Introduction: A Portable Power Box for Photographers

Picture of 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.

Materials:

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
Hose clamp
Zip ties
Flat piece of hard plastic
Copper ground wire stripped from an electrical cable
Battery clamps


SAFETY: Disconnect the battery when working on the circuit. Wear gloves when handling the battery.

Step 1: Angle Brackets

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.




Comments

TBKVideo (author)2011-05-18

This is awesome. I'm building mine this weekend. What kind of strobes do you use?

Divet (author)TBKVideo2011-05-22

I have used the power box with Alien Bee and Elinchrom strobes.

mrosenstok (author)Divet2011-11-28

hey Divet,

whish elinchroms you use?
I have 2 - 500ws BXri. Do you think it will work ok with my strobes as they are a digital model?

Is your DIY portable power pack still working ok after all this time?

Best regards
Miguel

Divet (author)mrosenstok2011-12-03

I've used the pack with the digital Elichrome Style 600RX, but I haven't ever used the Bxri and couldn't say for sure it would work (tho there is a decent chance it would).

Three months ago I loaned the pack to someone and they burnt out the inverter by shooting too quickly with a 1000 watt load. The design of Samlex inverters have been changed so they now shut down with a low voltage. I haven't found a decent replacement inverter locally to rebuild it with. I guess in Portugal another option might be the Paul Buff vagabond mini, if you have the cash. http://paulcbuff.eu.com/cms/

thepaul93 (author)2010-07-19

Very nice. I guess if you wanted to you could add a charge controller and a solar panel.

ElectricMan1 (author)2010-02-19

Dude, thats way overkill on the wire, 300watts/ 12volts= 25 amps DC.
25 amps for less than a foot= 12 guage flexable wire is fine.

I use #10 on my 400 watt inverter system.

Divet (author)ElectricMan12010-03-31

I tested with scraps of 12, 10 and 8  gauge wire when building the box. 8 gauge allows more watts.


darkride (author)2008-12-22

Um, a GFCI doesn't protect against a short, it protects you from being electrocuted by touching the hot wire while having a path to ground, such as when in a wet environment. Since this project is not using commercial power (which has a good ground path), I doubt it is of much use, just added cost, leave it off. Lose the ground spike (which is not long enough to be of any use anyway.) and there is no path to ground. With no connection to earth ground the only way you could electrocute yourself would be to touch both AC wires at the same time. By the way, it doesn't matter which battery lead you attach first, just make sure the breaker is off.

Divet (author)darkride2008-12-22

Using a power box in wet environment with a sync cord can result in a photographer receiving shocks through the camera. Installing a GFCI and a ground spike is an attempt to minimize the possibility of damage to either equipment or personnel.

ElectricMan1 (author)Divet2010-02-19

GFCI- GROUND fault circuit interrupter.
The inverter is poorly grounded if at all, the best thing to do is not get electrocuted.

vodo (author)2009-03-13

field bought a battery at radioshack. it is an alarm battery, here air the specks: charging voltage: 14.4-15.0v 13.5-13.8v. application:normal charge standby/trickle max charging current: 1.5a 1.5a. can I use it to build it? oh,an its 12v.

Divet (author)vodo2009-03-21

See if there is an AH marking. I think most 12 volt batteries at Radio Shack are under 8 AH. 10AH would be the minimum I would try.

ReCreate (author)Divet2009-04-22

I used a 700MAH battery for an inverter and it ran a 60 watt fluorescent light for a while,So its about 1 amp hour,if the Light consumed 100miliamp hours(i know it does not though)it would last for 7 hours,if it consumed 700miliamp hours it would last,1 hour

ElectricMan1 (author)ReCreate2010-02-19

In a perfect world, batteries don't drain 100%.
I learned that the hard way!

Sandisk1duo (author)2008-07-27

The battery is a bit heavy? anyone got links to cheap Li-Poly batteries?

ReCreate (author)Sandisk1duo2009-04-22

HO NO,LITHIUM-ION COSTS THAUSANDs Really They are very expensive,and they are dangerous to charge too! just overcharging them by a hundredth of a volt and you get KABOOM

Sandisk1duo (author)ReCreate2009-04-23

if you over charge by 1/100 of a volt, you only slightly damage the battery

ReCreate (author)Sandisk1duo2009-04-23

Ha yeah,its a 1/10 of a volt not a 1/100

Sandisk1duo (author)ReCreate2009-04-23

1/100 of a volt actually decreases the cycle life by just a tiny bit if a battery is charged to 4.20v it has less capcity, but lasts longer then a battery charged to 4.235V

ReCreate (author)Sandisk1duo2009-04-23

the battery is rated 3.7 Volts,if it is at 4.20 that means that it is overcharged,big time

Sandisk1duo (author)ReCreate2009-04-23

and that....................... is where you're wrong 3.7V is just nominal cell voltage When fully charged, the battery is at 4.15-4.25 Volts same thing for nicad, nimh, and Lead-acid batteries, they are said to be a certain voltage, but they need to be charged more then that.

ReCreate (author)Sandisk1duo2009-04-23

I don't get it,so batteries are not really what they say? But much higher?

Sandisk1duo (author)ReCreate2009-04-24

not 'much' higher and only rechargeables regular batteries have the voltage that is printed on them

ReCreate (author)Sandisk1duo2009-04-24

SO lithium ion batteries to get overchagred,to 4.20 volts,when they are rated 3.6 volts,so what are their real voltage? 4?

Sandisk1duo (author)ReCreate2009-04-24

there is no 'real' voltage the voltage changes when fully or partially charged

ReCreate (author)Sandisk1duo2009-04-24

I guess so...So what would be an over charged lithium ion?

Sandisk1duo (author)ReCreate2009-04-24

An overcharged i-ion would be 4.2+ V a charged li-ion would be 4.2V a fully drained li-ion would be 3.6V

ReCreate (author)Sandisk1duo2009-04-24

I guess you learn something new every day!

Sandisk1duo (author)ReCreate2009-04-24

over the course of two days

ReCreate (author)Sandisk1duo2009-04-25

I meant that that you said that day,i thought that it was to 3.7.not 4

ElectricMan1 (author)ReCreate2010-02-19

HA ok they should come up with another name for Lithium Polys: BOMBS

dooj (author)2009-09-08

and plese give me i link

dooj (author)2009-09-08

where did you get the battery

brian3140 (author)2009-05-02

The concept is good, but electrical theory in the comments is very confused. definitely put the GFCI on the outside of the box and don't worry about rain or water. it'd never be ideal to do it on purpose, but the GFCI will protect you even if you sprayed it with a hose. In rain there is a high likely hood that it will never even trip the GFI on the outlet, but if it does... oh well. that is what it is designed for. And no rain cover is necessary. People trying to make the system lighter should just stick with the heavy battery, but a smaller cheaper inverter could be used. I bought a 400 watt inverter at Wallmart a few years ago for $40 and never had any trouble.

ReCreate (author)2009-04-24

The Inverter should provide you 300 WATTS for 1 hour...I think

ReCreate (author)ReCreate2009-04-25

So according to my calculations,IT should last... few days at 30 or so watts

josh.wingo (author)2009-04-24

you should cut a hole on the side for the gfci to face outwards. so you do not need to run power out of the box, the box can stay sealed. when facing the gfci out, get a weatherproof box cover, it will keep water from getting in it and you can still safely use it in the rain.

animes25 (author)2008-07-29

and the million dollar question is: How do I recharge the battery when is worn out?

Divet (author)animes252008-07-30

Use an intelligent car battery charger.

ReCreate (author)Divet2009-04-22

or any charger,or any supply of 12-15v

ReCreate (author)2009-04-22

what are the specs of the battery?(other than voltage of course its 12v) Lets see...50 ma is lethal,the battery being 12v will let only 0.20(about) ma through your skin,therfor.0.20 out of the lethal 50.0 ma is going through you,your safe touching the leads,just as long as you don't have salt water on your hands

mbainrot (author)2009-02-11

One thing i noticed is that you have set the inverter up in a way which puts it in an enclosed space, if your not careful you can overheat the inverter.

With the AH question, in theory you can get away with as low as 7AH, (which is what i use to power computers etc), the important calculation you need to make is how long you get, MINUS the safety cut off (most inverters will shutdown when the battery voltage gets too low), also you need to compensate for the efficiency of your inverter, batteries, etc.

For example :)
To calculate approximately how long a 7AH battery will last on a 300 watt load (my inverter @ 100%), you would do the following :)

1) Convert the watts into "dc amps", this is down by dividing the wattage by the operating voltage of the inverter (so in this example its 300 divided by 12, which gives you 25A)
2) Then you add the inefficiency amount (so if its 95% eff, the inefficiency is 5%) to the amperage (so in this example, 25*1.05, which gives you 26.25A), then you add the approximate efficiency of your battery (Sealed lead acids trend around 90%, so in this example, 26.25*1.10, gives you 28.875A)

3) With the final figure from the last calculation (in this example its 28.875A). you divide the amp hour capacity of your battery (or total capacity of your battery bank) by the amp figure (e.g. 7AH / 28.875A = 0.24 hrs or approx 15 minutes)

Please note that the voltage cut off will usually prevent the battery from getting lower than about 10.5volts, which leaves you with piss all power from the battery.

Other notes are with measuring equipment to determine your needs, please note that inductive loads sometimes cannot be measured (no idea why)

Hopefully my 5 cents is of use to people :)

brandegor (author)2009-01-16

Would this be safe to run video cameras as well?

Divet (author)brandegor2009-01-19

Yes. Power supplies for video cameras are usually under 20 watts.

lgcalex (author)2008-09-06

this is great! I didnt see how much shooting you can do off of one of these. How many watts are you strobes/ number of times you can fire them? I will most likely be building one of these in the near future. The strobist method doesn't allways work if you need more light

Divet (author)lgcalex2008-09-10

The portable power box with a 33AH battery will run two 600 watt strobes (not at maximum settings and with some use of the modeling light) for over 210 flashes.

mattccc (author)2008-08-22

can i use a 4.5 ah battery instead

Divet (author)mattccc2008-08-26

A 20AH battery is the lowest I've seen used for photography purposes. Some people have said that 15AH is the minimum.

Dipankar (author)2008-08-02

Your Photography is of excellent quality.

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