In-line capacitors?

[willsonman]

I've been investigating this for a little while now and would like some general feedback. David... you there?

I'll start off by saying, like Josh B, I'm cheap. I buy the cheap ESCs and batteries like most all of us here do. I still do my 80% margin of error on my power setups but am still not satisfied with the longevity of my batteries and how hot my ESCs get. I have a little bit of electronics background but only as a hobbyist. I know that the ESCs we all buy do not always come with the best of components. Particularly my concern is with the battery to ESC relationship.

Our ESCs typically do not use ultra-high-quality capacitors. I'll start with a good analogy I will quote from an RCGroups thread:

"Water running in a pipe and through a tap. Now turn off the tap quickly. You'll hear a loud knock/shock sound in the pipe. The water wants to continu flowing but it can't, for a moment the water pressure is much higher than the static water pressure. It's the same for a current that's switched off, because of the inductance it wants to keep on flowing, voltage gets higher. This is also what causes sparks (brush fire) in a brushed motor. The controller is like a watertap that's switched off (and on) very fast (8,16, 32kHz PWM) to get the desired current. Turning off the current, in combination with the battery wire inductance, causes voltage spikes because the current wants to continue on its course (ref. inertia of the moving watercolumn). Those voltages are higher than the battery voltage. The input capacitors (cylindrical) takes care of these spikes (they reduce the wire inductance). The longer the wires, the higher the voltage spikes induced in the wires, the harder on the input capacitors. They will get warmer, heat up and explode. This is caused by the wire inductance, not by wire resistance. Therefore, using thicker wire will not help much, it's not a bad idea either, but extra capacitors are the solution, thus reducing/compensating the wire inductance."

This quote is in relation to the effects of long battery or motor wires in an electric setup. That is not my concern. My concern is the effect of the spiking on the battery over time. With capacitors on the ESC that are not low ESR (equivalent series resistance) the spiking is potentially what is killing my batteries quicker than what I feel is ideal. Some batteries as quickly as 50 cycles.

Essentially my thought is that by adding capacitors will add in-line "water reservoirs" That will drain and fill during the spikes. More reservoirs or more capacity in the reservoirs will yield better results. The parts are cheap and weigh next to nothing. Even Castle Creations has a CapPack that is geared toward the long-wire applications.

For the more expert people out there... am I on to something, or just on something?

 

[rcspaceflight]

I remember reading a science article on how capacitors would be soooo much better for electric cars than batteries. You could punch the throttle and have instant power/response. Plus capacitors last longer than batteries. (They just don't hold a charge as long from leakage.)

I also know that back in the late 90s / early 00s a lot of people used massive capacitors for their subwoofers. (I think they still do.) You got a few more decibels out of your system that way.

Capacitors can certainly even out the strain on an electrical system. That's what they're for. I'm assuming you would mount your capacitors directly to the battery because it's the only thing you're trying to save. Everything else in the system would take care of it's self, it just draws extra amps once in a while. I say give it a try. It wouldn't hurt as long as you do it right. But I'm not a electronics expert.

 

[bitogre]

I do not know much about Lipos but most batteries can handle these spikes. In fact, most batteries are constructed like capacitors (two metal plates with a dielectric in between). The big difference between batteries and capacitors is that a capacitor stores the energy as static charge where as a battery has a chemical reaction to store the energy chemically.

The items that you should be more concerned about such voltage spikes are the semiconductors components in things like the ESC, receiver, and servos. These components can be damage by static electricity smaller than you can feel (1000+V at 1 uA). The ESC should have circuitry to protect it from these spikes coming from the motor but the other components assuming that the ESC or BEC are providing the needed protection. In most cases, the ESC and BEC does provide the protection but it may not be enough on high current motors with cheap ESCs (especially if you are getting close to the ESCs max current capacity). So, if you are really concerned, put a capacitor on the power to the radio coming from the ESC.

Personally, I would not worry too much about it so long as you are keeping to the 80% margin of error on your power setups.

Chances are the harm to your battery is something else (possibly not using a high enough C rating or draining your battery too much or charging your battery too fast or not properly storing your battery between uses or ...). From my understanding, there are a lot of things that can impact how long your Lipos will last but I would be surprised if adding a capacitor to your battery will help. In fact, I can see it causing harm if the battery is always connected to the capacitor as the leakage current of the capacitor can increase the drain of the battery during storage (but only if the battery is being stored for over a month).

Now, again, while I know electronics, I am new to RC and do not know that much about Lipos so I could be wrong.

 

[willsonman]

I was into car stereo for a long long time. The Caps used there were more for system strain. The hard hits on the bass could potentially fry your alternator and kill your battery. Those are in the 0.25-2.0 Farad capacity. It alleviated dimming headlights and dashboard too.

The caps would be something you plug in between the battery and ESC. It would not be connected all the time. I have a battery that has around 150-ish cycles that is on its way out. Its not as punchy as a new battery due to higher resistance. I'll give it a shot in conjunction with something I whipped up and report back... if the weather ever cooperates.