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Old 10-10-2007, 08:12 AM   #1
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Questions?? battery performance - voltage vs amps

If I had a 7.4v pack with 8 amps of capacity should it perform as well as a 11.1v pack with 5 amps of capacity?

amps * volts = watts equation says they should be very simliar. From what I've read the 7.4v pack would heat up more than the 11.1v pack because of the nature of the current. The 7.4v and 11.1v packs are the same size +-1mm and I could use the extended run time of the 8 amp pack.

I've asked this question before (diff in voltage, without mentioning the difference in mah) I knew more about batteries and I was told the 7.4v pack won't perform as well... can anyone explain why, Thanks.
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Old 10-10-2007, 09:18 AM   #2
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You have another part to consider and that's the load (the ESC and motor). The battery will only discharge what the load will flow.

I don't feel qualified to discuss all the possible variations you could possibly be looking at. If you could narrow up your query a bit (ask a better question ), you might get some real answers.

Is this a battery for a micro-scale car? Most 1/10 batteries will do a lot more than 5 amps...they match them at 30-35.
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Old 10-10-2007, 09:26 AM   #3
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capacity = run time but in many ways can lower IR to give better performance. thing is you are not asking a level question because there is most likely hood be an advantage to haveing the higher voltage ......
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Old 10-10-2007, 09:40 AM   #4
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sorry about that, the application / environment that I want to use the batteries in is: a velineon brushless rustler stadium truck with stock gearing (goes between 45 and 50mph with my 5amp 11.1v lipo). Will the truck "pull" like it does with the 11.1v pack and if I change the gears around to go 50+ or 65+ will the 7.4 be able to "drive" the motor?

I think the velineon motor can draw up to 320 amps for a brief burst. Of course, neither the 2s2p or 3s1p packs can deliver that max draw but they're certified to run with the VXL by maxamps.

Thanks
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Old 10-10-2007, 09:42 AM   #5
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you are talking about capacity in terms of your amperage. you are talking about mah and AH not burst output
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Old 10-10-2007, 09:52 AM   #6
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you are talking about capacity in terms of your amperage. you are talking about mah and AH not burst output
not sure what you mean. I just know that the potential power in the 7.4v 8 amp pack around 56 watts. The potential power for the 11.1v 5 amp pack is 55.5 watts. I understand the differences in the pwer delivery with amps and volts with the analogy of amps is the diameter of a pipe and voltage is the speed that the water is moving through it.

Technically according to max amps, the 8 amp pack can discharge at a higher rate than the 5 amp pack because they're both 20c packs. So the 8 amps pack is pushing more power through the larger pipe than the 5 amp can push through it's smaller pipe even though it's flowing faster for the 5 amp pack...

I understand capacity (overall with volts and amps) doesn't equal burst power. I just want to understand why someone (me) would buy an 11.1v with less amperage over a 7.4v higher amperage pack.

Thanks
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Old 10-10-2007, 11:24 AM   #7
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capacity = run time but in many ways can lower IR to give better performance. thing is you are not asking a level question because there is most likely hood be an advantage to haveing the higher voltage ......
' Capacity = Runtime ' assigns capacity of a pack to a generalized outlook. Because your runtime will be dictated by load and pack capacity you cannot do this. So what manufacturers have done is assigned a load figure for you. They take the pack and calculate the max draw you can take from the cells for an hour. Then they assign that as your Mah figure.

Ie. 7.4 volts 5000mah pack will allow you to take 5 amps out of the pack for 1 hour. But if your load takes 10 amps you would only see a half hour use out of the pack. Understand?

Now the 'roof' or limit to how much you can take from the pack continuously is dictated by the discharge rating (C rating) For example if the above pack allows 4C discharge you can leech 20 amps from that pack and doing the math that gives you 15 minutes of pack runtime at that draw rate.

Now in an actual runtime enviroment temperatures of the pack, ESC and the load itself will vary and so will your final numbers so all the number crunching in the world won't help you. In the end what you can hope for is using the peak possible drain rate for your load as a worst case scenario number, do the math and come up with a worst case runtime for the pack.

The Velineon figures he got for burst current are from the ESC ratings and include SOA. I did a full accelleration test on the VXL rustler and got an average number of 19.5 amps of draw in sport mode (75% current delivery). To depelte a lipo pack of 7.4 volts and 3200Mah to 60% took roughly 10 minutes and the lipo cutoff engaged at a standing voltage of 6.7 volts which is a bit high in my opinion so I think they need to work on that. Leaving the last 20% behind just isn't right! Thats again a worst case scenario though. Traxxas engineers still have not gotten back to me with the dyno wattage and I'm assuming they never will. For a company that supposedly engineers everything in-house, they sure don't have much information available on it. Go figure?

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Old 10-10-2007, 11:34 AM   #8
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. I just want to understand why someone (me) would buy an 11.1v with less amperage over a 7.4v higher amperage pack.

Thanks
Because in the world of brushless every volt you add equates to more RPM's. As long as the pack can discharge the current necessary to keep the motor moving under load the excess won't matter.

If the Velenion draws 25 amps of current and both your 7.4v and 11.1v pack can deliver it, every volt over the 7.4 volt pack will deliver more RPM's. And kids wanna go FAST!!

That should sum it all up.

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Old 10-10-2007, 12:52 PM   #9
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' Capacity = Runtime ' assigns capacity of a pack to a generalized outlook. Because your runtime will be dictated by load and pack capacity you cannot do this. So what manufacturers have done is assigned a load figure for you. They take the pack and calculate the max draw you can take from the cells for an hour. Then they assign that as your Mah figure.

Ie. 7.4 volts 5000mah pack will allow you to take 5 amps out of the pack for 1 hour. But if your load takes 10 amps you would only see a half hour use out of the pack. Understand?

Now the 'roof' or limit to how much you can take from the pack continuously is dictated by the discharge rating (C rating) For example if the above pack allows 4C discharge you can leech 20 amps from that pack and doing the math that gives you 15 minutes of pack runtime at that draw rate.

Now in an actual runtime enviroment temperatures of the pack, ESC and the load itself will vary and so will your final numbers so all the number crunching in the world won't help you. In the end what you can hope for is using the peak possible drain rate for your load as a worst case scenario number, do the math and come up with a worst case runtime for the pack.

The Velineon figures he got for burst current are from the ESC ratings and include SOA. I did a full accelleration test on the VXL rustler and got an average number of 19.5 amps of draw in sport mode (75% current delivery). To depelte a lipo pack of 7.4 volts and 3200Mah to 60% took roughly 10 minutes and the lipo cutoff engaged at a standing voltage of 6.7 volts which is a bit high in my opinion so I think they need to work on that. Leaving the last 20% behind just isn't right! Thats again a worst case scenario though. Traxxas engineers still have not gotten back to me with the dyno wattage and I'm assuming they never will. For a company that supposedly engineers everything in-house, they sure don't have much information available on it. Go figure?

- Matt
yes i understand but like you just stated going into capacity like you just did and already knowing that there are many variables as to being exact i didnt find the need to explain any farther. do 3800's have a longer runtime than 3300 on the same load (car/motor/esc/gearing/track)? do 4200's have a longer runtime than 3800 (again same car, motor, esc, gearing, track, track conditions)? on a stock motor my t4 can run for ~9 minutes on 3300's ~10 minutes on 3800 and ~11 on 4200 running pretty identical lap times until the pack fades and the larger capacity packs also fade later into the run....
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Old 10-10-2007, 01:18 PM   #10
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Originally Posted by R1R 7humbs View Post
amps * volts = watts
This might be what is miss-leading you. The "amps" in this is the current being drawn by you motor/speed control not the capacity of the battery.
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Old 10-10-2007, 02:04 PM   #11
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yes i understand but like you just stated going into capacity like you just did and already knowing that there are many variables as to being exact i didnt find the need to explain any farther. do 3800's have a longer runtime than 3300 on the same load (car/motor/esc/gearing/track)? do 4200's have a longer runtime than 3800 (again same car, motor, esc, gearing, track, track conditions)? on a stock motor my t4 can run for ~9 minutes on 3300's ~10 minutes on 3800 and ~11 on 4200 running pretty identical lap times until the pack fades and the larger capacity packs also fade later into the run....
If English is your second language you are excused from what I just tried like hell to read. Else if you plan to carry on a discussion with me, I have to ask you put more of an effort into conveying your thoughts because I truely had to read that several times to get the jist of what your trying to say. I am no grammar Nazi by any means, but what was written above is a literal disaster and I'm trying to carry on a conversation with you. Please for me?

What you state above has no relevance as a rebuttle to my statement. You made a brief statement first claiming 'capacity = runtime' and simply put that statement is untrue and misleading. Thats why I gave a rebuttle and went into explanation for everyones benifit, not just for arguments sake.

You further go on to make more misleading statements such as 'my t4 can run for ~9 minutes on 3300's' - and I will rebuttle that is also misleading because you make a blanket statement without regards to specs or test conditions. If I take a generic 3300 and apply it to your setup and go full throttle up a 60 degree grade until the batteries deplete will I get 9 minutes? I doubt it.

The sole purpose of my rebuttle was to bring more fact into a blanket statement before it became the next 'common knowledge' among a group of people regardless of how inaccurate it may be. When you present a statement at risk of being spread among a community you owe it to them to be detailed, thorough, and correct to the best of your belief.

Some may think this is out of line and I need to lighten up. But I have to deal with misinformation and misinterpretation on a daily basis. What I have come to learn is that when someone asks for your help they are looking to you as an authority on the matter. And dare you chime in with an answer you should BE an authority, ACT like an authority, and PRESENT your information as such.


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Old 10-10-2007, 02:39 PM   #12
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Thank you:
TechWun
RBMike
mugenlife
Turbo Joe

Yeah, I'm an electric noob but I've learned quite a bit over the last few weeks and from this thread in particular.

TechWun, I just recently came to realize what the kV stands for on a motor's specification (number of rpm's per volt - related to the number of "turns" - with lower "turns" giving a higher kV and less torque). So if I want to go as fast on a 7.4 pack I would need to change my motor to one with a higher kV than the stock velineon.

Thanks again.
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Old 10-10-2007, 03:15 PM   #13
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If English is your second language you are excused from what I just tried like hell to read. Else if you plan to carry on a discussion with me, I have to ask you put more of an effort into conveying your thoughts because I truely had to read that several times to get the jist of what your trying to say. I am no grammar Nazi by any means, but what was written above is a literal disaster and I'm trying to carry on a conversation with you. Please for me?

What you state above has no relevance as a rebuttle to my statement. You made a brief statement first claiming 'capacity = runtime' and simply put that statement is untrue and misleading. Thats why I gave a rebuttle and went into explanation for everyones benifit, not just for arguments sake.

You further go on to make more misleading statements such as 'my t4 can run for ~9 minutes on 3300's' - and I will rebuttle that is also misleading because you make a blanket statement without regards to specs or test conditions. If I take a generic 3300 and apply it to your setup and go full throttle up a 60 degree grade until the batteries deplete will I get 9 minutes? I doubt it.

The sole purpose of my rebuttle was to bring more fact into a blanket statement before it became the next 'common knowledge' among a group of people regardless of how inaccurate it may be. When you present a statement at risk of being spread among a community you owe it to them to be detailed, thorough, and correct to the best of your belief.

Some may think this is out of line and I need to lighten up. But I have to deal with misinformation and misinterpretation on a daily basis. What I have come to learn is that when someone asks for your help they are looking to you as an authority on the matter. And dare you chime in with an answer you should BE an authority, ACT like an authority, and PRESENT your information as such.


- Matt
hmmmmmm full throttle for 9 min? i was talking about LAP/RUN times on the same track. sure my ~#'s are generic but the run times are close. how do i know? because i spent a few races running against 1/10 nitro trucks in 15-20 minute mains at the local track.
i am sorry if you feel that when i say same conditions that you seem to think one will be running in 32 F heat and the other at over 100 F. the simple fact that one battery can store more current supports my argument (with the same vehicle AND motor). do you see racers run 10 turn and under motors with 3300's as often as you do now with 3800 and 4200? are the larger capacity packs running longer than 5 minutes? do they also run stronger in their 5 minute race? keep in mind i am talking about cells people actually use and not some fuddy duddy cells that arent main stream for racing.

"If I take a generic 3300 and apply it to your setup and go full throttle up a 60 degree grade until the batteries deplete will I get 9 minutes? I doubt it."

if i take a generic ib 3600 and then take a generic ib 4200 and run them up the same hill with the same set up, are you trying to tell me without there being anything wrong with the 4200 it wouldnt run longer and stronger over the test time? or are you going to say if one was run in the dead of winter and the other on the planet mars that it will show in favor of the 3600? lol

lipo per capacity (3800lipo vs 3800 nihm) has a longer run time but it is also a totally different make up. a lipo's voltage doesnt roll off as steep during discharge. also at the same time if you are talking about cells that have the same capability C wise with lipo again the higher capacity would still run longer. example http://www.maxamps.com/proddetail.ph...74-Pack&cat=13
http://www.maxamps.com/proddetail.ph...74-Pack&cat=13
and if this is from the same company that is doing the 5000 mah and the 8000 the 8000 can dieliver more current if needed in a crunch..... that would be one heck of a motor lol

Last edited by mugenlife; 10-10-2007 at 03:54 PM.
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Old 10-10-2007, 10:25 PM   #14
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hmmmmmm full throttle for 9 min? i was talking about LAP/RUN times on the same track. sure my ~#'s are generic but the run times are close. how do i know? because i spent a few races running against 1/10 nitro trucks in 15-20 minute mains at the local track.
i am sorry if you feel that when i say same conditions that you seem to think one will be running in 32 F heat and the other at over 100 F. the simple fact that one battery can store more current supports my argument (with the same vehicle AND motor). do you see racers run 10 turn and under motors with 3300's as often as you do now with 3800 and 4200? are the larger capacity packs running longer than 5 minutes? do they also run stronger in their 5 minute race? keep in mind i am talking about cells people actually use and not some fuddy duddy cells that arent main stream for racing.

"If I take a generic 3300 and apply it to your setup and go full throttle up a 60 degree grade until the batteries deplete will I get 9 minutes? I doubt it."

if i take a generic ib 3600 and then take a generic ib 4200 and run them up the same hill with the same set up, are you trying to tell me without there being anything wrong with the 4200 it wouldnt run longer and stronger over the test time? or are you going to say if one was run in the dead of winter and the other on the planet mars that it will show in favor of the 3600? lol

lipo per capacity (3800lipo vs 3800 nihm) has a longer run time but it is also a totally different make up. a lipo's voltage doesnt roll off as steep during discharge. also at the same time if you are talking about cells that have the same capability C wise with lipo again the higher capacity would still run longer. example http://www.maxamps.com/proddetail.ph...74-Pack&cat=13
http://www.maxamps.com/proddetail.ph...74-Pack&cat=13
and if this is from the same company that is doing the 5000 mah and the 8000 the 8000 can dieliver more current if needed in a crunch..... that would be one heck of a motor lol
Another beautiful and irrelevant rant..

I actually took the time to write up this long drawn out rebutle, and before I even posted it decided that if you can't retain what you read, interpret it properly, or entertain a legible and thought out discussion with me, that I'm not even going to waste my energy or mind on you. SO I deleted it and opted for this final statement and I will say nothing more on the matter until you can manage to legibly do any of the above stated things to entertain said discussion.

You stated:

'capacity = runtime'

I state here and now in short:

'capacity = industry standard measurment system based on amount of current that can be drawn from a battery in one hour's time and that it does NOT deviate between different battery technologies and although it can *assist* you in determining runtime it does not directly equate to runtime'

Everything else I stated was factual and pertinent to determining runtime using several factors to provide a general outcome. You have failed to disprove any of it, and are now wasting my time. If and when you can bring yourself to have this discussion with me, I would be happy to school you all day, every day and twice on sundays via PM.

The poster got what he wanted, it's time to bury the post and continue it in PM if you insist on being right..

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Old 10-11-2007, 09:59 AM   #15
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Another beautiful and irrelevant rant..

I actually took the time to write up this long drawn out rebuttal, and before I even posted it decided that if you can't retain what you read, interpret it properly, or entertain a legible and thought out discussion with me, that I'm not even going to waste my energy or mind on you. SO I deleted it and opted for this final statement and I will say nothing more on the matter until you can manage to legibly do any of the above stated things to entertain said discussion.

You stated:

'capacity = runtime'

I state here and now in short:

'capacity = industry standard measurement system based on amount of current that can be drawn from a battery in one hour's time and that it does NOT deviate between different battery technologies and although it can *assist* you in determining runtime it does not directly equate to runtime'

Everything else I stated was factual and pertinent to determining runtime using several factors to provide a general outcome. You have failed to disprove any of it, and are now wasting my time. If and when you can bring yourself to have this discussion with me, I would be happy to school you all day, every day and twice on -Sundays- via PM.

The poster got what he wanted, it's time to bury the post and continue it in PM if you insist on being right..

- Matt
you have not disproved it at all in fact the basic laws of physics agree with me. battery voltage and current relate to the charge (you cant disprove that)
v*c=power. only way you are "right" is if you change the other parts of the vehicle, drastically change the parameters of the test (which you already show that you try to do) and or use cells that one wouldn't normally use to run in an RC car.....not very hard to manipulate equations. show me a 3300 nihm that will run longer than a 4200. just one example with an equation lol. a lipo of the same mah as a Nihm on average has a longer balanced run time.
do you think explaining one part of a battery terminology down to it's roots is important? or do you think of what the end result/benefit would be as important? IMO the only reason to learn this is to understand the benefit but at the same time i think one would quickly learn that the batteries the industry uses for the discipline of RC that they do are generally well thought out where most of your mumbo jumbo wouldn't even come into effect to the degree you are wanting it to.
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