Can someone explain how higher Mah batteries make a car faster? - RC Groups
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Feb 15, 2013, 05:22 PM
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Can someone explain how higher Mah batteries make a car faster?


I am pretty well versed in electricity, so it intrigues me how increasing battery mah affects anything other then run time. Increasing voltage would increase output (until failure), but the motor draws whatever current it draws. How does the batteries milliamp capacity affect performance?
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Feb 15, 2013, 05:28 PM
Rocket Programmer
jasmine2501's Avatar
It reduces the voltage drop under load. The C rating is actually defined as the maximum current you can pull without suffering a significant voltage drop. It is an actual number, but it's expressed as a factor of the capacity, so when the capacity goes up, so does the C rating, not the factor, the actual number.

So, a 30C 1000mah has a C rating of 30 amps, but the 30C 2000mah has a C rating of 60 amps. So your 30 amp car will cause a significant voltage drop on the small battery, but not on the big one. Same c factor, different c rating.
Feb 15, 2013, 07:28 PM
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wparsons's Avatar
^^ What he said. Voltage drop under load at close to the packs happy limit will be pretty bad, so having more amperage available will greatly reduce the voltage drop.
Feb 15, 2013, 10:04 PM
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jasmine2501's Avatar
There is a point of diminishing return...

Say you have a 30-amp truck and you're running a 5000mAh 35C battery, like I run in my 1/10th truck. That battery won't experience a significant voltage drop until you pull around 175 amps, so my truck isn't even stressing that battery at all. If I put a 10,000 mAh battery, all that will happen is I will add weight, and the truck will go slower. So, it's only true if your truck is currently being limited by the battery you're running. If that is not the case, then yes, a bigger battery will slow you down. That's why racing is so much fun... so many trade offs to shave those seconds
Feb 18, 2013, 06:43 PM
Stop me before I build again!
Quote:
Originally Posted by dma251
I am pretty well versed in electricity, so it intrigues me how increasing battery mah affects anything other then run time. Increasing voltage would increase output (until failure), but the motor draws whatever current it draws. How does the batteries milliamp capacity affect performance?
RC batteries are generally rated by Ah (mAh) and "C". "C" is a horrible rating since it's really just a relative discarge rate and what is more important is Ah*C which computes the amp rating of the pack. Pulling more amps than Ah*"C" on a continious basis will cause strain on the pack, premature aging, voltage sag, and limit the total capacity you can safely extract from the pack.

If your battery Ah*C is higher than the amps that your ESC/Motor can pull then going with a bigger battery will probably not produce any differences. There would be a tiny difference in voltage drop over time which would probably have a minimal effect in overall speed.

If your battery Ah*C isn't high enough you can get more amps in two ways, go with a higher C rating or a larger Ah, both will allow you to safely pull more amps.
Feb 18, 2013, 07:31 PM
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jasmine2501's Avatar
Brontide, that's why i made it clear that the C rating is the actual number of amps, not the factor. That's just how it's expressed, so the C rating is 40 amps or whatever, it's not the number before the C.
Feb 19, 2013, 03:16 AM
Fan of just about anything RC
SoloProFan's Avatar
Quote:
Originally Posted by jasmine2501
It reduces the voltage drop under load. The C rating is actually defined as the maximum current you can pull without suffering a significant voltage drop. It is an actual number, but it's expressed as a factor of the capacity, so when the capacity goes up, so does the C rating, not the factor, the actual number.

So, a 30C 1000mah has a C rating of 30 amps, but the 30C 2000mah has a C rating of 60 amps. So your 30 amp car will cause a significant voltage drop on the small battery, but not on the big one. Same c factor, different c rating.
Quote:
Originally Posted by jasmine2501
Brontide, that's why i made it clear that the C rating is the actual number of amps, not the factor. That's just how it's expressed, so the C rating is 40 amps or whatever, it's not the number before the C.
I understand you are trying to make it easier to grasp, but talking about a seperate C-rating and C-Factor isn't helping, I think, it just makes it more confusing. A C-rating is not in Amps, it's just a number. The C number is simply what you need to multiply the battery's capacity with to get the current this battery can deliver without it's voltage collapsing and/or the battery overheating.

You can't simplify that, the only way to determine how much current a battery can deliver is by simple math, a single calcucation. The outcome of that calculation is what matters. C-rating without taking into consideration the capacity of the battery means nothing, and capacity alone means nothing either. As brontide explained, it's a relative number, and as such, you can be tricked thinking into just looking at the C-number. But it is as it is, and once you know how it works, it's easy to compare batteries.
Feb 19, 2013, 10:39 AM
Registered User
wparsons's Avatar
In reality, the C rating is a representation of the internal resistance of the cells. Lower resistance means higher C rating.
May 16, 2017, 02:19 PM
Registered User
Hey guys, I know I'm a bit late to the party with this thread being from 2013 and that but I have a question. I am using a 200 amp ESC with a motor, the motor is brushless and the website (the motor Is on a remote control boat) recommended a 20 C 4000 mAh battery, but that was for a 120 amp ESC. I am making this boat into a scale boat, so will not be running it very fast but occasionally want to go fast to demonstrate that it can (in front of friends et cetera) . I really want a battery that will give me a lot of running time so I have been looking at the 16,000 M a H 10 C battery. Am I correct in thinking that this will give me 160 amps of out put (16amps x 10C), and so therefore should not damage my ESC?
Many thanks
T
PS, the battery is 4S
Last edited by Tintin242; May 16, 2017 at 02:21 PM. Reason: Add content
May 16, 2017, 02:33 PM
Suspended Account
a 16k battery will slow boat down.. and decrease run time..more than likely burn stuff out like esc motor...a 4500mah at 40c will do...depends on setup..aka.pro/motor.so on
May 16, 2017, 02:58 PM
I hate waiting for parts
Mike_Then's Avatar
Quote:
Originally Posted by Tintin242
Hey guys, I know I'm a bit late to the party with this thread being from 2013 and that but I have a question. I am using a 200 amp ESC with a motor, the motor is brushless and the website (the motor Is on a remote control boat) recommended a 20 C 4000 mAh battery, but that was for a 120 amp ESC. I am making this boat into a scale boat, so will not be running it very fast but occasionally want to go fast to demonstrate that it can (in front of friends et cetera) . I really want a battery that will give me a lot of running time so I have been looking at the 16,000 M a H 10 C battery. Am I correct in thinking that this will give me 160 amps of out put (16amps x 10C), and so therefore should not damage my ESC?
Many thanks
T
PS, the battery is 4S
Welcome to RC Groups. Understand that a 16,000 mAh battery is going to be significantly heavier than a 4000 mAh pack (it's technically 4x the capacity of the 4000 mAh pack). That should be taken into consideration as far as the way the boat is going to perform.

Also, as with any electric power system, you should do an amp draw to see exactly what your system is pulling. I assume you'd have to do that while the prop is in the water to get a true reading. And the ESC has little to do with the maximum draw of a power system, with the exception of whether it has enough overhead to handle it. Definitely do an amp draw to see what the system pulls. The 4000 mAh 20C battery will theoretically deliver a max of 80A and the 16,000 10C battery will theoretically deliver a max of 160A.

Finally, remember that with any BL power system, it's always delivering the maximum power; it just switches it on and off thousands of times a second when not at full throttle. So the thinking of an 80A ESC in a system that pulls 100A at WOT and "it will be OK because I'm not going to use full throttle" is wrong. Always use an ESC that has at least a little overhead... and that's another reason why an amp draw is so critical.

Sorry for rambling on, hope this helps!
May 17, 2017, 03:22 AM
Registered User
Thank you so much for replying so quickly! I am aware that the 16 amp battery will be heavy and so affect performance however I needed the model to sit lower in the water so I could put a bow thruster in anyway so this does not bother me too much. About working out the amp draw, how would I do this? I am changing the propellor (it is a boat not a plane however) to a smaller size so that when it gets on a plane and glides over the water it does not leave a rooster tail of water behind it, so I'm guessing that this should lessen the amp draw? Thank you for all your help. One last thing, will the battery only being 10C when the motor says it wants to draw 20C be a problem? I.E. will it cause swelling or a less long bettery life? Many thanks
Ps if I wired up two 4000 mah batteries in parallel so the Mah is doubled would this double the amp draw?
Sorry for all the questions! Many thanks
T
May 17, 2017, 08:49 AM
I hate waiting for parts
Mike_Then's Avatar
Quote:
Originally Posted by Tintin242
Thank you so much for replying so quickly! I am aware that the 16 amp battery will be heavy and so affect performance however I needed the model to sit lower in the water so I could put a bow thruster in anyway so this does not bother me too much. About working out the amp draw, how would I do this? I am changing the propellor (it is a boat not a plane however) to a smaller size so that when it gets on a plane and glides over the water it does not leave a rooster tail of water behind it, so I'm guessing that this should lessen the amp draw? Thank you for all your help. One last thing, will the battery only being 10C when the motor says it wants to draw 20C be a problem? I.E. will it cause swelling or a less long bettery life? Many thanks
Ps if I wired up two 4000 mah batteries in parallel so the Mah is doubled would this double the amp draw?
Sorry for all the questions! Many thanks
T
Probably will be easier to put this in a list form, if that's OK. Hope this helps.
  1. The most important tool in any electric modeler's toolbox is a wattmeter. They can be had for $20, if not less. You put the wattmeter in between the power source (battery) and load (motor) and measure the amp draw at full power.
  2. I'm not sure about smaller boat props but I would assume it (loosely?) follows airplane props.... your amp draw will change as the diameter and pitch of the prop changes. So I would also assume that a smaller-diameter prop of the same pitch as the larger-diameter prop would draw less amps. Again, the wattmeter will tell you.
  3. Using the numbers above, the 16,000 mAh pack has the capability to deliver more amps due to its larger capacity. But once again this is something a wattmeter can tell you. Try both batteries and see which one has the larger amp draw and holds better voltage under load.
  4. Wiring two 4000 mAh batteries in parallel gives you double the capacity, yes. Each battery will halve the load, so think about two 4s 4000 mAh batteries as a 4s 8000 mAh battery. Assuming both are 20C (which they should be!), that means you'll have a 4s 8000 mAh 20C pack that can deliver 160A. So that is definitely an option as well.
May 17, 2017, 09:14 AM
Suspended Account
a list form will not help the needed amp draw.. you need to do a actual water test with gear you are going to use.. on paper it might be one # and real life will be another..most amps drawn will be getting to plane. ..Use a watt meter inline with battery to esc to get amps needed..then you can go from there..

personal experience
i have had many things from motors saying 60 amps .. put a higher power esc on it with watt meter only to see 80 amps draw... so lists are ok but real life use can be more..
Last edited by rcdog69; May 17, 2017 at 09:27 AM.
May 17, 2017, 10:16 AM
Registered User
Thank you both for being so helpful, when all the kit arrives I will put it all together and will check that it doesn't draw too much, I can't wait to get all the kit and test it out and will definitely make a blog post about making the boat and making it realistic with lights and working radar etc, when I do (which will be in late June after I Finnish my GCSE exams) I will be sure to post a link. Thank you both and hope to see you around on the forums!
Yours
T
(PS sorry for all the questions, it's just previously I've always used different batteries to Lipo and they have always been barges where amp draw is not such an issue.)


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