Points in Focus Photography

Powering Projectes – Batteries


I just finished soldering together my first test LED light panel, it’s only 492 lumens worth of LEDs, but it’s good enough to start testing a lot of the electronics. So things are slowly moving forward. Today though, I want to talk a bit about power, or rather batteries.

I first started thinking about power back when I conceived of the idea. Back then, I was still targeting a >6000 lumen 114 W 1×1 panel, and the idea of using a 19 V laptop power brick for the power supply. I’m still working on that idea for large panels and as a non-battery option, however, when I started thinking about smaller test panels, I started thinking about using batteries.

My first iterative test panel is only 4×6 inches, and with standard density LED strips, only draws 6 watts. Even if I scaled it up to high density LED strips, I’m still only talking about 12 watts of draw. Factor in some overhead for inefficiencies and a good estimate is somewhere around 15 watts, which is well into the range of batteries.

The question then becomes choosing a battery pack and dealing with the engineering needed to get it to work.

From a design perspective there are two major considerations I’m working under here. First, since this is a DIY type project, costs are a consideration; so I don’t want something that’s incredibly hard to source or expensive. I’d also like something that I don’t have to design the charger for, or build a charing circuit for. The second consideration is that since this is a photography project, I want to use photography/videography batteries if I can.

My first though was to use Canon’s LP-E6(N) batteries, the same batteries my 5D mark III, and Small HD DP4 EVF use. Sure I’d need more batteries than I currently have, but any batteries I bought I could also use in my camera and monitor.

The only trouble with the Canon batteries is that they’re a bit under powered for this. The LP-E6 batteries are 7.2V 1800mAh, yielding 13 Wh. The LP-E6N batteries extend the capacity to 1865 mAh, or 13.5 Wh. With the panel drawing an estimated 15W, that gives me less than an hour of run time on the Canon batteries. That’s not a situation that I’m really all that happy with.

My second though was to step up to 18V DeWalt or Ryobi Lithium Ion tool batteries. Since they’re 18V batteries, I would be able to omit the boost converter that I need to raise the 7.2–7.4 V output of camera batteries to 14–15V for the regulation circuit. That saves some parts and work, but it’s not ideal either.

From a mechanical standpoint, tool batteries are relatively heavy and have an unusual stalk that has to be addressed. Plus nobody sells a socket for those batteries, so I’d have to hack apart cheap flashlights or something similar.

From an electrical perspective, there’s precious little reason to bother. As a slight aside, I’ve seen at least one person build tool battery camera power packs, in the vein of a Anton Bauer battery, and they’re really not all that great for that solution either.

A DeWalt 18V compact lithium ion battery is only rated for 27Wh, which yields a little less than 2 hours of panel runtime. For $30 they’re not horrible, but they’re also not native camera batteries either.

The best option in my opinion, is actually to use off brand Sony NP-F97x batteries, like these Kastar NP-F975. These batteries are 7.4 V, 8700 mAh, yielding 65 Wh of capacity. With the on camera panel drawing around 15 watts at most, that gives about 4h20m of run time on a battery which isn’t bad and isn’t totally insane either.

Mechanically, the NP-Fxxx batteries mount flat, and mounting plates can be had for the myriad of devices from this $6 [Watson Adatper][] that would need some adapting, to adapters that have DC coaxial jacks already attached. All of which are relatively easy to mount to a custom project with epoxy or even screws.

What about Anton Bauer or V-Mount batteries?

I don’t really see them as a viable option other than potentially as an external power source, unless you’re already using them for your camera. Their small packs are in the 90 Wh range, which is way more power than a 12W on camera light needs (7.5 hour run time), and not enough power for a 114 W 1×1 panel. Plus on a dollars per watt-hour scale they’re expensive at almost $3/Wh, compared to the <$1/Wh of the 3rd party NP-Fxxxes.

Of course there’s still a lot of this up in the air since I haven’t finalized any designs. There are also still some outstanding questions about the Kastar packs I linked above, such as whether or not they have over discharge protection. However, at least based on my initial analysis, off-brand NP style batteries do appear to be the best design choice for a DIY scale camera related project, at least they’re something worth looking into if you’re building something like this.

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