A battery and capacitor to get best of both

[markheggie]

So I have an idea and was wondering if anyone here could help me understand some of the physics behind trying to make it work before I buy up some materials and experiment. All the google searches I did with various keywords kept hitting here so...

I have a battery, let's say it is 7.4V and 8000 mAh and rated at 20C continuous with a 4 wire kelvin resistance measurement at around 4 mOhm for each cell (2 cell battery). I want to add capacitors to the battery to increase the burst performance of the battery. Do I add capacitors in series or parallel? If in series, does it matter if it is done on the positive terminal or the negative terminal? And I am aware of inrush current and I think this needs to be addressed with either a resistor or thermistor so that the capacitor doesn't try and soak up electrons (or charge, whatever the correct term is here) too fast. The battery will be used off and on, but it needs to be lightweight, so that is why I am trying to marry the positives of the battery and capacitor. Obviously, low esr capacitors will be used or just capacitors rated at maybe 0.1 to 1.0 mOhm would work for the burst current needed.

Any ideas or thoughts out there?

Regards,
Mark

 

[DragonPetter]

The capacitors need to be in parallel.

The main problem with this is that you will drain your batteries faster in the long run.

Also, depending on your application, you might want to consider super capacitors. They hold charge much longer with large capacitances, but they are usually rated for low voltage and so can't really hold much energy, which is why it depends on the application.

 

[markheggie]

Dragon-

So is one reason for the higher drain have to do with coulombic efficiency? I know batteries, especially high drain type, can have very good coulombic effiency in low drain applications.

Do capacitors have low coulombic efficiency? Or is it more voltage breakdown in the capacitor over time. The charge won't be stored in the capacitor very long.

 

[DragonPetter]

I'm sorry, I think I gave you some incorrect information. Super capacitors won't necessarily hold their charge longer than a normal capacitor, but they can hold much more charge (charge density) at the same voltage level. They just can't go very high in volts like a normal capacitor might.

As far as your other question, I don't think its coulombic efficiency, although I'm not sure if that is applicable to capacitors. The reason I say your batteries will lose energy more quickly in the long run is that when you transfer the charge to the capacitor in between uses, then the dielectric insulation in the capacitor and the surrounding air will cause charge leakage that bleeds charge out of the capacitor over time that you could have kept in the battery.

 

[nsaspook]

http://web.mit.edu/first/kart/

http://web.mit.edu/first/kart/circuit.jpg

 

[metiman]

I'm curious about your application. Usually battery = steady state power and capacitors = pulsed power. I'm picturing a whole bunch of those 2 Farad audio caps in parallel. You might want to also consider putting a large inductor in the circuit to better shape the pulse. Sounds like a fun project.