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Old 18th July 2010, 09:53 AM   #1
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Default needing some ideas for a radio controlled car re capacitors!!

I have a 1/8th scale truck.

it is running a 120amp speed controller, off a 4 cell lithium polymer battery.

SO, the batteries nominal voltage is 14.8volts, and fully charged voltage is 16.8volts.

I put a clamp on amp meter on the battery cable...

Click the image to open in full size.

yes, thats 160amps at about 14 volts... 2.2kw from a 500 gram battery.

Now! the issue is, that the speed controller seems to go into "protect"

it wasn't until a few test runs, that I realised the heatshrink on the speed controllers capacitors had split, I can only assume that the capacitors got to some stupidly high temperature.

what capacitors would I realistically need to handle a 150amp discharge rate, repeatedly?

thankyou for ANY input!

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Old 21st July 2010, 07:47 PM   #2
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halp!
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Old 21st July 2010, 08:09 PM   #3
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The speedo is rated due to the MOSFETs not the caps, it will never be a 160A controller no matter how many caps or what kind of them you use. Buy the correct speedo.
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Old 21st July 2010, 08:16 PM   #4
godfrey is online now godfrey  South Africa
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Disclaimer: I don't really have a clue, don't even know what the circuit looks like, however...

Maybe you can use the existing caps as a guideline. Since whatever cap is in there now is good for 120 amps, I'd guess that two of them connected in parallel would be good for 160 amps.

Of course if you do that, something else might decide to go bang.

Alternatively, if it's still working, maybe you can stick with the existing caps and focus on trying to keep them cool with heatsinks or a fan or something. Getting rid of the plastic sleeve would help ventilation for a start.

How do you get 160A out of a 120A controller anyway? Isn't it supposed to have current limiting or some kind of overload protection?

Cheers - Godfrey

Edit: Just saw Richie00boy's comment. Reminds me - you need to keep the MOSFETs cool too.

Last edited by godfrey; 21st July 2010 at 08:21 PM.
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Old 22nd July 2010, 06:34 AM   #5
RJM1 is offline RJM1  United States
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The speed controller works by pulse width modulation. The capacitors on the modulation board just control the frequency of the modulation and do not carry the >100 amp current. If your controller is going into protection and shutting off and you are drawing 140 amps through a 120 amp controller it would seem that you have a motor with shorted windings that would need to be replaced. You can replace the capacitors with low ESR high frequency 105C capacitors of the same value. The plastic has split because of the repeated over heating of the controller and will happen again unless you replace the motor.
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Old 22nd July 2010, 07:19 AM   #6
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the controller is rated for 120amps constant, and some stupid, impossibly high current peak, that it will never do.

the esc and motor are fine together, the 160amps is the peak current.

The mosfets aren't actually getting hot (50 degrees celsuis for the heatsink), just the capacitors seem to be, they are directly across the battery, so I'm wondering if an insufficient battery isn't helping.

The motor doesn't have shorted windings, it only has something like 5 turns to begin with and functions perfectly, for about 3 minutes

YouTube - 18072010016.mp4
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Old 22nd July 2010, 09:28 AM   #7
godfrey is online now godfrey  South Africa
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If they're connected directly across the battery, they're just soaking up the ripple current. Adding more caps to share the workload should help and can't hurt anything.

It may be worth checking your wiring as well. Any extra resistance between the battery and the controller makes life harder for those caps. So long/skinny wires and wimpy/tarnished connectors are bad. Shorter, thicker wires are better.

The + and - wires should be close to each other as well to minimize inductance. (see pic below)

Better batteries might help too if all else fails, but I'd consider that a last resort as I'm guessing they're scary expensive.

Cheers - Godfrey
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Old 22nd July 2010, 09:59 AM   #8
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I had shortened the wires after posting this, and it didn't seem to help, I've got about 5cm from the speed controller to the socket, then about 12cm from the plug to the battery.

I'm using connectors rated for 160amps.

They are 10awg cable on the battery, and 12awg on the speed controller.

This battery is rated for 125amps constant, 175amps peak. Cost $US40.

I could get a battery rated for 175amps constant, 230amps peak for about $US70...
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Old 22nd July 2010, 11:04 AM   #9
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Quote:
Originally Posted by ThyDntWntMusic View Post
the controller is rated for 120amps constant, and some stupid, impossibly high current peak, that it will never do.

the esc and motor are fine together, the 160amps is the peak current.

The mosfets aren't actually getting hot (50 degrees celsuis for the heatsink), just the capacitors seem to be, they are directly across the battery, so I'm wondering if an insufficient battery isn't helping.

The motor doesn't have shorted windings, it only has something like 5 turns to begin with and functions perfectly, for about 3 minutes

YouTube - 18072010016.mp4
The caps are probably faulty, if the ESC is matched to the motor AND BATTERIES. What value and voltage rating are the caps?

Is the ESC rated for the ~16V that you're getting from the lithium batteries? It may only be intended for use with NICADs or MiMh. Lithium batteries are a comparatively new thing in RC models, a 10-cell pack of NICADs only runs 12.5V fully charged. Lots of buggies used to run 6- or 8-cell packs, IIRC. I'm a heli man myself, and a bit rusty at that.

w
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Old 22nd July 2010, 11:15 AM   #10
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Quote:
Originally Posted by wakibaki View Post
The caps are probably faulty, if the ESC is matched to the motor AND BATTERIES. What value and voltage rating are the caps?

Is the ESC rated for the ~16V that you're getting from the lithium batteries? It may only be intended for use with NICADs or MiMh. Lithium batteries are a comparatively new thing in RC models, a 10-cell pack of NICADs only runs 12.5V fully charged. Lots of buggies used to run 6- or 8-cell packs, IIRC. I'm a heli man myself, and a bit rusty at that.

w
35volt capacitors, 220uF, 4 in parallel, Rubycon brand, 105 degree rating.

esc is rated for 2-4s lipo, its intended for lipo use
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