH battery will run two 600 watt strobes (not at maximum settings) for over 210 flashes. I have not yet drained the battery shooting under normal conditions.