Diminishing Returns?
08-19-2009, 05:00 AM
#17
Buying a good quality/brand name pack is the only good answer to get a decent lipo.
Unheard of brands have nothing to loose and everything to ($) gain by C rating their packs higher than they actually are.
Buy cheap, buy twice I guess is the old saying....
Unheard of brands have nothing to loose and everything to ($) gain by C rating their packs higher than they actually are.
Buy cheap, buy twice I guess is the old saying....
08-19-2009, 07:58 AM
#18
Battery "C" ratings are a bit of a hype. Its like I remember in the 90's your home computer was "obsolete" every six months and everyone was upgrading to newest faster model. Few people actually step back and looked at what they used there computers for. Even now do you need a 5gig duo core processor with a high performance graphics card just to send emails and surf the net? So do you need a 35c lipo on a micro brushless motor?
Personally I have found that even my 5700kv motor doesn't strain my 25c SMC lipo's at all. I wouldn't drop below 20c constant though, just because below that, the battery temps start going up and the life of battery seems quite a bit less.
My personal experience with what brand of battery to buy. I use to buy Thunder power batteries but found that they didn't last any longer than my less expensive Flight Power batteries.
A reality check on price...MaxAmps 7.4/4000 40c lipo $132.95
Flight power 7.4/4000 30c/40c burst $32.18
I can buy 4 Flight power packs for the cost of 1 maxamps pack....I think I will take my chances and I have and never had a problem with them.
Personally I have found that even my 5700kv motor doesn't strain my 25c SMC lipo's at all. I wouldn't drop below 20c constant though, just because below that, the battery temps start going up and the life of battery seems quite a bit less.
My personal experience with what brand of battery to buy. I use to buy Thunder power batteries but found that they didn't last any longer than my less expensive Flight Power batteries.
A reality check on price...MaxAmps 7.4/4000 40c lipo $132.95
Flight power 7.4/4000 30c/40c burst $32.18
I can buy 4 Flight power packs for the cost of 1 maxamps pack....I think I will take my chances and I have and never had a problem with them.
Last edited by dhc8guru; 08-19-2009 at 08:11 AM.
08-19-2009, 08:10 AM
#19
Summarizing what I see above, if the battery is actually higher C with lower internal resistance Rs, then:
1. you get a bit more peak power on the infield with each reduction of Rs;
2. your battery lasts more time relative to a lower C rated battery;
3. you stay high on the voltage curve for a bit longer (delays power fade)
.......
To get more run time you need higher capacity ratings.
Is it worth the cost in stock racing? There are no $ signs in the equations of natural law, so that is a question open to moral debate.
Due to the high internal resistance of lower power stock motors, the benefit of a high C low Rs battery is a much longer run time, but not that much more peak power, if my intuition is right.
......
That said, you will understand now why a higher C rating battery does not necessarily mean longer run time. A higher capacity, yes (but not necessarily always either). As you have demonstarted a lower internal resistance for the battery makes all the difference. All else being equal, it would seem that a higher capacity battery would last longer but if the car draws say 100A constant, then a battery that can not deliver that current might actually last longer than one that can, simply because the weaker battery is going to limit the current drawn therefore slow the discharge rate. We would like to see battery internal resistance data when we buy batteries, but I have not found yet a manufacturer to give such figures. If you use a high end charger, then it may tell you.
08-19-2009, 08:29 AM
#20
To put these discharge rates, reguardless of capacity, into perspective:
20C = 3 minute runtime
25C = 2.4 minute runtime
30C = 2 minute runtime
35C = 1.71 minute runtime
40C = 1.5 minute runtime
50C = 1.2 minute runtime
As you can see there is a line of diminishing returns. There is a quite large difference between a 20C & 30C pack. Not nearly as much difference between 40C & 50C pack. Of course even the smallest change can be felt by some drives.
20C = 3 minute runtime
25C = 2.4 minute runtime
30C = 2 minute runtime
35C = 1.71 minute runtime
40C = 1.5 minute runtime
50C = 1.2 minute runtime
As you can see there is a line of diminishing returns. There is a quite large difference between a 20C & 30C pack. Not nearly as much difference between 40C & 50C pack. Of course even the smallest change can be felt by some drives.
08-19-2009, 08:36 AM
#21
SMC defines C rate as the max discharge rate that allows the pack to maintain 90% of it's capacity. This means a 5000mah 40C battery will still put out 4500mah before LVC when discharged at a 200amp load.
They say that the problem is that most companies don't follow their 90% rule. If you use an 80% rule, you can bump your C rates up and make your pack look better.
They say that the problem is that most companies don't follow their 90% rule. If you use an 80% rule, you can bump your C rates up and make your pack look better.
08-19-2009, 11:27 AM
#22
The problem with 'C' ratings is that they are total bollocks 
There is no standard for testing the ratings so each manufacturer can adjust things to claim whatever they like.
The important factor is the packs internal resistance, but I belive only SMC give a printout with that on it.
Skiddins
There is no standard for testing the ratings so each manufacturer can adjust things to claim whatever they like.
The important factor is the packs internal resistance, but I belive only SMC give a printout with that on it.
Skiddins
Nicely put skidderly doo daa
Malc TCCFM
08-19-2009, 11:32 AM
#23
I didn't think you (or the initial post) were considering a reversible ESC. Higher performance ESCs don't have reverse.
In your explanation I am not sure what Vbe is, but in principle you are right. On the other hand, good ESCs don't switch directly to reverse to avoid the problem you elaborate. They have a delay function which introduces a few seconds (on the V12R it is 3seconds) delay before you can go into reverse after applying the brake.
But the way I read it, the initial question is where is the expense of buying higher C rating batteries no longer justified for the purpose intended (i.e. what is the maximum C rating useable in R/C cars before wires become a limiting factor). I think (form personal experience) the wires can easily hold higher currents than any ESC on the market today can deliver, so I don't worry too much about it. Again from personal experience I don't think we use currents higher than 100A under normal conditions (save for accidents, etc).
That being said, there are other problems with pulsating high currents. Your theoretical explanation does not take into account the behaviour of the winding which is a coil and as such has a variable impedance depending on the frequency of the current (including the parasitic Emf). Parasitic reverse voltage spikes are managed by various methods (such as Schottky diodes, but again only on non-reversible ESCs) so hopefully some of that effect is alleviated but I wouldn't be able to put a figure on how efficiently.
And yes, on top of everything there is a lot of doubt regarding the C ratings manufacturers put on their batteries.
In your explanation I am not sure what Vbe is, but in principle you are right. On the other hand, good ESCs don't switch directly to reverse to avoid the problem you elaborate. They have a delay function which introduces a few seconds (on the V12R it is 3seconds) delay before you can go into reverse after applying the brake.
But the way I read it, the initial question is where is the expense of buying higher C rating batteries no longer justified for the purpose intended (i.e. what is the maximum C rating useable in R/C cars before wires become a limiting factor). I think (form personal experience) the wires can easily hold higher currents than any ESC on the market today can deliver, so I don't worry too much about it. Again from personal experience I don't think we use currents higher than 100A under normal conditions (save for accidents, etc).
That being said, there are other problems with pulsating high currents. Your theoretical explanation does not take into account the behaviour of the winding which is a coil and as such has a variable impedance depending on the frequency of the current (including the parasitic Emf). Parasitic reverse voltage spikes are managed by various methods (such as Schottky diodes, but again only on non-reversible ESCs) so hopefully some of that effect is alleviated but I wouldn't be able to put a figure on how efficiently.
And yes, on top of everything there is a lot of doubt regarding the C ratings manufacturers put on their batteries.
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