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High current drain on supercapacitors in series

Old 08-26-2019, 05:47 AM
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trijoco
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Default High current drain on supercapacitors in series

Not sure if this is the right place to post this, but I'm an undergrad engineer looking for some advice. We have a project to build an RC car from scratch and are only allowed to use NiMH batteries, no LiPo. We're hoping to power the car with an EDF (ht#tps://hobbyking.com/en_us/durafly-vampire-v2-rcaf-edf-and-motor.html or similar) which runs at 4s or ~14.8V, 60A. Instead of using two heavy (1kg) 8.4V batteries in series, we would like to try and power the EDF with 6 x 2.7V 60F supercapacitors, charged before the race with the aforementioned heavy battery packs.

So my questions are: Do capacitors in series need some sort of balance charging like LiPo's do? Do they all discharge at the same time or one at a time, i.e. will we be constrained by the rated 20A discharge rate or the low voltage cutoffs of the ESC?

Any advice is much appreciated, thanks!

Last edited by trijoco; 08-26-2019 at 05:50 AM.
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Old 08-28-2019, 12:59 PM
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ticedoff8
 
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Originally Posted by trijoco View Post
Not sure if this is the right place to post this, but I'm an undergrad engineer looking for some advice. We have a project to build an RC car from scratch and are only allowed to use NiMH batteries, no LiPo. We're hoping to power the car with an EDF (ht#tps://hobbyking.com/en_us/durafly-vampire-v2-rcaf-edf-and-motor.html or similar) which runs at 4s or ~14.8V, 60A. Instead of using two heavy (1kg) 8.4V batteries in series, we would like to try and power the EDF with 6 x 2.7V 60F supercapacitors, charged before the race with the aforementioned heavy battery packs.

So my questions are: Do capacitors in series need some sort of balance charging like LiPo's do? Do they all discharge at the same time or one at a time, i.e. will we be constrained by the rated 20A discharge rate or the low voltage cutoffs of the ESC?

Any advice is much appreciated, thanks!
Out of curiosity: Why are you making a jet powered car? I guess it depends on the point of the exercise, but jet engines are inefficient at low airspeeds (like a car). I would think a car would require torque and direct drive of the wheels.

Capacitors do not need to be balanced like a LiPo.
The 4S LiPo is 4 separate 4.2v cells in series. LiPo packs are balanced during charging because if any of the individual cells is charged too quickly, it will overheat and experience a thermal run-away (aka: catch on fire). The reason a cell will charge quicker than its neighboring cells is due to the each cell's Internal Resistance (IR). Cells with a lower IR can be charged / discharged faster than cells with higher IR. This mainly comes into play when the cells age and have been abused (charged too quickly or discharged too quickly for their quality).

You are basically creating a 6S SuperCap pack.
I would check to see what their maximum discharge rate is. They may not be able to deliver the current needed to spin the turbine fast enough for your use case.
I don't know what their maximum discharge rate might be, but if you charge them at that their "C-rating", they should be fine.
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