Trying to understand battery needs for lights!
 

I'm looking at some lights to put on my cam and I will obviously need a battery pack to go with it. Problem is, trying to figure out what my needs are... specifically, capacity!

For example, an NRG 56001 Varalux Pro, which is a dimmable 100w light (from 10% to 100%). It will obviously consume more power at 100% than at 40%. But how can I calculate what I need from a battery?

Looking at packs, they are rated with 86.4Wh, 144Wh, 4.5Ah, 12Ah... I can't make heads or tails out of this!

I got a some great help from Mark Sasahara on this board for a very similar question. He said that you take the watt hours of the battery and divide the wattage of the bulb into it. So if you have a 85wh battery and a 20 watt bulb you get 4.25 hours of light. Now, with using a dimmable (sp?) light I would think that the rule would still apply, but I could be wrong, but I would think that it would be a good starting point.

Maybe see about shooting Mark Sasahara an e-mail or maybe he'll pick up on this thread.


To Mark:
Thanks again for your help Mark, the shoot went well and I had good light all night...

Kevin

*EDIT*

Mike, after looking at the light on B&H it seems that the above rule would apply. At 10% the light would use 10 watts, so an 85wh battery would last 8.5 hours. Looks like a great light, hope it works out for you.

In an ideal world, yes you can just do the math. Unfortunately, batteries are in the realm of "voodoo," much like RF equipment. Sometimes, they behave exactly as you'd expect (and as numbers would suggest), and other times they do whatever they feel like.

Batteries provide their best power when kept warm. In the cold, you'll find that you get half, or perhaps less than half the time you expect from a battery. Also, depending on the age and upkeep of the battery, you may find that it holds less and less power.

In terms of draw, the draw of the lamp itself doesn't take into account the inefficiencies of wiring or dimming circuits - it's simply a measure of the draw of the lamp, not the working unit. Unless you have obscenely long cables, the added resistance shouldn't be much of a problem, but the method of dimming will affect the draw. PWM dimmers take FAR less power than simple resistive dimmers... but you're more likely to find simple resistive dimmers on inexpensive on-cam lights.

So, now that we know the numbers don't always mean what they say.... The best advice is to use the 100% draw (lamp) and brand new capacity (battery) figures as your initial guideline.... and then build in a safety factor. Never, EVER go into the field with a single battery, even if you're positive you won't need 2. Murphy's law dictates that you'll forget to charge one, you'll shoot a 20hr. day, someone will accidentally leave a light on.. There are myriad ways a battery can be discharged, and chances are it'll accidentally happen when you have no room for error. Play it safe, and plan for twice the theoretical maximum you'll ever shoot. You may have extra batteries lying around most of the time, but for that one-in-a-thousand shoot that you run over, it'll pay for itself.

Mike,

The simplest way to figure it is; 'volts times amps equals watts'. Or 12A X 12V = 144W. Now divide the lamp's wattage into the 144 to figure the time, or 144/10w=14.4 hrs. Also, 144/100w=1.44. So, a 12Ah battery should get you anywhere from 1.44 to 14.4 hours depending on your light setting (10w to 100w). For those of you critical of my formula, I know, I know, but I'm trying to simplify it for use in the field and for those without any electronics background. (no insult intended toward you, Mike) Having written all of that and not wanting to restate what's already been said, read Jaron's response. It's a good one. Generally, I just make sure to have on hand way more battery power than I'll possibly need for the shoot. Plus, I keep spares 'just in case'. I hope I've been helpful!

Actually, nothing is wrong with your formula! One thing to keep in mind is if the load is loading the battery really bad (like a high power 100W light), the battery does not deliver its rated capacity. In other words, if you have a 30w light powered by a 100wh battery, you would get about 3 hours out of it. But if you connect a 100W light to the same 100Wh battery, you will get less than 1 h out of it. (may be 20-30% less). This is why using a dimmer extends the run time of a battery, by reducing the stress level on it.

One other thing, you may want to go with a 50, 35, or 20 watt light. that way you'll get proper color temp and longer battery life. when you dim the light, you also lower the color temperature, making the light warmer. Unless, you white balance for that setting.

Kevin, glad I could help.

Jaron brings up a good point, the formula gives a rough estimate, your actual mileage may vary, as they say.