As a follow on to a previous thread called "Ripple Voltage Explained" I figured I could add on some notes about cap packs - Do you need them and how do the work?
What are Cap Packs?
Cap Packs a capacitors wired in parallel to increase the total capacitance of the "pack". For example 6x 225uF capacitors wired in parallel will have and effective capacitance of 1350 uF.
Yeah, so What?
Caps are like small cache of charge that you can use when the batteries can't source enough current. When that happens the voltage droops (AKA Ripple Voltage) and less voltage mean less power.
A Simple Example
For this discussion, I will keep it simple and use lots of pictures.
I created a very simple circuit to demonstrate what caps do. It only has Voltage source, a resistor (R1) and a cap (C1). The initial values are arbitrary just to illustrate a the concept. The voltage source is nominal 12.6V and ramps from 0V to 12.6V in 40 mliseconds (0.040 S). Yes when you have a battery it's instant-ish, but I used the slower ramp value so we can see the cap charge.
The battery source is in red and the wire between R1 and C1 (green) represents the voltage you would get at the ESC.
The capacitor takes a lot of current (not shown) in the beginning and ramps quickly, then slows down as it charges. Note that it's fully charged after ~650ms. Again this is a simple illustration and your specific situation will be different.
Now if we add a simulated load that drops the voltage to 11V for 25ms v. In this case, for the sake of simplicity I'm altering the input voltage. This isn't really how it works, but its and easy way demonstrate what the caps do. When the red line drops to 11V, the green line follows but doesn't dump completely. This would be seen a .8V "ripple voltage".
So what happens when you pin the throttle?
Here we simulate pinning the throttle for 2 full seconds under ideal conditions. As you can see the voltage to the ESC (Green) decays to 11V in about .500 mS. Then after 1.5 S more the throttle is released, the cap starts to recharge and the voltage to the ESC starts to return to 12.6V.
Wait a 1/2 Second Here!
What do you mean the voltage to the ESC starts to return to 12.6V and why don't I get all my volts back when I release the throttle?
Just like when the capacitor keeps your voltage higher by holding charge, it will take time to recharge and it will keep the effective voltage to the ESC diminished until it's fully charged. Remember it's a small cache of current that effectively dampens fast changes in current demand.
Is More Better?
Can you add more caps to withstand the 2 second of pinning the throttle? Theoretically yes, but more caps mean more re-charge time and more space.
In fact you could run you car off of nothing but capacitors if you had enough of them. But it's not practical and it's probably a bit more exciting than you would want because capacitor can shock the crap out of you. There are some details involved, but it's basically a stun gun at that point. It would also be a one shot wonder because there is no way to recharge the capacitors.
If you really wanted a cap powered car, you could charge up your capacitor bank to a few hundred volts... or maybe a few thousand. Then run a DC-DC converter to step your meg-voltage capacitor bank down to something usable. It will run until the capacitor voltage drops below the required input voltage of your converter. Right now that's tough technology to find and build, but given enough time and money anything is possible.
The Less Over-Simplified Model
Keep in mind that the example above over simplified, so lets make it a little less simple. Unless you have zero load on the motor the current draw will not be constant. It will jump up and down depending on the load. In the next example I will generate some random numbers between 90-100% of 12.6V in 5ms increments, simulating 10% ripple.
OK so now it's getting messy.
Looking at the same "pinned" throttle with the load variation you can see that the effective voltage doesn't stay at 11.34V (90% of 12.6V). In fact the average is around 12V, which effectively reduced the voltage droop (ripple voltage) to about 5% or about 1/2 of the range I gave it.
The capacitor adds charge when the battery is below the capacitor's voltage and gets charged when the battery is above the capacitor's voltage.
If it's Just Average Voltage, Why Do I Need Caps?
All of the numbers are between 11.34 and 12.6 and they average to 11.97V. So why do I need the caps?
The math is correct. What the caps do is create a more constant voltage (smooth the input) to the ESC. Without the caps you would see a much wider range of voltage numbers and that could effect performance. Of course there are cap in the ESC that will smooth (average) it out, but sometimes external caps do help. It really depends on your car, driving style and your electronics.
What about the Real World?
For the example I useed a .01F capacitor; which is very close to the value of 13200uf from the Powerhobby 6 Pack 13200uf RC Cap Pack. 13200uf = 13.mF or .013F. Different values will change the model. However, in the real world you have about a thousand variables and 13,200 F or 10,000 F probably doesn't make a difference until you sort out all the big ticket items like your batteries, ESC, motor, wiring etc.
Does the size or length of the wire matter?
No, it's more about how you use it. The wire and connectors add inductance, resistance (voltage drop) and even a tiny bit of capacitance. It how electrical things are in the real world.
Inductance can impede the capacitors ability to provide charge. However the values of the wire are very small. For a 6" piece of 12 AWG wire is the inductance is 149 nH. That's nano Henri's, as in 149*10E-9. So even it you add in twice the wire and connectors and ... let's call it 5x that value, the inductance means nothing for this application. If you building high speed PCBs it matters, but For for 12.6V and very slow current changes, it's literally in the noise.
However, looking at the resistance of the same 6" piece of 12 AWG wire you have a resistance is 0.8165 mΩ. Which means, .8165 mΩ @ 160A results in a .125V drop. So your 12.6V is now 12.47V. Which is real and can effect your performance if you are at the edge. However, if you shorten the wire to 3", you cut your resistance in 1/2 and your voltage drop too, with only .063V with an effective voltage of 12.53.
Likewise you can change the 12AWG to 8 AWG, keep the longer wire and get a lower resistance - 0.314 mΩ. Which results in only 50 mV drop.
In Summary
Most capacitors have polarity (+/-) and if you hook it up backwards bad stuff happens. Modern electrolytic (aluminum cans) caps typically have built in failure points so you don't get shrapnel flying around your garage, but they do light off and give you a really neat smoke show.... Eh, yeah, don't try that at home. Smaller solid state caps can still fragment. So be aware of your polarity on any type of cap.
If you decide to make your own cap pack make sure you use the correct voltage for your application. With that said you need to derate most cap voltage values. They typically print the maximum voltage on the cap; however, that is not the working voltage. The working voltage is well below that number because of heat, type of cap and other factors. Derating is 20-30% in most cases, but in some instances it can be as high as 50%. So check the datasheet and if you're not sure assume it's 50% of the voltage specified.
Final safety tip. Always make sure you caps are discharged before you handle them. The caps we use are relatively low voltage and moderate capacitance; however, they can give you a bit of spark if you aren't careful. So make sure you discharge them before you start giving them to the kids to play with.
What are Cap Packs?
Cap Packs a capacitors wired in parallel to increase the total capacitance of the "pack". For example 6x 225uF capacitors wired in parallel will have and effective capacitance of 1350 uF.
Yeah, so What?
Caps are like small cache of charge that you can use when the batteries can't source enough current. When that happens the voltage droops (AKA Ripple Voltage) and less voltage mean less power.
A Simple Example
For this discussion, I will keep it simple and use lots of pictures.
I created a very simple circuit to demonstrate what caps do. It only has Voltage source, a resistor (R1) and a cap (C1). The initial values are arbitrary just to illustrate a the concept. The voltage source is nominal 12.6V and ramps from 0V to 12.6V in 40 mliseconds (0.040 S). Yes when you have a battery it's instant-ish, but I used the slower ramp value so we can see the cap charge.
The battery source is in red and the wire between R1 and C1 (green) represents the voltage you would get at the ESC.
The capacitor takes a lot of current (not shown) in the beginning and ramps quickly, then slows down as it charges. Note that it's fully charged after ~650ms. Again this is a simple illustration and your specific situation will be different.
Now if we add a simulated load that drops the voltage to 11V for 25ms v. In this case, for the sake of simplicity I'm altering the input voltage. This isn't really how it works, but its and easy way demonstrate what the caps do. When the red line drops to 11V, the green line follows but doesn't dump completely. This would be seen a .8V "ripple voltage".
So what happens when you pin the throttle?
Here we simulate pinning the throttle for 2 full seconds under ideal conditions. As you can see the voltage to the ESC (Green) decays to 11V in about .500 mS. Then after 1.5 S more the throttle is released, the cap starts to recharge and the voltage to the ESC starts to return to 12.6V.
Wait a 1/2 Second Here!
What do you mean the voltage to the ESC starts to return to 12.6V and why don't I get all my volts back when I release the throttle?
Just like when the capacitor keeps your voltage higher by holding charge, it will take time to recharge and it will keep the effective voltage to the ESC diminished until it's fully charged. Remember it's a small cache of current that effectively dampens fast changes in current demand.
Is More Better?
Can you add more caps to withstand the 2 second of pinning the throttle? Theoretically yes, but more caps mean more re-charge time and more space.
In fact you could run you car off of nothing but capacitors if you had enough of them. But it's not practical and it's probably a bit more exciting than you would want because capacitor can shock the crap out of you. There are some details involved, but it's basically a stun gun at that point. It would also be a one shot wonder because there is no way to recharge the capacitors.
If you really wanted a cap powered car, you could charge up your capacitor bank to a few hundred volts... or maybe a few thousand. Then run a DC-DC converter to step your meg-voltage capacitor bank down to something usable. It will run until the capacitor voltage drops below the required input voltage of your converter. Right now that's tough technology to find and build, but given enough time and money anything is possible.
The Less Over-Simplified Model
Keep in mind that the example above over simplified, so lets make it a little less simple. Unless you have zero load on the motor the current draw will not be constant. It will jump up and down depending on the load. In the next example I will generate some random numbers between 90-100% of 12.6V in 5ms increments, simulating 10% ripple.
OK so now it's getting messy.
Looking at the same "pinned" throttle with the load variation you can see that the effective voltage doesn't stay at 11.34V (90% of 12.6V). In fact the average is around 12V, which effectively reduced the voltage droop (ripple voltage) to about 5% or about 1/2 of the range I gave it.
The capacitor adds charge when the battery is below the capacitor's voltage and gets charged when the battery is above the capacitor's voltage.
If it's Just Average Voltage, Why Do I Need Caps?
All of the numbers are between 11.34 and 12.6 and they average to 11.97V. So why do I need the caps?
The math is correct. What the caps do is create a more constant voltage (smooth the input) to the ESC. Without the caps you would see a much wider range of voltage numbers and that could effect performance. Of course there are cap in the ESC that will smooth (average) it out, but sometimes external caps do help. It really depends on your car, driving style and your electronics.
What about the Real World?
For the example I useed a .01F capacitor; which is very close to the value of 13200uf from the Powerhobby 6 Pack 13200uf RC Cap Pack. 13200uf = 13.mF or .013F. Different values will change the model. However, in the real world you have about a thousand variables and 13,200 F or 10,000 F probably doesn't make a difference until you sort out all the big ticket items like your batteries, ESC, motor, wiring etc.
Does the size or length of the wire matter?
No, it's more about how you use it. The wire and connectors add inductance, resistance (voltage drop) and even a tiny bit of capacitance. It how electrical things are in the real world.
Inductance can impede the capacitors ability to provide charge. However the values of the wire are very small. For a 6" piece of 12 AWG wire is the inductance is 149 nH. That's nano Henri's, as in 149*10E-9. So even it you add in twice the wire and connectors and ... let's call it 5x that value, the inductance means nothing for this application. If you building high speed PCBs it matters, but For for 12.6V and very slow current changes, it's literally in the noise.
However, looking at the resistance of the same 6" piece of 12 AWG wire you have a resistance is 0.8165 mΩ. Which means, .8165 mΩ @ 160A results in a .125V drop. So your 12.6V is now 12.47V. Which is real and can effect your performance if you are at the edge. However, if you shorten the wire to 3", you cut your resistance in 1/2 and your voltage drop too, with only .063V with an effective voltage of 12.53.
Likewise you can change the 12AWG to 8 AWG, keep the longer wire and get a lower resistance - 0.314 mΩ. Which results in only 50 mV drop.
In Summary
- Cap packs can help performance in cases where the current needs are consistently high and changing. However, a cap pack cannot replace good batteries, ESC and good wiring.
- The caps smooth the input voltage with respect to current needs.
- Keep the wires short, not because of the caps, but because of the voltage drop.
Most capacitors have polarity (+/-) and if you hook it up backwards bad stuff happens. Modern electrolytic (aluminum cans) caps typically have built in failure points so you don't get shrapnel flying around your garage, but they do light off and give you a really neat smoke show.... Eh, yeah, don't try that at home. Smaller solid state caps can still fragment. So be aware of your polarity on any type of cap.
If you decide to make your own cap pack make sure you use the correct voltage for your application. With that said you need to derate most cap voltage values. They typically print the maximum voltage on the cap; however, that is not the working voltage. The working voltage is well below that number because of heat, type of cap and other factors. Derating is 20-30% in most cases, but in some instances it can be as high as 50%. So check the datasheet and if you're not sure assume it's 50% of the voltage specified.
Final safety tip. Always make sure you caps are discharged before you handle them. The caps we use are relatively low voltage and moderate capacitance; however, they can give you a bit of spark if you aren't careful. So make sure you discharge them before you start giving them to the kids to play with.
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