Profoto Acute LiFePO4 custom battery

26650 4.5Ah 14V LiFePO4 yay, I made a battery

I had a previous post detailing my research on an alternative power source to the Profoto branded battery for the Acute B.

After exhausting my search for a battery within spec for the job and in some form of personal protest I made a battery. Not so much made as assembled a battery of batteries with some protective circuits but still, it’s a project!

WARNING: Lithium batteries can explode or catch on fire, they can dump massive amounts of power, will melt things and burn up your devices. Build and use at your discretion, this is a record of what I have done not a suggestion that you do it.

 

26650 4.5Ah 14V LiFePO4 battery build

First off, my design spec… it had to fit in the original battery cassette, handle at least 26A draw and I didn’t want to exceed 14V which pointed to LiFePO4 cells over LiPo which would have given 16… and done who knows what to the electronics. I wanted it to cost less than $650, and preferably not take hundreds of hours to build. It’s a battery, not a car. It took about $100 and probably too many hours but if I want to make another…. maybe two hours not including print time.

In the first photo you can see the cassette, with the original Pb gell cell removed. You can 3d print the cassette for the 26650 cells from this design.

25560 cells can be bought in several chemistries. LiFePo4 gave me comparable voltage to the original PB. I had wanted to use a different cell made by Nitecor originally but thought 3.7V cells combined to make 16V might increase the chance of catastrophic failure. I used Tenergy 26650 cells. You can buy A123 cells, same capacity for double the price. The reviews were favorable on Tenergy, I have used their NiMh cells in previous projects, I’ll tell you next year how the 26650’s hold up.

26650 4.5Ah 14V LiFePO4 battery build

BMS Board- Battery balance and protection PCB. This allows you to charge the battery using a relatively stupid LiPo charger and to not blow things up. It took a while to source a board that could supply up to 30A continuous and fit in the box. This just made it, barely! It can peak discharge over 100A, kind of scary but there’s a fuse on the cassette as well to limit it to 40A. Note, the board didn’t work at first. After removing it, I found a reference online that mentioned hooking it to a charger to reset it. Bit of a pain that I had desoldered it but after successful testing with a batch of alligator clamps I put it back together and went on to live testing. How to wire it or “the secret spec sheet!”

 

 

 

Testing….

Just because I can… I had previously printed a custom jumper set that allowed me to power up the Acute B with the battery outside the generator that was able to handle serious amperage. I was concerned that battery clamps might short out on the bench. It’s overkill but that’s what you do when you have a 3d printer. It has lights to indicate correct polarity and a power meter to catch peak draw. I metered the original Pb cell at 24A, a drop in replacement LiFePo4 I bought five years ago which is no longer available at 25A, then my new homebuilt cell at 31A. Pretty nifty, likely I’ll get a bit faster recycle on the unit. I might consider that as the cassette has a 40A fuse on it and hopefully the pack has it’s own internal temperature monitoring I’ll be safe… but then the heads for this system are not fan cooled so I’ll have to be aware to not overheat the heads/pack. It’s rare that I’m rapid firing at full power for any length of time, and for that matter on huge shoots I usually rent equipment.

battery normally goes inside the rig powerpack, power meter/test rig in use

Note: the . I have it wired externally to test it… I’ll have to upload the files for the rig perhaps…

Thanks to Jamie Allegre for initially putting the test rig on an oscilloscope to get power readings and see charge profile. The initial tests kept me from buying a bunch of possible replacement that would have just shut down on start up. Thanks for Michael Curry for encouraging me to fry my pack… enough to make me second guess the LiPo pack I bought and never test it on the rig.