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Hacking PRO jumper block to feed 14V to the ALEXA

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#1 Frederic Chamberland

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Posted 22 October 2013 - 10:59 PM

Hi all,


Here is a little modification I made to my sled to better power the ALEXA. It works like a charm so I thought I'd share:


Been flying the same setup for the past 3 years; ALEXA with Cooke lenses (5i's), cinetape, teradek, 2 motor preston with MDR2 on my HD PRO GPI sled.

To keep the post short, I fly 1 Dionic HD in the front (below the post) and 2 Dionic HC at the rear : I have been using the 2 rear batteries in 24V mode to power the ALEXA until last week.


The 2 HC's at the rear would drain very fast feeding the ALEXA ; I needed at least 8 of them to get thru morning. Since the sled is very well balanced the way it is right now, I considered using the front battery in 14V mode to feed the camera. The big Dionic HD battery in front is from the rental house and we already use them in studio and handheld mode.

Since the 3 jumper blocks from PRO are not designed to feed the camera from the front plate, I called David Hable and he re-wired one of my blocks to do just that : front is now camera only, and the 2 rear plates are ACC and MONITOR only (see pic).

At the same time , he also re-wired one of my Arri power cable to feed the camera in 14V instead of 24V. Only thing to remember if going with this mod is to go into the ALEXA menu and change the "low battery" warning on the BAT1 position to somewhere around 11v instead of 22V: if not, the camera will "think" the voltage is way too low to power the camera.

The mod is simple and cheap(re-wiring a jumper block and a power cable) and the difference is huge in power management: not only am I saving my batteries (using the rental house instead), the camera will last almost 70 minutes on one single fully charged Dionic HD .

On paper it seemed like a win-win modification and I can now confirm it works great with this setup.


Would love to see if people here have made such modifications and what they did.


Fly safe,





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#2 Richard James Lewis

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Posted 23 October 2013 - 04:18 AM

I've had an Aux to Arri cable made. I can put a single 12v battery on the Aux line on the battery directly under the post and feed the Alexa. Just change the menu so it knows it's getting 12v from the cable.
I also have a 24v Hawkwoods plate that I can put on the Aux line under the post to feed the camera 24v if the rental house has them.
The Gen 3 base has been very useful for me with its swappable plates..
I did consider having a jumper block modified but I didn't find it necessary in the end.
Good thinking tho.
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#3 William Demeritt

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Posted 23 October 2013 - 09:06 AM

I'm glad to hear this mod of working out for you. Out of curiosity, why not just save one step and just modify your Arri power cable to 12v so it powers off the camera battery on the 3 position power block?

That way, the camera battery only powers your camera, monitor for monitor, etc. Of course, I'm also having trouble understanding how 2 batteries in 24v (offering 178wH) didn't last as long as a single battery (offering 89wH).

I feel like I'm missing a step in your thought process for why this mod.
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#4 Jens Piotrowski SOC

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Posted 23 October 2013 - 10:17 AM

Dionic HD has 183 Wh

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#5 Jens Piotrowski SOC

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Posted 23 October 2013 - 10:18 AM

Why not using all 3 batts in 12v, share the load...?
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#6 William Demeritt

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Posted 23 October 2013 - 10:32 AM

Sorry, didn't catch that you're mixing HC and HD Dionics. However, Jens also presents a good idea.
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#7 Frederic Chamberland

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Posted 23 October 2013 - 12:40 PM

Hi Richard ,
I did not think long enough to consider the Aux power connector on the sled ... And i already had extra arri power cables , i went the lazy way I guess .
And answering Jens question : i much prefer to use batteries in a separate way so they don't die all at the same time . It's a very personnal way to see it and I would understand someone who would prefer sharing the load between all of them.
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#8 Mike Germond SOC

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Posted 23 October 2013 - 12:51 PM

I have a 3-battery 12v Parallel jumper block from David Hable which is great for hot-swapping and you get 3 batteries running every port on the sled. But with the Alexa I get better results using the traditional 3-battery 24v block from PRO. I can only speak towards PRO sleds and wiring sizes, but the batteries seem to last longer for me in 24v. I use Dionic 90's.

For hot swap, I build the camera with my SO-XLR plate on the back and plug the VCLX into that for shore power while I shut my sled off or change batteries. That's my "hot swap" method for 24V mode.

One thing you have to consider on ALL sleds is that providing increased Voltage reduces the Amperage load on the batteries (100% necessary if using Dionic 90's as they are rated at 6A MAXIMUM).

We know the minimum draw of the Alexa with EVF is about 85W. Knowing the voltage options available to us, you can use the equation
Watts / Volts = Amps

So 85W / 12V = 7.08A (too much for Dionic 90's)

And 85W / 24V = 3.54A

So then you could calculate what that difference in draw does to your batteries. For the sake of argument, just assume a Dionic 90 has the advertised Typical 93wH capacity (even though we know real world values differ due to lots of variables).

Watt Hours / ( Voltage * Current in Ampere ) = Battery Life

So if you were running 3 Dionic 90's in 12V Parallel, the equation would be:
279wH / ( 12V * 7.08A ) = 3.28 hours
(But remember now you're running more than just the camera, you're running everything on the sled. The Amperage will be higher than this)

If you were running the camera with 2 Dionic 90's making 24V, the equation would be:
186wH / ( 24V * 3.54A ) = 2.19 hours
(Again, Amperage will be slightly higher because the first battery in the series is still providing 12v power to the sled connectors EXCEPT the Monitor)

But again, the Dionic 90's are only rated for 6amps. So beyond the issue that you're probably damaging your battery when running the Alexa in 12v mode, I've found the camera rips through 3 batteries in 12V a lot faster than the equation suggests. Especially since you're probably pulling closer to 10amps on them with wireless FF, wireless HD, sled monitor, CineTape, and whatever else you've got.

Your results may vary with Dionic HC's (83wH, 10A max) or Dionic HCX's (118wH, 10A max). Both can run the Alexa in 12V safely. Just plug the wH into the equation to find the run time.

Honestly, there are so many variables between Aks and Sled wiring sizes that this makes my head hurt. For me, 24v is preference. For small rigs like the Zephyr, 24v is probably mandatory to run the Alexa.

You're going to chew through batteries all day. Just don't exceed the Amperage limit on your batteries! Or if you must, make sure it's the rental house batteries ;)

Edited by Mike Germond, 23 October 2013 - 01:00 PM.

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#9 Jarrett P. Morgan

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Posted 24 October 2013 - 03:50 PM

Here is my take on the math: the amperage is increased in 12v parallel mode but you are running all three batteries so the load is spread evenly. Thus the amperage draw may be less per-battery than 2 in 24v and one powering monitor:



12v Parallel:
hypothetical 10 amps total (7 for camera, 3 for AKS/Monitor) = 3.3 amps on each battery



hypothetical 6.5 amps total (3.5 for camera, 3 for AKS/Monitor) =

Cam = 3.5 amps

Aux = 5.5 amps (3.5 for camera, 2 amps for AKS)

Monitor =  1 amp

(*caveat* this is given my understanding on how series circuits work, please correct me if I have gotten something wrong.)

My bet would be that the camera rips through 12v faster than 24v because it is designed to work with 24 and therefore has to do more work (for lack of a better phrase) in order to work in 12v mode. Case in point (of the camera having to work harder): the 24v RS outputs are unconverted from 12 to 24v when the input voltage is below 24. When 24v (or higher) is inputed, the outputs are merely a passthrough.

Just my 2 cents.

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#10 thomas-english


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Posted 27 October 2013 - 06:18 AM

With regards to the Alexa being 24v. I have been told by a few people that the main body is actually 12v and it is only upconverting the RS 3pin 24v  out for accessories. This was the argument I heard for Panavision going 12v AB on UK cameras. It should be noted that Arri Media allso use 12v AB batteries on their cameras. 


Basically I am not sure the Alexa is a 24v camera. certainly its very compatible with 24v to fit in with the rest of the Arri line going back decades. 


Can anyone elaborate or dispute this hearsay with actual facts? Since I am only working on rumour.

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#11 Brian Freesh

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Posted 27 October 2013 - 10:07 AM

From this page: http://www.arri.com/...earn/alexa_faq/


"Please note that using a 24 V power source is more efficient than using a 14.4 V source, as ALEXA uses 24 V internally."

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#12 Lawrence Karman

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Posted 27 October 2013 - 04:43 PM

Frederic, it sounds o me like your Dionic HC batteries may be nearing the end of their life cycle. I run the Alexa in 24v mode off the rear 2 Dionic HC batteries and get about 45 minutes including all the same extra accessories you show on your rig. And my batteries are almost 2 years old.

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