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Cap packs on drag cars - really?

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Cap packs on drag cars - really?

Old 08-09-2020, 06:31 PM
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Default Cap packs on drag cars - really?

Went to check out the "No Prep" drag cars yesterday. Everybody is running 4.5 turn motors with a huge capacitor pack - I mean they are the size of a motor. I suspect they have minimal impact on performance, and probably negative due to the extra weight - and a lot of cars have a shelf to raise the ESC so they can get the cap pack in. Any EE types that can explain?
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Old 08-09-2020, 09:10 PM
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Im still waiting for someone to show data to confirm that sizing a capacitor properly does not store and return power to a motor. But I’ll say two things as it relates to this concept and your scenario. you would need allot of the right capacitors for them to add anything to the available power. secondly it would not work in a constant acceleration application. it would only work in continuous acceleration and deceleration applications which is not drag racing.

so then what you have left is a capacitor acting somewhat like a resistor that is frequency dependent. imagine that your motor is designed to work best with when it gets a pulse of electricity that looks like a trapezoid. the trapezoid is the shape created by lots of little pulses to create the shape. it has an angle on the slope up and an angle on the slope down. in the middle it should be flat. if instead that motor gets something that looks like a square it doesnt have the right angle up and down. from the motors perspective anything that sits outside the shape it was designed for (trapezoid) is turned into heat. thats not a problem if you dont have a heat problem. but anything that sits outside the ideal waveform can do more than create heat. If that is something interesting im sure I can help but at this point im pretty sure you will get more answers on that whether you like it or not. but the cleaner the power is through the esc and to the motor the more power your motor can deliver.

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Old 08-09-2020, 11:20 PM
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Current batteries when right treated have just 1mohm resistance per cell. I need to see a capacior bank that can come close to it to help that 1st peak current.
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Old 08-10-2020, 12:57 AM
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Theory: The current draw is at the absolute maximum when accelerating hard from a dead stop, which will cause the battery voltage to drop. If the capacitor bank is large enough it could help to reduce the voltage drop. It'll only be of any use during the initial moments of acceleration as it'll discharge pretty quickly.

Practice: I don't have any experience with no-prep drag racing, but in 4WD TC it's pretty easy for a 4.5T motor to break traction when accelerating. I'd be surprised if a capacitor bank would be of any practical use as I'd have thought that traction would be the limiting factor during initial acceleration, not battery voltage.
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Old 08-10-2020, 01:00 AM
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Got this link to such a cap:
https://www.killmoderc.com/online-st...-***-p84360572

The price of 55 dollar is insane for just 6 low ESR capacitors (if they are really low ESR...)

A low ESR datasheet.
https://nl.mouser.com/datasheet/2/31...22-1144690.pdf
OK, those are 220uF and I asume 35v. -> 160mohm With 6 parallel -> 26mohm. A good 6S pack will be lower than 10mohm so calculate how the 1st peak current is split.

A normal cell will have 2~3mOhm so lets say 15mOhm for a 6S pack..... It can take some of the 1st peak.

But how long:
https://sparks.gogo.co.nz/capacitor-formulae.html
Take 26v as start and 15v as end (max voltage dip), at 150A it will be 1msec
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Old 08-10-2020, 01:01 AM
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Originally Posted by Roelof View Post
Current batteries when right treated have just 1mohm resistance per cell. I need to see a capacior bank that can come close to it to help that 1st peak current.
The IR of the capacitor doesn't actually need to be close to the battery IR to provide a benefit. The capacitor bank is placed in a parallel connection with the battery. Adding resistors in parallel actually reduces the overall resistance. As long as the capacitor provides current (which happens when the battery voltage drops) it's going to help with 1st peak current.
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Old 08-10-2020, 05:14 AM
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cap pack has nothing to do with battery IR. the cap pack helps when battery voltage sags under load.as in taking off the start line..I use a 32 cap pack 4 x 8 on 1 of my drag cars..I dont do the no pre thing...tracks are clean with leaf blowers the sprayed with a drag track stickum solution(i just call it that)...I run a 2 turn brushed motor on the 32 cap setup .unless you run a drag car on might not know the added benefits of a larger cap pack untill you take off on the lights and your car hops like a rabbit(cap pack can help that)..
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Old 08-10-2020, 05:54 AM
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Originally Posted by the rc guy View Post
cap pack has nothing to do with battery IR.
Sure it does....With most ESC's the capacitor is acting as a filter to filter out the high frequency ripple on the power lines with the result the battery and ESC will stay cooler.

But when the capacitor is acting as a power buffer other aspects need to be looked at. If the IR of the capacitor is higher than that of the battery then still the battery wil provide the most current and determs the voltage drop because the higher IR of the capacitor will not help to keep the voltage on level. Current Lipo batteries have an amazing low IR and can handle high currents, more than such a cap pack can handle.

I can imagine they can help with that first push but I wonder if it is really needed with current good batteries, that it is still a thing still used from the past with NiMh batteries.
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Old 08-10-2020, 08:53 AM
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Originally Posted by gigaplex View Post
Practice: I don't have any experience with no-prep drag racing, but in 4WD TC it's pretty easy for a 4.5T motor to break traction when accelerating. I'd be surprised if a capacitor bank would be of any practical use as I'd have thought that traction would be the limiting factor during initial acceleration, not battery voltage.
This is exactly what I observed - seeing how the rules require the chassis to be based on 2WD short course trucks, the 1st milliseconds of acceleration are the least of their worries. Surprising how many crashes there was considering they only have to go in a straight line.
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Old 08-10-2020, 09:02 AM
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it only acts as a voltage stabilizer..keeps voltage up stops momentary sag..
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Old 08-10-2020, 09:13 AM
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Do we have a good conversation going about capacitor use? I think so.

all the theories are pretty good and explained well (im glad I didnt add to the confusion). They are filters and power sources but how each plays into the performance in no prep drag or rc in general still needs a data log.

Someone will have to be insulted or challenged enough to actually do the test and see what is happening. it would have to be a really good one to motivate me. LOL

Last edited by Bry195; 08-10-2020 at 05:05 PM.
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