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Cap pack?

Sethman919

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Don't take me wrong I am looking into them also and it's question of understanding how them Jewls work. I like the idea of adding them but what I am trying to understand is the load to the amount of caps or uf. I believe like joostin42 in doing everything to ensure the best performance
I'm trying to understand this as well. I'm an electronics engineer in the DC to DC converter realm for 15 years. This is why I want to do bench testing. I suspect that ESR is more important than the amount of uF. Meaning the caps ability to deliver high current for short durations is more important than the ability to deliver lower currents for longer durations. The only way to know for sure is to hook up my scope in the lab and see what is really happening. Duration and amplitude or current spikes and resulting dips in voltage.
 

Drtybird

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I have read on one site that caps take away radio frequency but what I know about about caps on a amp it takes voltage spikes and balances it. Wouldn't there be a particular uf to either volt or amp draw
 

Sethman919

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No. uF is simply the capacity of the cap. Like the battery. More uF means it can deliver current for longer. If the spikes are short before th current draw settles down to its nominal running current then a lot of uF isnt needed. The battery will deliver the running current.
 

Sethman919

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Let me get back to my office and I'll type out a better explanation
 

Sethman919

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ok,... There are two main factors contributing to the voltage drops at the ESC. the Resistance and the Inductance. The resistance is a combination of the resistance of the wire and the ESR of the lipo pack itself. The inductance which is mostly the wires though there is some inductance in the pack and the cells themselves.

The resistance.

Resistance drops the voltage relative to the current through it. So (V) volatage drop = CUrrent (I) x Resistance(R). For example say you have 50A current draw and overall resistance of 10mOhms. this means you will see a voltage drop equal to 0.01(R) x 50(I) of 500mV or 0.5V! at the ESC. during the startup of the motor they have a much higher and short duration startup current. for example, say you have a 300 amp start up current. so you will see a voltage drop of 0.01(R) x 300(I) of 3V at the ESC. now these numbers are all hypothetical at this point.

The inductance.

Ill leave out the math here. Think of your wires as inductors. Inductors store energy in the form of magnetic fields. while the field is charging the voltage at the other end drops. your battery can only deliver so much current and some of that is being sucked up by the inductance. once the field charges fully this effect goes away and only the resistance drop is present.

To counteract these effects you can do a number of things. the best is to shorten the wires and make them bigger. this lowers their resistance and their inductance. they have less resistance and cant store as much magnetic energy. The other thing to do is to provide a current supply right at the load (the ESC). This is where caps come in. they supply low esr/inductance current during the inrush and while the inductance is charging. if your caps are connected via big wires then they are much less helpful as you are adding back in inductance and resistance to those current supplies. they still help because they are in parallel with the battery and helping to supply some of that current. with the case of electrolytic caps their ESR plays a much bigger role as well as they tend to be in the 100mOhm range compared to the 6-10mOhms of your Lipo pack. Since these spikes tend to be short you don't need a ton of capacitance to absorb them. its better to have much lower esr and Inductance. this is why smaller polymer caps placed right at the ESC would be much more usefull and compact. Polymer caps are in the 15-30mOhms range.
 

jamesbcn

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ok,... There are two main factors contributing to the voltage drops at the ESC. the Resistance and the Inductance. The resistance is a combination of the resistance of the wire and the ESR of the lipo pack itself. The inductance which is mostly the wires though there is some inductance in the pack and the cells themselves.

The resistance.

Resistance drops the voltage relative to the current through it. So (V) volatage drop = CUrrent (I) x Resistance(R). For example say you have 50A current draw and overall resistance of 10mOhms. this means you will see a voltage drop equal to 0.01(R) x 50(I) of 500mV or 0.5V! at the ESC. during the startup of the motor they have a much higher and short duration startup current. for example, say you have a 300 amp start up current. so you will see a voltage drop of 0.01(R) x 300(I) of 3V at the ESC. now these numbers are all hypothetical at this point.

The inductance.

Ill leave out the math here. Think of your wires as inductors. Inductors store energy in the form of magnetic fields. while the field is charging the voltage at the other end drops. your battery can only deliver so much current and some of that is being sucked up by the inductance. once the field charges fully this effect goes away and only the resistance drop is present.

To counteract these effects you can do a number of things. the best is to shorten the wires and make them bigger. this lowers their resistance and their inductance. they have less resistance and cant store as much magnetic energy. The other thing to do is to provide a current supply right at the load (the ESC). This is where caps come in. they supply low esr/inductance current during the inrush and while the inductance is charging. if your caps are connected via big wires then they are much less helpful as you are adding back in inductance and resistance to those current supplies. they still help because they are in parallel with the battery and helping to supply some of that current. with the case of electrolytic caps their ESR plays a much bigger role as well as they tend to be in the 100mOhm range compared to the 6-10mOhms of your Lipo pack. Since these spikes tend to be short you don't need a ton of capacitance to absorb them. its better to have much lower esr and Inductance. this is why smaller polymer caps placed right at the ESC would be much more usefull and compact. Polymer caps are in the 15-30mOhms range.
Sethman, did you manage to test those peaks and find which kind capacitor suits better? this way we can buy bulk of first class caps and make our own cap-packs. Thx!
 

Sethman919

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I haven't had a chance. Had surgery the day after Christmas so I've been down. I may borrow a portable scope from work and do it at home.
 

Drtybird

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I haven't had a chance. Had surgery the day after Christmas so I've been down. I may borrow a portable scope from work and do it at home.
What are variables of loads? Does kv and voltage very when motor is not running at based rpm?
 

joostin420

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Searched for 5x months for the 40mm nitro exhaust boat mounts to use as motor mount/braces like I used in my Kraton with the tp 4070cm 2200kv. Finally found a seller in England and bought the last 3x he had real fast ;)
AFB0AF84-BC8F-4E27-889B-64BFB2878796.jpeg
EB0B5B84-1B1D-4A8A-8601-5653437F9BC8.jpeg
 

Drtybird

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I have found some 45mm mounts, I wonder if some 40mm heatsink find could be modded to make spacer for clamp
 

joostin420

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I have a 45mm brace as well that is going to be used for something else since it was much too large. It may be too wide and hit the rear center driveshaft.
 

Drtybird

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I have read a post about someone looking at the TP motor but said something about the cans breaking or coming apart. Such a long motor the mount seems to be needed
 

joostin420

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I wouldn’t run any motor this long without the braces.
 

Bogis9

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ok,... There are two main factors contributing to the voltage drops at the ESC. the Resistance and the Inductance. The resistance is a combination of the resistance of the wire and the ESR of the lipo pack itself. The inductance which is mostly the wires though there is some inductance in the pack and the cells themselves.

The resistance.

Resistance drops the voltage relative to the current through it. So (V) volatage drop = CUrrent (I) x Resistance(R). For example say you have 50A current draw and overall resistance of 10mOhms. this means you will see a voltage drop equal to 0.01(R) x 50(I) of 500mV or 0.5V! at the ESC. during the startup of the motor they have a much higher and short duration startup current. for example, say you have a 300 amp start up current. so you will see a voltage drop of 0.01(R) x 300(I) of 3V at the ESC. now these numbers are all hypothetical at this point.

The inductance.

Ill leave out the math here. Think of your wires as inductors. Inductors store energy in the form of magnetic fields. while the field is charging the voltage at the other end drops. your battery can only deliver so much current and some of that is being sucked up by the inductance. once the field charges fully this effect goes away and only the resistance drop is present.

To counteract these effects you can do a number of things. the best is to shorten the wires and make them bigger. this lowers their resistance and their inductance. they have less resistance and cant store as much magnetic energy. The other thing to do is to provide a current supply right at the load (the ESC). This is where caps come in. they supply low esr/inductance current during the inrush and while the inductance is charging. if your caps are connected via big wires then they are much less helpful as you are adding back in inductance and resistance to those current supplies. they still help because they are in parallel with the battery and helping to supply some of that current. with the case of electrolytic caps their ESR plays a much bigger role as well as they tend to be in the 100mOhm range compared to the 6-10mOhms of your Lipo pack. Since these spikes tend to be short you don't need a ton of capacitance to absorb them. its better to have much lower esr and Inductance. this is why smaller polymer caps placed right at the ESC would be much more usefull and compact. Polymer caps are in the 15-30mOhms range.
What gauge wire can we get away with using when making cap pack?
 

2015GTRBE

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I wouldn’t run any motor this long without the braces.
Jesus bro.... max 5??!!!!! You are nuts. I just put one in my new X Maxx and I figured it was overkill with stock motor and sufficient when I go leopard
 

joostin420

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Dutch213

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Searched for 5x months for the 40mm nitro exhaust boat mounts to use as motor mount/braces like I used in my Kraton with the tp 4070cm 2200kv. Finally found a seller in England and bought the last 3x he had real fast ;)
View attachment 27110
View attachment 27111
oh man... totally genius! You are a true RC'er.... Nicely done with those spit rings!!!!
 

Dauransx

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I see what Cap packs are kinda for but why would i need one? Will it save my lipo from stress? my 1480kv caused my 6s SMC to puff a lil, least people here said it was because of the motor so would a cap pack help? Im looking at adding one for peace of mind, if that is one way of using cap packs. I also want to add a TP motor, maybe the 4070 1520kv one day and maybe 8s but not likely but you never know.
 

sadlerbw

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There are two places that we commonly see extra caps added to RC cars. First, you can put some caps on the power wires running from the battery to the ESC. Often there are some of these built into the ESC, but sometimes the manufacturer wants you to install them externally, or folks might want to add more. These help when the car draws a big spike of power and the battery can't deliver it fast enough. Adding caps puts a bit less stress on the battery, but how much it helps is open for debate. Still, this can technically help pretty much any RC car since it adds a small bucket of power that can discharge really fast when the motor turns on and draws a big spike of current.

Second, folks add cap packs to the output of the BEC (generally plugged into an empty port on the receiver) which is used for powering the receiver and other accessories. These mostly just help with voltage sag when the battery voltage gets too close to the BEC output voltage. Say you are on an almost dead 2s pack that is putting out 6.8v, and the BEC voltage is 6v. When you slam the throttle, the voltage out of the battery may drop low enough that the BEC can't manage to keep putting out 6v during that initial spike of power draw. this is where a cap pack helps, as they extend the time that everything stays over 6v without needing the BEC to keep up. Cap packs probably don't do a whole lot for voltage ripple. If you want to use capacitors to smooth out ripple, you have to choose the value of the caps according to the frequency of the ripple you want to stop. You can't just throw any random cap in there. So, cap packs help when battery voltage and BEC voltage get too close to each other. The bigger the difference between your battery voltage and BEC, the less a cap pack should matter.

Technically, cap packs shouldn't be doing a whole lot when you are running 3S or more because the battery voltage shouldn't be dropping down to the point where it affects the BEC voltage. However, they do seem to help some cars, so something else must be going on. My best guess is that it can also help if there are spikes in current draw from a servo or something that the BEC can't handle.

So basically caps are added to help handle big spikes in current draw. Either they are added before the ESC to be kind to the battery, or added after the BEC because it isn't coping with the power draw and browning out the receiver. There may be other, trickier reasons, but I don't know them!
 
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