Supercap rapid charger + balancer

Thread Starter

Just Another Sparky

Joined Dec 8, 2019
Just looking to pick some brains for a project I'd like to do later on down the line.

I've got 6 Maxwell boostcaps rated 3kF, 2.7V and would like to assemble them into a functional jump-start pack for cranking a frozen engine in extreme cold. I'll be fabricating a portable enclosure from 12 or 14 gauge steel and figuring out a non-metallic mounting arrangement for them since the cans are at potential with respect to one another.

I'd like some help designing a rapid charging circuit and a means of balancing them. From my testing, 50 amps is sufficient to charge up the bank to 14~14.5 volts from zero in about two minutes. I'd like to utilize a 120V, 15A branch circuit as a power source as 99% of the places I find myself parking have access to a convenience receptacle in one form or another. Parking ramps, residences, jobsites, gas stations, airports, etc.

Would it be possible to build a buck converter matching these specifications without too much difficulty? I have a bunch of 400A, 1200V IGBTs lying around and a mystery choke which fits comfortably in the palm of my hand with #8AWG leads extending from it. Some 350 volt caps rated about 300-400uF too off the top of my head. Cooling the IGBT would probably be by means of thermal mass - sinking the losses generated by a 2 minute charge cycle into the steel enclosure to be dissipated at a leisurely pace. Subsequent charges will by necessity be several minutes apart and likely from a partially charged state of 8 volts or more.

I would like to avoid the use of microcontrollers (beyond those little BASIC stamps - those are great. Or maybe a Parallax Propeller?) as I've never had any luck with mainstream coding languages outside of pBASIC and the ladder logic used in industrial PLCs. Software and I just don't see eye to eye.

Has anyone got any analog buck control logic they could recommend to begin my experiments with? CC and CV control modes will be necessary since these caps present what is essentially a dead short across the charger terminals. ESR ~290 μΩ.

Finally, are there any suggestions for a per-cell balancing circuit? Something to dissipate excess charge through a small 1-2 watt resistor when the cell exceeds 2.4~2.5 volts.


EDIT: P/N on the inductor reads 'Cramer 0139, 309495-C01'. Can't find any information on it but it's got a laminated core and weighs a few pounds. Maybe not a high frequency inductor but probably good for a few kHz.
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Joined Sep 9, 2010
Hey, I don’t have answers but I’m following along.
Be patient. It can take people with the know-how a while to check in and notice your request. I'm not one of them. All I can recommend is to not reinvent the wheel. I'm certain this problem has been solved and there's probably even an off-the-shelf commercial product you can use.