One ONsite Relay C can replace 8 batteries

May 5, 2023

Udi Tirosh

Udi Tirosh is an entrepreneur, photography inventor, journalist, educator, and writer based in Israel. With over 25 years of experience in the photo-video industry, Udi has built and sold several photography-related brands. Udi has a double degree in mass media communications and computer science.

One ONsite Relay C can replace 8 batteries

May 5, 2023

Udi Tirosh

Udi Tirosh is an entrepreneur, photography inventor, journalist, educator, and writer based in Israel. With over 25 years of experience in the photo-video industry, Udi has built and sold several photography-related brands. Udi has a double degree in mass media communications and computer science.

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Onsite relay C title card

You’ve been there. You are shooting a long-form video or running a full night timelapse, and boom! Your battery dies. It’s one of those sad things that comes with digital photography. The need for power. If you are shooting something where you cannot move your camera, swapping batteries can be a real project killer. This is where the ONsite Relay C (link) comes in. It provides an uninterrupted power supply to your camera via the battery chamber. (And that battery port thing matters. I’ll get to that later).

The ONsite Relay C is a battery coupler with some smarts to connect more than one battery to your camera. More than one battery means that even if one battery runs out, the other battery will keep pushing power to your camera, so the camera never feels the swap. From the camera’s perspective, it just has one huge power reservoir to drink from.

The problem that the ONsite Relay C fixes

Back in the days when I started shooting, batteries were never an issue. A battery would typically last dozens of shoots and definitely outlast 37 frames. But with cameras now doing video, timelapse, and other long-term projects, the needs have changed, and a battery may last as little as a single shoot. Or sometimes, even less. This is not always an issue, but for some projects, it’s destructive.

If you shoot long-form video, a battery change is a workflow killer. It’s not just the camera stopping to record. It’s an entire ritual that you have to go through. Typically, the first step would be to turn off the monitor, the audio, and any wireless device. Then, you’d swap the battery. Then you’d need to reconnect everything again. Not fun. Or, even if you are shooting a concert, you just want to ensure you have uninterrupted power for about two hours.

If you shoot a timelapse or stack long-exposure astrophotography images, you’ll get a similar issue. It’s not that there is much of a ritual here, but changing a battery means that you have to move the camera, and this is not something you want to do when precise positioning is critical.

This is where the ONsite Relay C comes in.

What is the ONsite Relay C?

In simple terms, the ONsite Relay C is a way to power your camera from a USB power bank. It looks like a small adapter that is not much bigger than two fingers. On one side, you have a barrel jack (DC) cable that goes into a battery coupler (aka battery stud). On the other side, you have two USB-C cables that should each go to a different battery. And it all comes wrapped in a lovely orange band.

Now, here is the magic. Once you connect a battery to one of the USB-C cables, the DC jack will hold a voltage of 8 volts. Coincidentally, this is just the right standard for camera batteries. This means that once you connect the DC jack to a battery coupler, you can power almost any camera from a USB-C source. This can be a battery or a USB-C wall adapter.  Now, while the ONsite Relay C supplies 8 volts, it should support devices running from 7.4V to 12V. Such is the magic of electronics.

Onsite Relay C - building a rig
A rig powering a camera, a transmitter and a small light

Uninterrupted power

There is more. Remember that the ONsite Relay C has two USB-C cables? You can connect two batteries to the unit, and once battery one runs out, you can continue using battery two and hot-swap battery one. As far as the camera is concerned, nothing happened, and it got power all through the swap.

The only thing you need to be aware of here is that the ONsite Relay C drains both batteries simultaneously. So here are your options:

  • Use one battery until it’s in the “red zone”, add a second battery, and remove the first battery.
  • Use two batteries for double the run time.

But if you are using two similar batteries, and both batteries are close to drainage, disconnecting one may kill the other.

Onsite Relay C - Using two batteries
Using two batteries in parallel

The Couplers

As I mentioned, the ONsite Relay C alone is not enough. You would need to add a battery stud (coupler). This is a fancy name for a plastic box that looks like your battery and will fit inside the battery chamber. Those are pretty cheap and are less than a cost of a battery. You can get them over at Tether Tools or any other photography store. Just here, I have FZ100 and FW50 couplers for the Sony alphas. But I also have couplers for the RX100 and an old Nikon 5100 from when I used them on timelapse projects. In fact, that Nikon would have probably been trashed if not for the timelapse usage. It is a great way to breathe new life into an old camera.

If you have a device that uses NPF batteries, there is even a coupler for that.

The only thing to worry about with battery couples is that they each come with their own sized DC jack. The ONsite Relay C comes with plenty of DC to DC adapters, so you should be covered. It does miss a small pouch for storing all those adapters, so remember where you placed them. (Yeah, right! You’ll lose them 10 minutes after opening the box)

When would you use an ONsite Relay C?

As I mentioned, the ONsite Relay C is great for times when you can’t move your camera or need it running for a long time. But there is one more advantage to feeding your camera via the battery port.

If you have an old camera that does not support USB-C charging, then, of course, you have no choice. And this was the case with my old Nikon and RX100. But if you can use your camera while charging it from a USB-C port, this means you would lose the USBC port. If you depend on that port for streaming, or tether shooting, you are in a pickle. I can see how this will be an issue for astrophotographers who want to shoot tethered. But also for streamers needing the USB-C port to connect to a computer.

But aside from freeing the USB port for other needs,  you should probably use that USB port as little as possible. USB ports are fragile, traitorous little things. And pulling too hard on that tiny cable can damage your camera. So given the option, I would opt not to use it.

Lastly, you now already have a battery in your setup, so if you have extra ports, you can use this battery to power a monitor, a microphone, or a small light.

Standard battery vs. ONsite Relay C vs. other solutions

There are a few options if you go with an external power solution. (ONsite Relay C is not the only option, though it is my favorite and our go-to for long interview days). You can, of course, stay with the internal battery. If you don’t shoot much, it’s a good option. You can also use other external power solutions, like the Anton/Bauer 68Wh Titon Base or the Core SWX Powerbase EDGE. While I respect the place of those solutions in the market, I think that an external power source with a coupler is superior in every aspect and is significantly more budget-friendly.

I am making this table for a Sony FX3 kit for convenience, but you can make similar calculations for any camera. The result would not be far off.

SolutionsizeweightCapacitycostcost per WHCost per extra WHcomments
Original FZ100tiny85.05 g16.4 WH$78$4.8$4.76
3rd party F100tiny82 g16.2 WH$49.95$3.1$3.08
Titon Base10.1×2.9×14.2cm520 g68 WH$446.5$6.6$6.57
Powerbase EDGE14.2×9.9×4.3 cm600 g70 WH$274$3.9$3.91
ONsite Relay Csee below299 gSee Below$164See BelowSee Below
ONsite Relay C + Tether Tools 100W batterysize of power bank849 g100 WH$343.99$3.4$1.79including ONsite Relay C
ONsite Relay C + Belkin Boost Charge 30W batterysize of power bank819 g74 WH$218.99$3.0$0.73including ONsite Relay C
Onsite Relay c - regular batteries vs Onsite Relay c
8 Sony FZ100 on the right, replaced with an Onsite Relay + a power bank beast on the left

For me, the benefit is there not only because the camera lasts longer. There is also a huge benefit that you get from reducing the miner of batteries that you need to manage. A 26,000mHa USB battery (here are some options) has about 99WH capacity. (this is not a coincidence, this is the max you’re allowed in a single battery in an airplane). A standard Sony FZ100, has about 16WH. Just imagine reducing the number of batteries you need to deal with by eightfold. Not only a single USB power bank is cheaper (Tether Tools’ 100WH is $179; this sticker brand is $99; Eight FZ100’s are $624). They are. But you now have only one battery to charge at the end of the day instead of eight. You no longer need multi-plate chargers.

There are two points of info that you need to consider when you are using the ONsite Relay C and not a standard battery.

The first is that the camera will not show the current charge on the battery. Since the ONsite Relay C delivers constant 8V, the camera will think the battery is always “almost full”. You must check the charging status on the battery display / LED bar.

Onsite Relay C - battery indicator
The battery indicator always shows about 85% battery

The second thing is that at least with some cameras (FX3 included), the internal battery will deplete if you charge the camera via the USB port and use extreme settings. This is not the case with running USB power via the battery port.

How to use the ONsite Relay C adapter

I would say that the ONsite Relay C is at its best when you are at a relatively stationary setup. Ideally, you would connect it to a tripod, just near the battery it uses. Tether Tools sells a simple clever wrap that connects to a tripod leg and holds a power bank. Between the orange strap of the ONsite Relay C and the black wrap for the battery, you are looking at a very low-profile setup.

Onsite Relay C - tripod setup
This is my tripod setup. I use one battery and connect a second battery before removing the first one

Start by turning the camera off. Once the USB cables are plugged into a battery, you can open the battery compartment door and slide the coupler in. Most cameras will have a small “cat door” to let the coupler cable out of the chamber. For the Sony FX3, you push a small button that removes the entire door. You should probably store it with the DC plugs so it won’t get lost ;)

Sony A7III + Sony FX3
The Sony A7III has a small cat-door for the cable. For the Sony FX3, you need to remove the battery door


Changing batteries in a middle of a shoot is a drag. You should avoid it if you can. If you are doing any work that needs power for long periods, the native batteries simply won’t do. The number of concerts and tradeshows I covered with a battery strapped to the leg of my tripod is beyond count. For about $164 for the system + $54.99$179.99 for the battery, it’s a no-brainer.


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Udi Tirosh

Udi Tirosh

Udi Tirosh is an entrepreneur, photography inventor, journalist, educator, and writer based in Israel. With over 25 years of experience in the photo-video industry, Udi has built and sold several photography-related brands. Udi has a double degree in mass media communications and computer science.

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One response to “One ONsite Relay C can replace 8 batteries”

  1. Daris Fox Avatar
    Daris Fox

    Weirdly, I ended using a UPS(!) recently attached to a Canon power adaptor not very elegant, but the UPS could the camera for years. The other advantage of the power adaptor was that I could use any power source such as AC, battery pack, generator without worrying about compatibility and other gubbins. I’ve shot time lapse using a Bluetti power back connected to the camera and it powered other devices for me to work whilst the camera carried on it’s business. The other advantage less cables and there less kit to forget and more importantly less points of failure.