UltraCapacitor Charging Voltage

[newman180]

I am working with a hybrid car which charges a bank of ultracaps from regen braking.
I read an article which I believe was written by someone who is misinformed, but it had me thinking...

Just to clarify some theory about charging voltage,

Scenario

Max ultracap voltage= 14V
Current ultraCap Voltage= 10V
Motor BEMF Voltage = 20V

It is safe to charge the ultracaps with the 20V source until the caps are at 14V, correct?
Potentially, the greater the difference of voltage (potential) is related to how much current can be transmitted. V=IR

 

[MikeHalloran]

Last time I looked at supercaps, they were limited to ~5V. ... and the Equivalent Series Resistance was substantial.

I guess things have changed some, but do read up on ESR before you get all lathered up about jamming a lot of charge into a supercap in a hurry.


Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

So, 14V:

http://www.tavrima.com/pdf/6_14.pdf

However, that capacitor is power-rating limited to 171A, but your proposed condition would apply about 1000A to the capacitor.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[RyreInc]

I don't understand how the cap holds only 6kJ @ 14V. E = .5*C*V^2 = .5*153*14^2 = 15kJ. What am I missing?

 

[VEBill]

The only voltage that the caps would "be aware of" (so to speak) is their own terminal or internal voltage. During charge, the internal voltage might be ever so slightly lower than the external terminal voltage due to voltage drop across the internal lead wiring (something to consider with very high currents).

If you're going to have a complex controller, why not include a DC-DC converter as part of the architecture?

Hopefully someone has confirmed that there's an environmental pay back point within the life of these systems. It'd be a shame if this complexity, expense and extra mass results in a larger cradle to grave environmental footprint (as is reportedly the case with the most famous hybrid).

 

[newman180]

I should clear some things up. I posted it as a rough example to avoid confusion.

The real setup I am working with 12 2.7 2000F capacitors in series with ESR on each cell of .03mohm.

These caps can withstand 1700 amps plus for 1-2 sec. Its power rating of 170 amps is for continuous operation. In my case braking will be at most 400-500 amps at 1-2 second intervals.


The motor controller applies its pwm frequency by voltage, hence raising the voltage of the spinning motor for regeneration. Specifically its V * 1/PWM.

The question is, will the caps be safe if you charge with a higher voltage source (ex. 42v) than the packs rating(32.4V) as long as you stop charging when the caps hit an internal voltage limit. It seems that VE1BLL has confirmed my assumption. I'm in the process of talking with the manufacturer.

 

[ijl]

I don't understand how the cap holds only 6kJ @ 14V

6kJ might be the energy that is available before the voltage drops too much.

 

[LionelHutz]

I'm not following why increasing the voltage on the motor side would cause this difference in charging voltage. If I understand correctly, you have a DC to PWM controller running the motor. The DC side will be the capacitor terminal voltage and you have to quit braking when the terminal voltage of the cap bank reaches 32.4V. I'm not sure how you expect to stop charging at some internal voltage limit if you can't measure it.

 

[epeeman]

Your ESR is only 0.03 mOhm, and your maximum safe current limit is 170 Amps, so the differential voltage between your supply and that of the capacitor is 170 x 0.00003 = 0.051 Volt. What that means is that you must have a current limit on your charger. In other words, your power source is both current and voltage limited. It acts like a current source for most of the charge but becomes voltage limited at near full charge.

 

[iop995]

Must check if ultracaps (UC) allow some % overvoltage for some amount of time; you may use a buck conveter with limited output voltage and need very fast control schema.

 

[waross]

Motors are not an infinite source. The internal impedance of the motor will limit the current and the internal voltage drop will lower the terminal voltage of the motor. Step back and look at the whole circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter