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  1. #1
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    Added Capacitors

    For the majority of people this post will be worthless but for the more technically inclined/amateur electronics engineers it might be worth noting.

    Ok in one of my previous posts I mentioned that I had ordered some ultra low E.S.R capacitors that I had intended to tie into my MMM to further increase the total capacity.

    (For the results skip to the end to avoid the technical blabber)

    The MMM has built into it a total of 3 35v 390uF Capacitors that i'm fairly sure are also low E.S.R. capacitors. Upon reading with my Fluke 115 actual capacitance was in the neighborhood of around 1290uF. That's actually more than rated so that's good.

    Now to my experiment...I figured if the MMM is so deathly (read: death=fire) afraid of ripple current why not help the situation so I used two 35v 680uF Ultra Low Esr Caps that I ordered from Jameco as a pack of 10 and wired them in parallel to net a total of 1360uF, actual measured of about 1421uF, which are again higher than rated so also good components.

    Alright taking into account that just as in the case of resistors and their parallel resistance being reduced by half, I figured if two caps are connected in parallel then just the same their E.S.R. should theoretically be reduced as well. (Here is the formula if you would like to see it for resistors).

    For equal resistance values;


    This is for unequal value resistors;


    Ok so on with the experiment.
    So after wiring in all the caps in and getting everything connected to my satisfaction (took some extravagant soldering techniques that resembled a yoga position trying to hold everything in place without being burned) I came up with a total capacitance of 2714 max. Im sure my CC UBEC also played a part in that last figure also to some extent. I doubled the overall capacitance in the circuit.

    So far with the results I haven't been able to do any extesive testing nor did I get temperature readings of the esc before doing this mod so I cannot give any hard evidence of lowering the temperatures any but what I can say is that after just about 5 minutes of running with a gearing of 18/56 with 4s through my front yard in some low grass/damp mossy areas with really good traction, the external caps that I added actually do get above ambient temperatures showing that they are in fact doing their intended purpose of helping to filter out ripple. But whats more interesting is that if these caps are getting slightly warmer than ambient then its no wonder that the MMM can fail due to ripple current because as the internal caps heat up with their own internal heat and heat from the fets as well, the electrolyte starts to boil off decreasing the available capacitance and fail. As the caps fail the MMM loses its built in ablity to filter ripple current. Its just a chain reaction that starts at the front (battery input - caps) and causes failures longer down the line (motor phases - FET's)


    Sorry for the long read, my friends tell me I like to go into detail too much, I tell them without details the world is just a dull fuzzy place

  2. #2
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    This is good. I had an idea about these ripple currents but never thought much about them. Thanks.
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  3. #3
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    pictures will be also beneficial and some diagram to for newbies thank you for the info
    maxx 2.5r, maxx 3.3, revo 3.3 and revo .28

  4. #4
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    lol ok guys as soon as I get the chance ill post up some pictures.

    EDIT - Here are the pics you asked for, I would have put them in the original post but I can't edit it for some reason and sorry for the crappy quality, all I have is my cell phone for pics.

    Here is the caps themselves


    Here is the two caps soldered in parallel with 14 awg. power leads.


    Here they are soldered directly to where my deans are soldered to the factory 10awg wiring. You can see them sitting between the main power wires. When I figure out a way to better secure them I will, this is just preliminary trials however.


    1 More picture, showing the Low ESR stamp.
    Last edited by Swindez85; 07-06-2009 at 10:25 AM.

  5. #5
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    Ok well....ill be the first to say you voided your warranty. That said I can see you were very worried about the equivalent series resistance. As you tried to keep it as low as possible you did increase it some, there for you may have caused the resistance value of the total filter to have changed and thus change the way it works. The fets and mosfets required depend on very precise inductance and resitance value which all have been designed to work work on that capacitance, its just a simple RLC circuit and as you know changing anyone of those values will cause different effects.

    Im not sure how well you MMM will work after this, sure it may be able to handle more ripple current but the MOSFETS may cause undesired effects now that they ripple filter output impedance has increased. Just wondering why do this when you can just use better batterys that dont cause as much ripple current but hey to each there own.
    Last edited by binaryd; 07-06-2009 at 10:30 AM.
    If you dont like it, just change the warranty.

  6. #6
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    The warranty may still be good as long as the soldering is not done to the PCB directly. You can add a small bank of caps to the input wires easily enough though.

    The caps on the ESC have nothing to do with "timing" (RLC circuits), it's all about filtering. Adding more caps just improves the filtering of the ripple currents.

    Small value but multiple caps are chosen for a number of reasons:

    - Space. Several physically smaller ones can be more easily arranged than one big fat one, especially when designing something as small as an ESC.

    - Heat. Several caps have more surface area overall to dissipate heat.

    - Transient response. Multiple caps "react" faster than a single cap of the same overall value.

    You generally want caps with low ESR because there are really high currents being used when driving motor loads, and high resistance ultimately means high heat.

    BTW: the existing caps on the ESC are indeed low-ESR. Easy way to tell is the 105*C (or higher) temperature rating. "Regular" electrolytics are typically rated 85*C.

  7. #7
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    Quote Originally Posted by binaryd
    Ok well....ill be the first to say you voided your warranty. That said I can see you were very worried about the qquivalent series resistance. As you tried to keep it as low as possible you did increase it some, there for you may have caused the resistance value of the total filter to have changed and thus change the way it works. The fets and mosfets required depend on very precise inductance and resitance value which all have been designed to work work on that capacitance, its just a simple RLC circuit and as you know changing anyone of those values will cause different effects.

    Im not sure how well you MMM will work after this, sure it may be able to handle more ripple current but the MOSFETS may cause undesired effects now that they ripple filter output impedance has increased. Just wondering why do this when you can just use better batterys that dont cause as much ripple current but hey to each there own.
    Eh I believe I voided the warranty when I soldered on my own connectors and slightly shortened the input wires to clean things up...next is to cut and shorten the receiver lead to clean up the wiring even futher . And as far as why I did it, simply because I can and I wanted to see what kind of result I would get by doing so. I haven't been able to leave stuff alone ever since I was old enough to hold a screwdriver, im always tinkering with something. I may not be doing any good as I say but it certainly cant be hurting anything either. I dont have the caps soldered directly at the MMM board also so techinically I havent even messed with it internally, just externally using my own method just as the Novak caps are wired in.

    Edit - Thank you BrianG

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    Hold ON, never said you shouldnt have done it, just wondering why, I mod crap all the time. Just stating the facts. Second the filter has RLC components with certain values, its not as simple as just adding more caps, you must also change the rest of the filter or the design will no longer be optimized. Theres alot more then just the caps that limit the ripple currents and the total inductance does change the way the circuit functions.

    Also you should be using a scope to make your changes so can see whats going on, without a scope you cant see the real world numbers, remember paper numbers and real number dont always agree...


    Simple transfer function H(z)=[Y(z)/X(z)] so changing values makes a differeance.

    Do you guys know what kind of filter you guys are even working with, I mean order of magnitude, type, output waveform? These are important and you should know these, I dont know them but I would find them out prior to making changes to the filter circuit.
    Last edited by binaryd; 07-06-2009 at 10:45 AM.
    If you dont like it, just change the warranty.

  9. #9
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    I agree, inductance does play a role in the circuit design but im not worried about that part of the equation simply because I dont have the necessary equipment to measure exact specs of the filtering circuit. I just simply added more caps, albeit a pretty long ways away from the actual circuit board as well so whether they play a large role in how the MMM itself was designed and will function is most likely little to nill because technically my own filtering stage happens at the connector, not at the circuit board itself where the FET's themselves are located.

  10. #10
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    Quote Originally Posted by binaryd
    ....the filter has RLC components with certain values, its not as simple as just adding more caps, you must also change the rest of the filter or the design will no longer be optimized. Theres alot more then just the caps that limit the ripple currents and the total inductance does change the way the circuit functions....
    In a "tuned" RLC circuit, that is absolutely true. However, this is not tuned circuit. An ESC is meant to drive a multitude of various motors (all of which have quite different inductance and DC resistance values). How could the caps be tuned to a certain inductance when it changes with the motor? Also, the same motor will have different inductive reactance depending on the frequency being used in the PWM at that moment. Furthermore, ESCs are designed to have as low "R" (copper losses and losses due to FET slew rate and rdson) as possible to reduce heat losses at these high currents - the "R" is not designed in on purpose. Ideally, R should be 0 (whihc of course will not happen).

    No, these caps are being used only for their filtering characteristics, similar to how caps are used in power supplies.


    BTW: I hope you are not taking this "debate" personally. I actually enjoy conversing with other techies.

  11. #11
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    Quote Originally Posted by BrianG
    BTW: I hope you are not taking this "debate" personally. I actually enjoy conversing with other techies.
    To tell you the truth I dont know for sure much about this particular ESC, but I do believe that the input DC source needs to be filtered like it is to weed out the ripple current. I think the ESC uses a switched mode regulator to do the job of supplying power to the esc through from the power supply (battery). It does require a filter circuit and though Im not sure how the design is I belive those caps are parts of the switching mode power supply filter and if so changing inpedence of that filter changed the way the output signal waveform looks and acts throughout the rest ESC.
    If you dont like it, just change the warranty.

  12. #12
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    we need the MMM pcb schematics then we really could have some fun
    If you dont like it, just change the warranty.

  13. #13
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    I agree that would be fun haha. However though...keep this in mind, I agree with binaryd on this one as well, you state a good point, and you are correct technically an ESC is a PWM circuit except where as most PWM power supplies do need a tuned filter, those power supplies usually power other sections that supply power to a load...a great example of this is an audio amplifier. You have a PWM that takes your input and either boosts it or in some cases lowers the voltage but more importantly it supplies both a Negative and a Positive voltage potential.

    Inside an ESC there is no positive and negative voltage potential, just what it sees from the battery which can be refrenced as simply ground and positive.

    An ESC is a PWM power supply BUT it does not feed a transformer in normal PWM supplies, instead it directly powers the load (motor in this case) and as BrianG said, inductance and resistance changes throughout the RPM range of the motor so in an ESC all the caps basic function is to filter unwanted voltage transients to maintain as close as possible a true DC voltage without any induced ripple in supply caused by the FET's turning on and off supplying the varying inductance which is the motor in this case, at different and increasing frequencies as the motor's rpm's climb. Ripple current would be greatly seen in the supply at lower motor RPM's where each phase is conducting more for longer to spin the armature. As RPM's increase so does the switching speed of each phase, increasing the frequency of the induced ripple on the supply.

    Whew ok I think I just overcomplicated that...but you get the idea haha.

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    That sounds like a very good idea and you must have taken some serious math courses. I just put in a Tekin RX8 with a 2250kv motor and got away from that.

  15. #15
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    Quote Originally Posted by Swindez85
    I agree that would be fun haha. However though...keep this in mind, I agree with binaryd on this one as well, you state a good point, and you are correct technically an ESC is a PWM circuit except where as most PWM power supplies do need a tuned filter, those power supplies usually power other sections that supply power to a load...a great example of this is an audio amplifier. You have a PWM that takes your input and either boosts it or in some cases lowers the voltage but more importantly it supplies both a Negative and a Positive voltage potential.

    Inside an ESC there is no positive and negative voltage potential, just what it sees from the battery which can be refrenced as simply ground and positive.

    An ESC is a PWM power supply BUT it does not feed a transformer in normal PWM supplies, instead it directly powers the load (motor in this case) and as BrianG said, inductance and resistance changes throughout the RPM range of the motor so in an ESC all the caps basic function is to filter unwanted voltage transients to maintain as close as possible a true DC voltage without any induced ripple in supply caused by the FET's turning on and off supplying the varying inductance which is the motor in this case, at different and increasing frequencies as the motor's rpm's climb. Ripple current would be greatly seen in the supply at lower motor RPM's where each phase is conducting more for longer to spin the armature. As RPM's increase so does the switching speed of each phase, increasing the frequency of the induced ripple on the supply.

    Whew ok I think I just overcomplicated that...but you get the idea haha.
    Not over complicated just better understood, the more details the better. Your right but remember were have "dirty" power supplys here, lots of changes in internal resistance varing with loads, temp etc... The filter must account for this and it does to a point. I said in another post that was deleted(MMM thread) Castle could make a ESC that handles much more ripple, more then we could ever throw at it, but it adds more complexity and thus more price to the controller. I think someone should come out with a in-line filter between the esc and batterys that smooths out the input voltage and current more so then the esc alone, that way it would allow you to run it with any esc and battery combo, ummmmmm maybe someone may make one tonight???? wheres my little black book of functions
    If you dont like it, just change the warranty.

  16. #16
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    Quote Originally Posted by binaryd
    Not over complicated just better understood, the more details the better. Your right but remember were have "dirty" power supplys here, lots of changes in internal resistance varing with loads, temp etc... The filter must account for this and it does to a point. I said in another post that was deleted(MMM thread) Castle could make a ESC that handles much more ripple, more then we could ever throw at it, but it adds more complexity and thus more price to the controller. I think someone should come out with a in-line filter between the esc and batterys that smooths out the input voltage and current more so then the esc alone, that way it would allow you to run it with any esc and battery combo, ummmmmm maybe someone may make one tonight???? wheres my little black book of functions
    Actually I dunno if I would call them dirty...more like...lazy power supplies. Batteries are simply a chemical means of supplying a voltage, and it takes time for those chemical reactions to happen. If the battery cannot produce enough reactions to keep up with demand then voltage falls. I believe this is where most ripple current comes from...we can actually test this though, how fast those reactions can occur can be measured in the IR of the battery itself.
    And after thinking about it further, ripple current at the ESC itself will be worse if the wire size leading up the ESC isn't capable of supplying the amperage demanded of it. Small wire/higher resistance with large current demand equals bigger dips in voltage at the ESC and more ripple that the caps themselves can actually induce and are trying to filter out from the undersized supply wire.
    Last edited by Swindez85; 07-06-2009 at 12:15 PM.

  17. #17
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    Quote Originally Posted by REVO7
    That sounds like a very good idea and you must have taken some serious math courses. I just put in a Tekin RX8 with a 2250kv motor and got away from that.
    MATH 140 Calculus I
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    and a few others booo my head still hurts
    If you dont like it, just change the warranty.

  18. #18
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    Quote Originally Posted by Swindez85
    Actually I dunno if I would call them dirty...more like...lazy power supplies.
    And after thinking about it further, ripple current at the ESC itself will be worse if the wire size leading up the ESC isn't capable of supplying the amperage demanded of it. Small wire/higher resistance with large current demand equals bigger dips in voltage at the ESC and more ripple that the caps themselves can actually induce and are trying to filter out from the undersized supply wire.
    your very right about wire size, biggest bottle neck next to the connector. some battery wire is WAY undersized for the current carrying requirements of the esc, 12-10 gauge on a 8000mah 30c 3s batt easy but most run 14-16 gauge and thats not enough.

    my one prof always said that theres no ideal power supply, some are better then others and some are just plan dirty, so I tend to use the same vernacular.
    Last edited by binaryd; 07-06-2009 at 12:18 PM.
    If you dont like it, just change the warranty.

  19. #19
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    Most, if not all, of these ESCs are pretty simple when you break it down. Really, the only difference between ESCs of different makers is the control circuits, and maybe the choice of FETs.

    A little tutorial on controller operation (this is for anyone who want to know how these work):

    The FETs (the part that does the work) are arranged in multiples of 6 in a brushless controller. Three are switching the battery (+) to the load, and three are switching the battery (-) to the load. Classic H-bridge configuration. A brushed controller has a minimum of 4 FETs (which you can see in tons of schematics), but since a BL motor has 3 phases, you need a minimum of 6 FETs for the H-bridge.

    The MMM has 18 FETs; 6 banks, each with 3 FETs in parallel to increase the current capability and reduces the rdson value (think of the rdson value as a small resistor - more resistors in parallel decreases total resistance). Less resistance means less heat (I^2*R losses). Ideally, an FET should have 0 ohms of resistance, but they don't, and that is the rdson value. You could parallel a ton of FETs to get a really low rdson value, but then the controller would get large, expensive, would be too much copper (PCB traces should be kept as short as possible when dealing with hi currents), and the controller circuit would have to be adjusted to drive that many FETs.

    FETs are simply voltage controlled switches (at least that's how they are being used in an ESC). The brains circuit turns on/off these FETs in a specific order to power the motor phases in succession.

    If that's all the ESC did, the motor would spin at WOT or be stopped - just on or off. So, to get variable speed, the brains also further switch the FETs many more times to send partial power to the FETs (and then the motor). The amount of time they are on vs the time they are off (or duty cyle) determines the speed. If the FETs are "on" 10% of the time and "off" 90% of the time, the motor effectively sees that as 10% of the battery voltage, so it spins slower. If the on/off ratio is 50%-50%, the motor is spinning at half speed. And so on. This is the PWM portion of the switching.

    Now, FETs do not switch directly on and off instantly, there is a "ramp up" and "ramp down". This is also known as the slew rate. During these ramp ups/downs, there is V*I losses. And this is why ESCs are not as efficient at less than WOT.

    That's the simple part. The controller brains has to read the motor position (by reading the back-EMF from the unenergized phase) and apply PWM switching to the proper phase, and do so many times per second. And then you add things like programmability and switching BEC. I am honestly amazed at how these ESCs can channel so much energy yet be so small. And with that kind of power it controls, it is not surprising when one burns up. All it takes is an improperly set up vehicle and poof! But yet, these things take some pretty heavy abuse!

    And I'm not going to get into phase shifting of the voltage and current due to inductance (the motor).

    Anyway, back to the cap question:

    The switching of the FETs to variably drive a motor pulls a lot of current. And since there is a lot of switching going on, there are a lot of high current pulses. Most batteries are not able to handle this type of abuse, so the caps are used to momentarily store the charge (during the "off" part of the switching). When the FETs are switched on, the caps discharge rapidly. All this charge/discharge action happens MANY times per second (FETs have a PWM switching speed usually in the 12kHz-18kHz range). This constant on/off action is the ripple current, and the subsequent rapid charge/discharge of high currents is what heats up the caps. If they are not sized right (or the battery dips too much under load), they heat up excessively. Adding more caps help to further share the heat and current load, and provides greater ability to smooth out the ripple.

    Incidentally, this is why gearing up too high is bad. The motor is forced to do more work, so it pulls more current and causes more capacitor and FET stress (heat). Also, using low quality batteries is making the caps work harder because the battery voltage dips more under load, and when that happens, the caps discharge deeper causing heavier ripple.

    So, the caps in an ESC (at least the 2 or 3 on the battery input wires) are being used as a filter. No RLC tuning for resonance or anything.

  20. #20
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    If I could clap I would BrianG. You have obviously had your share of classes, I just wish I could sit still long enough to take those classes...and be good enough at math to calculate all that stuff but everything I know I taught myself through countless hours of boredom reading up on stuff . Anyway, you explained that better than I could have, I knew all that but I have problems putting what I know into words.

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    Quote Originally Posted by BrianG
    So, the caps in an ESC (at least the 2 or 3 on the battery input wires) are being used as a filter. No RLC tuning for resonance or anything.
    Very nice explanation for the layman. I still feel like there would be some issue with adding more caps which would require some filter tuning but I dont like to speculate. I will look alittle more into this with my Electronics Workbench Circuit Board Design and Simulation Software.
    If you dont like it, just change the warranty.

  22. #22
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    All way to complicated to me. It starts to look like an Novak that way. I'm not sure i like that

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    I always heard about adding capacitors to an esc to help keep the tempretures down and prevent it failing under heavy load. Both brushed and brushless. Is this why i take it?
    I was born to break the mould

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    bricks: I tend to agree. I'm generally of the opinion that if temps are too high; gearing is too high, there is some other setting issue, or the system is too small for the vehicle (not the case here). Here is how I added a couple more caps to a different ESC:



    A couple more is easy to add and not too cumbersome. And, adding them to the wires doesn't void the warranty (soldering to the PCB does).

    Hardcoretam: Exactly

    Note: Adding more caps will increase the connection spark when you hook up your batteries, which will eat away at the connectors (from the arcing) faster. If the connectors get pitted, you should replace them.

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    So in simple terms is > 680uf cap better assuming all caps are 35v?

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    Quote Originally Posted by BrianG

    Hardcoretam: Exactly

    Note: Adding more caps will increase the connection spark when you hook up your batteries, which will eat away at the connectors (from the arcing) faster. If the connectors get pitted, you should replace them.
    Are there any other negative effects from adding a couple of caps to a overworked speed control?
    Seems like an easy way to squeeze some more performance from a struggling speed control.
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    PBO: Technically, more is better, but you run into physical size and the constraints it creates on placement.

    Hardcoretam: Negatives? Just figuring out where they are gonna go. Ideally, it should be as close to the ESC as possible, which limits things. However, I personally would not use a cap for a "struggling" ESC, but to improve the cooling on an existing good running setup. Doing things like adding caps and fans only addresses the symptom, not the problem.

  28. #28
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    I was under the impression from the RC-monster discussion on ripple current and over-rated batteries. That It was caused during hard braking, and trying to "force" that generated current and voltage back into the batteries. Since they are becoming the load.

    If that is the case are the caps in the MMM not going to see the generated "V" And "I" before the extra ones anyway?. Would they not still be damaged?

    Sorry if I'm way off base here, I have some electrical background but nothing as substantial as what you guys are talking about.

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    This kinda reminds me when I was into car audio. A lot of noobs like adding caps to their electrical system thinking they can run bigger amps/subs and not have dimming head lights. Then the pros in the car audio scene would laugh and tell the noobs to put bigger alternator and stronger batteries so that their electrical system can provide enough amperage to their amp. They say putting caps cures the symptoms not the problem.

    When I was doing car audio, we used to say "little power smokes big amps". A lot of noobs burn their huge amp and thinking why the **** that's happening to them. they got the best amp out there and it just went poof. Problem is they don't back their amp with enough amperage causing it to work too hard and go poof. Same thing with the MMM, I really think MMM esc is like one of those competion amp and it really needs strong electrical to perform it's job. I you run smaller batteries, it will go poof. So in this case, "little power smokes big ESC's." You have to have big batteries for the MMM. There's no other ways around it.

  30. #30
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    Yeah, the caps used in car audio were just mainly to reduce headlight dimming. Doesn't do much for the amps other than provide a slightly more even current flow. They work ok when the bass is "punchy", but those long bass notes will drain a cap very quickly (way before the bass note finishes) and just add more load to the battery/alternator. Better to invest in a better power delivery system IMO (alternator and battery). I ran a cap just because I was annoyed at the slightly dimming lights at red lights.

    But for ESCs, they are absolutely needed. Even the best battery in the world will exhibit v drop under heavy loads. Gotta remember that most car amps have a nice regulated power supply, and the car battery can handle ripple much better.

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