Car Battery As Power Source

John Crow · March 9, 2009

Ok, so here's my situation: for reasons that are complicated and not worth going into I need to light a scene with no generator and no house power. It's a low-level light scene and we're talking about just using 5 or 6 CFL bulbs and trying to power them with a car battery. I've never attempted to use a car battery as a power source and have several questions:

- how many car batteries would it take to power 6 13 watt CFLs & how long would it last?

- Could I get away with just using 1 battery, or would I need to wire 10 together since they're 12 volt to make it a 120 volt circuit?

- As far as wiring it goes, I've never done this. Is it just a matter of stripping some zip cord of the insulation and wrapping the wire around the terminals? Do any other steps need to be taken to get power flowing?

- since the battery is DC and the bulbs are AC I imagine I would need to use an inverter of some sort, correct?

- finally, how much of a pain in the ass is this? Is it worth my time to seriously just try my best to convince production to suck it up and get a generator?

Thanks for reading

Edited March 9, 2009 by John Crow

Adrian Sierkowski · March 9, 2009

Could you just drive a car up there and power it off of an inverter? I've run CLFs off of an inverter before and it was fine.

Also an inverter would convert the 12vdc from a battery to 120v.

John Crow · March 9, 2009

It's sort of a cave and we can't get a vehicle near enough to it to do that - the power source basically needs to be transportable by hand and our generator is too big and there's not enough cable to run it. I'd like to just get a little 2k putt putt to bring in their but the sound guy threw a fit, plus it's an enclosed space so I think fumes were an issue. So production's really sold on the car battery idea and it's up to me to figure out how to make it work. So... yeah. Fun stuff.

Adrian Sierkowski · March 9, 2009

Sounds like a PITA (Pain in the...).

I would look into wiring a few batteries in Parallel and then going through an inverter to convert your 12VDC to 120V.

Michael Collier · March 9, 2009

Send a PM to Michael Bergstrom, He rigged up a cool pelican case with a sealed gel battery and inverter in one unit. Its heavy, but carryable by one person. Drives about 300 watts for a few hours. You might find the cost of building something like that comparable to a generator rental if its just for one shoot however.

John Crow · March 9, 2009

Thanks for your responses. I would assume the inverter would take care of the voltage needs, so would I be wiring several together to make sure I get enough power? How many should I use? We're shooting here for a few days and can basically only fit the DP and two actors in there, so once it's set up we'll likely just leave it for a few days. Once it's powering the lights does anyone know how long it will last so I can tell production how many to order? I figure I'll just have the DP disconnect and reconnect the wires from the terminals every time he goes in and out, but they may be left running for several hours at a time so I'm just wondering how often we'll need to come in with replacement batteries. I think we'll wind up running about 6 13 watt CFLs, so 80 watts total, and if I can squeeze enough juice out of them we may run an additional 2 flourescent tubes - probably just go with regular over Kino since the wattage is lower, something like 34 watts x 2 so another 70 watts. So looking at about 150-200 watts total, wondering how many batteries I need and how long they'll run for.

Thanks guys

Adrian Sierkowski · March 9, 2009

You're talking hours not days.. for days.. perhaps a proper genny and a long cable run from outside would be a better choice...

John Crow · March 9, 2009

Send a PM to Michael Bergstrom, He rigged up a cool pelican case with a sealed gel battery and inverter in one unit. Its heavy, but carryable by one person. Drives about 300 watts for a few hours. You might find the cost of building something like that comparable to a generator rental if its just for one shoot however.

Thanks for the tip, I'll try hitting him up. Cost isn't really the issue, it's really a logistical reason for not getting a generator.

John Crow · March 9, 2009

You're talking hours not days.. for days.. perhaps a proper genny and a long cable run from outside would be a better choice...

It would be days, but not running the whole time. They're shooting in there for like 6 hours a day 3 days in a row. I expect it to go over schedule as well, so maybe like 8 hours a day for 3 days.

Again, the issue with the genny is we don't have enough cable, and we're out in the middle of nowhere so production is under the impression that it would just be easier and faster to do it with the car battery rather than having more cable shipped - time is an issue on this one more than money and we can pick up the batteries any time. I've tried to convince them to either go with a smaller genny or just have more cable shipped for the big one but they're fairly stubborn and pretty sold on having me do it with car batteries. So it looks like I'm stuck with this option and need to figure out how to make it work.

Edited March 9, 2009 by John Crow

Adrian Sierkowski · March 9, 2009

Ahh producers.

Well I'd look into Deep Cycle Marine batteries, as they tend to last a bit longer. Then you need to work out how many Amp Hours the batteries have (might be rated) and then basically average your amps that you're pulling off of it, and divide AH/Amps=Time. I believe that is the proper formula.

Stuart Brereton · March 9, 2009

You should really use lead acid Leisure batteries, rather than car batteries. Car Batteries are not designed to be run flat, whereas Leisure bats are deep cycle. There are plenty of people selling them on ebay. You should look for one in the 80 -100 amp hour range. You should also be able to pick up a 300w inverter pretty cheap too.

how many batteries should you buy? Well, your load of 200w divided by 12v = 16.6amps. A 100AH battery should run that for 6 hours, although, realistically it'll be more like 4, so I'd buy a couple and have one on charge all the time.

Michael Collier · March 9, 2009

Keep in mind John that there is an efficiency overhead for the inverters, so even if your lights are 16amp, you're likely to need more to cover the inefficiency of the inverter.

I know bergstrom used sealed gels because they can mount in any direction, which is helpful for transpo. Don't know about leasure, but I would avoid anything that isn't sealed because it could leak acid if not stored properly. I would assume marine batteries are sealed, but check to be sure.

but in general yes, that is a good rough equation to figure approximate runtime. I would factor a 10% overhead to cover your bases.

Also be sure your battery can source the amps you need. It may be a 100amp hour battery, but only able to safely source 10 amps continuously (for 10 hours). Batteries have no fuse or breaker pannel to keep them from exceeding their current limit, so over driving them is a good way to shorten their life span or get them to catch fire. Bergstrom's setup if I remember properly is able to drive 25 amps at a time (enough to cover the 300w inverter.) If you need more current than any one battery can supply, you can wire them in parallel to up the rated current, but you might need a shunt diode to prevent cross charging of the cells at the end of their run.

John Crow · March 9, 2009

I'd buy a couple and have one on charge all the time.

So you can charge these like with a car battery? So theoretically if I just always kept one charging I could just keep cycling through them every few hours and wouldn't have to get a brand new one each time?

Thanks for the response

Stuart Brereton · March 9, 2009

You can charge them with a regular car battery charger, but bear in mind they take a long time (8+ hrs) to charge fully.

John Sprung · March 9, 2009

Rather than buy this stuff -- and then where do you keep it after the shoot -- ask your rental house if they have deep cycle battery kits with chargers and inverters. They're nice for car work, you just stash the batteries and inverter in the trunk.

A couple of 200 Amp-hour batteries should do it, one working, the other charging.

-- J.S.

Hal Smith · March 9, 2009

Some inverters generate a lot of electrical noise which can find its way into sound gear. Don't let the sound guy try to save some battery by tapping into your inverter generated 120 volt AC.

Unsealed batteries can get quite smelly in close quarters when under heavy load. But gel cells are so much more expensive than marine deep cycle batteries that it'll probably be good enough to locate the battery/inverter package more towards the mouth of the cave and use a stinger (extension cord) to run the lights themselves.

All CFL's are not created equal, run some camera tests with the ones you buy before you haul them out to your location. There are some "bargain" ones with pitiful light output even though they make claims like "equal to 100 watt incandescent", etc. I've got two allegedly equal to 60 watt bargain CFL's in my garage door opener's head that don't put out anywhere near as much light as the single 60 watt incandescent they replaced.

Satsuki Murashige · March 9, 2009

You could also always go with 12v block batteries and 12v dedos and kinos right?

William Coss · March 10, 2009

John,

The deep cycle battery sounds like the right choice to me. I would get 4 of them and put 2 each in a milk crate and wire them in series to get 24 volts. You could then power many Dedolight DLH4 focusing heads in combination with DLH1X150 soft lights. These lights draw less than 7 amps each and the DLH4 puts out more than or equal to a 650 Arri head. You could burn these till you go into overtime.

Good Luck,

Bill