Profoto Acute B Lithium Battery Upgrade LiFePO4

I have a 1st generation Profoto Acute B, 600ws pack/generator. The original battery is a Lead Acid I’m documenting my second and current attempt to find a suitable LiFePO4 (Li) 3rd party replacement.

update…. I built a battery… check out the new post…

My beloved AcuteB so this post isn’t all words

 I have a 1st generation Profoto Acute B, 600ws pack/generator. The original battery is a Lead Acid (Pb) 4.5Ah 12V in the form factor of a home alarm backup battery. Several times I have re-celled the batteries and attempted to use 3rd party cells with mixed results detailed below in addition to a current attempt to find a suitable LiFePO4 (Li) 3rd party replacement. 

Profoto sells replacement PB cells for about $65 and a replacement LiFePO4 battery for $655. You can find 3rd party Pb cells for as little as $20 and LiFePO4 for $60 but don’t expect them to work so easily.

My first attempt at using an off the shelf Pb 4.5Ah (PowerSonic PS-1250) in 2010 was lousy, where the original would give about 160 full power flashes, it gave about 45 and never rose the battery level out of the red zone.  

A few months later I purchased a Tenergy 4.5Ah LiFePO4 which is no longer on the market and was getting over 200 full power flashes per charge. I should have bought four of them.

Recently as the performance of the Pb and Li cells have sharply dropped, I began shopping for another drop in Li and it’s turned into an engineering project. After a two month wait for the only drop in I could find out there, a CTC LiFePO4 I was disappointed that the cell would shut down when the generator was powered up. I’ll go into the details a bit more below and document my attempts to avoid dropping $655 which just seems like a steep hike… until you start doing research.

The Profoto AcuteB is rated at 600J output per flash and charges the capacitor bank in under 2 seconds using a stock PB 12V 4.5AH 

with a 40A automotive fuse on the battery case. Speaking with a tech on the phone at Silvino’s in LA I found the approximate surge pull on the battery was 30A. He was also searching for a feasible LiFePO4 solution.

Considering the lack of a drop in replacement I began looking at configured and tested packs that might fit into a 3d printed battery case vs building a pack using off the shelf cells and protection circuits.

So far, I’m unable to find a preconfigured pack that will supply over 22A peak discharge let alone the 30-40A I need. In conversation with a battery tech, we surmised that it would be possible to wire a large capacitor in parallel to the battery pack to buffer the peak draw, keeping it from shutting down during the initial surge and also lengthening the life of the batteries as a bonus.

Breaking out a calculator and the internet, I did a little math to see how this would work.

The AcuteB has a rating of 600WS or 600J
WS = Watt Seconds
J = Joules
S = Second
A = Amps

I’d like to cut the peak draw from 30A down to 20A, if a capacitor could hold 14.5V 10A it would appear that I might be able to make this work. A few more google searches and scribbles and I calculated I need a 15V 1F cap or (5)2.5V 5F caps in series parallel with the battery to do the job. Here’s where I need someone to chime in and tell me it won’t fry.

Q=Coulomb=1A/1S 1Amp supplied for one Sec
C=Capacitance (Farad F) 1F cap can store 1Q
C=Q/V for Parallel
1/Ceq=1/C1+1/C2… series

Alternative Solution: I spoke with a tech today at bioenno power about a 4.5Ah pack with 18A peak cutoff and a 6Ah with 24A peak cutoff . The tech said that they could be programmed to cut out at a higher current though it would decrease the battery life/charge cycles though pose no danger of overheating.

AcuteB battery $250, replacement cell $65, carrier $190? for what?