Efficient Powerpath Balancing for Fuel Cell Battery Hybrid Powertrains

For those following my work, thank you and sorry for the long wait for an update!  I am currently writing up my doctorate so as you can imagine I have been a social hermit recently!

Nevertheless, I have now had a journal paper published which may be of interest to those fellow electronics geeks out there and as a bonus it is open access so no charge to view it!

http://www.sciencedirect.com/science/article/pii/S0360319915308065

The paper focuses around balancing a hydrogen fuel cell with a battery with ideal diodes.  These give a tremendous efficiency advantage over traditional diodes.  The powertrain exploits the ability that a fuel cell has to operate at different voltages in order to create a potential long endurance system which still has all the power of a battery at an instant when needed.

Once again, thank you for your interest and I will do my best to answer any questions you have!  The paper is a creative commons license so feel free to use it and build upon it, giving credit for the original work where due!

Views: 1483

Comment by eduardo on February 8, 2016 at 8:16am

great article.

Thanks Simon.

Comment by JB on February 8, 2016 at 10:40am

Very nice! Awesome stuff!

Thanks for sharing.

Comment by Simon Howroyd on February 8, 2016 at 11:51am

Thanks gents.  If you're interested what it looks like here is a photo of the working prototype.  It is the same size as the raspberry pi and stacked on top of it.  Original concept was to fly it with a fuel cell inside a Skywalker X8...may still happen if this storm dies down!

Comment by Simon Howroyd on February 8, 2016 at 11:53am

oh, that one is spec'd for 1.8kW ish (i.e. designed for a max of a 4 cell with a 100A ESC).  It is very scalable however, just add more MOSFETs to handle larger loads..

Comment by Quadzimodo on February 8, 2016 at 11:11pm
Outstanding! Thanks for posting.
Comment by John Rambo on February 9, 2016 at 1:30am

I've been working on something similar, but for powering a machine with different capacity/voltage LiPos (i.e. 3S and 4S in parallel or 3S 4Ah+3S2.2Ah).

Industry has already came with a solution for your problem (see LTC4370 for example).

I, however, had to come with a more intelligent prototype - it enables me to manually balance/switch the current drawn by i.e. adjusting trim knob on transmitter, or by reacting to the throttle curve (hover from battery-A and maneuver or failsafe from battery-B automatically).


Developer
Comment by John Arne Birkeland on February 9, 2016 at 2:09am

Great stuff. I skimmed your article, and the peak charge limitation that is used as the reasoning for not doing the more traditional serial system, seems a bit dated. You can easily get LiPo batteries today that will accept a 5C charge, and some even claim to handle 10C charging.

Comment by Simon Howroyd on February 9, 2016 at 3:34am

Thanks for your comments gents.

Rambo, that looks very interesting congrats.  I am actually using the LTC4416, unfortunately the 4370 is designed for load sharing equality which my system isn't. The fuel cell is not electrically similar to a LiPo so wouldn't be able to share 50:50 like two batteries would, making the 4370 somewhat over-engineered for this purpose (although it would of course still work).  This project started a couple of months after the 4416 was released. (yes academia is that slow!)  The follow on project is to look at controlling the chemistry within the fuel cell to anticipate a load change (eg feed forward throttle for an upcoming manoeuvre) and therefore be able to sustain itself in order to reduce the battery capacity (therefore size/weight) with no drop in performance.  Basically an energy optimisation study.

Birkeland, yes I am well aware that 5C is achievable with LiPos.  Interestingly though if you look at battery electric cars they tend to be around the 0.3C mark for safety and battery life.  The peak charge limitation is to do with the fuel cell voltages.  As the voltage climbs, its current output reduces.  So in a series system the overall load sharing ratio between fuel cell and battery would be very low in the normal operating window because the fuel cell output cannot go above the charged battery voltage.  In a parallel/combined system you can have a fuel cell with a higher nominal voltage than the battery allowing for recharge and/or a better load sharing ratio under normal operating conditions.

Hope this makes sense, if not ask again and I will try and reclarify or revisit anything!  I am grateful for the interest.

I shall also point my successor (when appointed) in the direction of this so he/she can keep the community posted.

Comment by Christian on February 10, 2016 at 3:06am
Thanks for sharing!
These designs may be also useful for automatic power switching and not only for balancing?
Arguably the most common cause of drone crashes is a power fail due to empty/bad/damaged batteries !?
Therefore, i recently thought about redundant power supplies in drones.
In the Pixhawk Flightcontroller a LTC4417 PowerPath controller is used to switch between Power Module, Servo Rail and USB supply.
Buth this redundancy in the flightcontroller doesn't help at all if the drone is powered by a single battery - and this battery dies...
So we need at least a circuit that can automatically switch between the primary and a secondary battery, if the primary battery voltage drops.
But passive diodes are not a good idea for high currents - too much power loss...
So i searched for better solutions and found some promising ICs from LT:
LTC4417: Triple Prioritized PowerPath Controller
LTC1473: Dual PowerPath Switch Driver
LTC4416: Dual PowerPath Controller for Large PFETs
 
@Simon Howroyd:
The LTC4416 would also be my first choice!
Did you encounter any unexpected behaviour or problems with the LTC4416?
Which MOSFETs du you use in your design? Because i haven't found many high-current PFETs with low RDSon...
Did you add any additional features to your circuit? 
 
@John Rambo:
What is the approach in your design? Can you share some details? Which MOSFETs do you use?
I assume, you use a LTC1155 and an additional ATtiny microcontroller?
 
@all:
Any other ideas or projects?
I wonder why redundant power supply solutions are such a quite unregarded topic?
Comment by Simon Howroyd on February 10, 2016 at 3:16am

Hi Christian,

Very interesting idea, I think it probably hasn't been done much because of the limited payload.  However, if rather than 2x 4Ah batteries linked in parallel to double your endurance you linked them through a LTC4416 in parallel you would get the same increase i endurance but the security that if one pack fails then it won't knock out the whole power supply.  Bear in mind that with the LTC4416 both batteries would be used simultaneously in normal operation.

The LTC4416 has been the most robust part of my whole setup if I am honest.  It has worked exactly as expected from day 1.  I am using Infineon Power MOSFETs which are good up to 80-100A ish in my setup with passive only cooling, no dedicated heatsink except the PCB (specially designed).

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