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Advanced Lipo Voltage question

Advanced Lipo Voltage question

Old 04-24-2015, 08:08 AM
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If you were having current draw issues, then you would also see heat issues. You never mentioned a problem with your vehicle outside of trying to get better battery longevity. Balancing your battery packs are probably best if you are referring to the life cycle of your batteries. If you are referring to run time, then most of that is going to be from your gearing. It sounds like you think you have a problem and you never really stated one outside of batteries swelling. You probably have one of the most advanced electronic speed controls in your vehicle and having your brother do anything else is not going to make it better. These are competition race vehicles, not eco-hybrids made to go long distances or times. I assume you are trying to be use the same 2s packs across the 3 vehicles in your signature. Buy a dedicated 4s packs for the 1/8th scale and I bet you will not see the swelling occurring on it.
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Old 04-24-2015, 05:55 PM
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Originally Posted by jmackani
If you were having current draw issues, then you would also see heat issues. You never mentioned a problem with your vehicle outside of trying to get better battery longevity. Balancing your battery packs are probably best if you are referring to the life cycle of your batteries. If you are referring to run time, then most of that is going to be from your gearing. It sounds like you think you have a problem and you never really stated one outside of batteries swelling. You probably have one of the most advanced electronic speed controls in your vehicle and having your brother do anything else is not going to make it better. These are competition race vehicles, not eco-hybrids made to go long distances or times. I assume you are trying to be use the same 2s packs across the 3 vehicles in your signature. Buy a dedicated 4s packs for the 1/8th scale and I bet you will not see the swelling occurring on it.
What you say is probably true of most RC users.

But I do have dedicated batteries for each model. The Dex408T actually runs on 2x2S, and I have 3 sets of batteries which are always paired together--they're run together and charged/balanced together. (The desc410 uses saddle packs, and I keep 3 sets of those as well. And the dest210 has a couple old gens ace packs all to itself!)

There is no reason why the motor should be hot when the batteries are strained. A 1C battery pack would get destroyed by a truggy motor. The motor might get hot because of the flames, but not from electricity running through it.


Run times have been excellent with the standard LVC. Around 15 minutes, pushing it hard on the track. It's a 7200mAh @ 4S, after all...

By battery longevity I mean charge/discharge cycles.

The problem is the % voltage drop under load. This indicates the batteries can't handle the current draw. And this will dramatically reduce the life of the batteries.

Another problem is having to buy new batteries every season. These batteries don't "just" go bad...they get abused. I want to get the best performance out of them without ruining them.
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Old 04-24-2015, 07:19 PM
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Originally Posted by fredygump
This is my dex408t with tekin rx8 and t8 2250 kv.

I have been trying different gears. Last year I used a 17t pinion. Motor temps were like 140 or 150 most of the time. The local track is a modest 1/10 dirt track, so quite small for truggy--it's constantly twisting and turning, so no opportunity to get any speed.

Last year I managed to strip out the fancy spiral cut diff gear (rear), so I decided maybe I should increase the gearing a little to reduce the strain. I've run 18,19, and 20 this year. Temps are pretty low, but weather is still pretty cool around here..
I run 14:45 in a Losi truggy on 4S with a 1800KV motor and a 5300 mAHr battery and easily run 10 minute races with no battery or motor heat issues. Nor am I slow - having beat most nitros and other electrics I run against.

You tell us you are using a 2250KV motor and pinions ranging from 17 to 20?

It seems obvious to me that the cause of your battery and voltage sag issues is that you are overgeared.
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Old 04-24-2015, 10:21 PM
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Originally Posted by ta_man
I run 14:45 in a Losi truggy on 4S with a 1800KV motor and a 5300 mAHr battery and easily run 10 minute races with no battery or motor heat issues. Nor am I slow - having beat most nitros and other electrics I run against.

You tell us you are using a 2250KV motor and pinions ranging from 17 to 20?

It seems obvious to me that the cause of your battery and voltage sag issues is that you are overgeared.
It is not over-geared. It just isn't.

When I was testing yesterday, motor temp got up to maybe 110 degrees after 5 minutes, and that was with the 20t pinion.

Your comment does help, though. The difference between our trucks is easy to explain--you have a 1800kv motor, while I have a 2250kv motor!


I'm sure you already know that low KV = high turns, and high KV = low turns. Motors with higher KV (fewer turns) have larger diameter windings that are shorter in length, which results in less resistance to current flow. So higher KV (lower turn) motors draw more current than a similarly sized motor with a lower KV. (But the lower KV motor can produce the same power (watts) if voltage is increased appropriately. Low KV wants volts, high KV wants amps...)

A couple posts ago the suggestion was made that I might have the wrong motor, and what I gather from your post is that you have had good success with a much lower KV motor. That is helpful!

What motor are you using? I was browsing the various choices today, but it is hard to know how they all compare. There is a Tekin 2000KV also recommended for 4S, but that seems almost the same. Or I could run a buggy motor at a similar KV?
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Old 04-25-2015, 08:04 AM
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Default You are over geared!

Originally Posted by fredygump
It is not over-geared. It just isn't.

When I was testing yesterday, motor temp got up to maybe 110 degrees after 5 minutes, and that was with the 20t pinion.

Your comment does help, though. The difference between our trucks is easy to explain--you have a 1800kv motor, while I have a 2250kv motor!


I'm sure you already know that low KV = high turns, and high KV = low turns. Motors with higher KV (fewer turns) have larger diameter windings that are shorter in length, which results in less resistance to current flow. So higher KV (lower turn) motors draw more current than a similarly sized motor with a lower KV. (But the lower KV motor can produce the same power (watts) if voltage is increased appropriately. Low KV wants volts, high KV wants amps...)

A couple posts ago the suggestion was made that I might have the wrong motor, and what I gather from your post is that you have had good success with a much lower KV motor. That is helpful!

What motor are you using? I was browsing the various choices today, but it is hard to know how they all compare. There is a Tekin 2000KV also recommended for 4S, but that seems almost the same. Or I could run a buggy motor at a similar KV?
I checked one of Lutz's setups for this truck and he is running a 2000KV motor on a much much larger track than you most likely are and he is running a 20T pinion. If you account for the fact that you are running a 2250KV motor on a much smaller track then you most likely should be running a 16-18 Tooth pinion max.

It is great to do testing but understand the capabilities of the equipment we have to work with and keep in mind the pros are running their equipment to the max capabilities already... learn from them and make wise decision...

Just my 2cents worth.
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Old 04-25-2015, 10:03 AM
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Originally Posted by JFuel11
I checked one of Lutz's setups for this truck and he is running a 2000KV motor on a much much larger track than you most likely are and he is running a 20T pinion. If you account for the fact that you are running a 2250KV motor on a much smaller track then you most likely should be running a 16-18 Tooth pinion max.

It is great to do testing but understand the capabilities of the equipment we have to work with and keep in mind the pros are running their equipment to the max capabilities already... learn from them and make wise decision...

Just my 2cents worth.

Hey, I don't want to miss out on that you are thinking. You say "understand the capabilities of the equipment"...that is the question I'm asking.

I am observing a mismatch in my equipment. And I'm trying to correct it before I completely ruin my batteries. So yes, I am trying to figure out what the capabilities of my equipment actually are. The manufacturers aren't being very helpful--Randy Pike from Tekin, for example, says that posting specs on their motors is "confusing". He said you should just pick the "KV" you "want" and forget about it.

It's more complicated than "just" a pinion gear, "just" a "KV" rating, which varies between brands...or the stupid inflated "C" ratings on batteries. Did you know that my SMCs are theoretically capable of delivering 500amps continuously? Danny at SMC calls them "Factory Spec", because the 70C rate is what his supplier claims. But he says that the actual discharge rates on the best lipos are only 25-30 amps.


So I ran a 17T last season. That is the middle of your estimate. This performed well in a variety of conditions, but the torque on the drive train damaged the rear differential gear, a gear which was specially designed to handle the power of 1/8 truggy.

Last season I tried a 15T gear. At max motor RPM I could get around the track pretty well. But it was slow. And jumping was a problem, because there was no extra rpms to make corrections in the air. Going up to 17T was a big improvement, but it is still a conservative gear. You wouldn't believe how conservative I am with gearing.

(In my dest410, I am running an 11T...but a guy I know with the same truck and same motor is running a 15T...he thought I was an idiot for running such a low gear. He is the guy with two tracks in his back yard, an AMB timing system, and he runs a racing series...)

I made a conservative increase this spring, trying 18T, 19T, and I made one run at 20T.

Everyone talks about motor temps with religious fervor, as though it is the only indication of what gear ratio to use. And judging by temperature, I am well under geared. My ESC temp info indicates max temps of 120-130 degrees, when the motor can take around 200 degrees.


Is there a way to make my equipment play nice together, other than change the gearing, which doesn't solve the problem? The more I look into it, the more convinced I am that I need to replace something. Either I need a smaller/lower kv motor, or batteries that can handle the current motor better. Which reminds me, SMC has 4S shorty packs--maybe I could run two of them in parallel? That would be fun. It would change the weight balance of my car, but they wouldn't care what motor I was running...
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Old 04-25-2015, 01:09 PM
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Gear diffs are not as low maintenance as they may appear. It's unlikely the motor torque damaged the diff gear if it was properly maintained. Gear diffs need to be shimmed properly, and not just once. The mesh, lube, bearings, even the diff housings, etc, need to be checked and maintained regularly for trouble free performance.

There may be some confusion, perhaps just a typo, between discharge amp capability and discharge C values. Typical lipo packs can handle well above 25-30 amps. Perhaps Danny was referring to 25-30C for the pack. Most of the lipo cells actually can handle quite a bit more than that, the pack connections are normally the limit. The more capable cells still provide a benefit in maintaining voltage.

Here is a graph of a 50C rated cell from 5 years ago being discharged at up to the full rating with no ill effects. 110 amps from a smallish 2200mAh cell. Of course I'm not saying all brands use legitimate ratings, but the good cells are capable of rather amazing performance.

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Old 04-26-2015, 06:12 PM
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Originally Posted by fredygump
It is not over-geared. It just isn't.

When I was testing yesterday, motor temp got up to maybe 110 degrees after 5 minutes, and that was with the 20t pinion.

Your comment does help, though. The difference between our trucks is easy to explain--you have a 1800kv motor, while I have a 2250kv motor!


I'm sure you already know that low KV = high turns, and high KV = low turns. Motors with higher KV (fewer turns) have larger diameter windings that are shorter in length, which results in less resistance to current flow. So higher KV (lower turn) motors draw more current than a similarly sized motor with a lower KV. (But the lower KV motor can produce the same power (watts) if voltage is increased appropriately. Low KV wants volts, high KV wants amps...)

A couple posts ago the suggestion was made that I might have the wrong motor, and what I gather from your post is that you have had good success with a much lower KV motor. That is helpful!

What motor are you using? I was browsing the various choices today, but it is hard to know how they all compare. There is a Tekin 2000KV also recommended for 4S, but that seems almost the same. Or I could run a buggy motor at a similar KV?
Maybe you are over-geared and maybe you are not.

I used the speed calculator at http://scriptasylum.com/rc_speed/top_speed.html and it shows you were geared for about a 50% higher top speed with your combo than I am with mine. Since I am as fast on the track as all the other trucks (electric and nitro truggys) I race against, being geared for a 50% higher top speed sounds like being over-geared.
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Old 05-07-2015, 05:45 PM
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Originally Posted by ta_man
Maybe you are over-geared and maybe you are not.

I used the speed calculator at http://scriptasylum.com/rc_speed/top_speed.html and it shows you were geared for about a 50% higher top speed with your combo than I am with mine. Since I am as fast on the track as all the other trucks (electric and nitro truggys) I race against, being geared for a 50% higher top speed sounds like being over-geared.
After doing a little testing last week, I found that in a lap of the local track, my motor RPM occasionally spikes to nearly 60K, which is the limit set in the ESC. This was with a 19T pinion. So I am using the full RPM range of the motor, without going over, which I think is a good sign. Motor temps are good--160 degrees after 20 minutes running practice laps at a ~90% pace. Ambient temp was 80 degrees.

Another consideration is that I always run on outdoor dirt tracks.
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Old 05-07-2015, 06:32 PM
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Back to the "lipo" part of this post..

I purchased a "Watts Up" current meter and did a couple brief tests today before it rained. This is with my dex408T, 4S, Tekin 2250kv



On gravel, driving similar to how I would drive on the local track, I got a peak current draw of 45 amps.

Driving through freshly mowed grass at full throttle resulted in a peak of 75 amps.

And full speed run on asphalt resulted in a peak of 80 amps.


These numbers are quite low compared to the ratings of Lipo batteries. My 70C 7,200mAh batteries technically have a 500amp continuous discharge rate, with up to 1000amp peak draw.

But my motor's peak current was measured at 80 amps, which is only 11.1C draw from my batteries.

And the thing is that running at the track should be quite close to that 45amp peak--lets call it 50amps. That translates to only "7C"!

So my initial conclusion is that the "truth" is that my batteries are being pushed (or rather, being pulled) to their limit. And that limit is ~7C. This is 10% of the pack rating, and it is only 25% of the theoretical 25-30C limit of lipo technology, as stated by Danny of SMC.

I am a little surprised that the numbers are so different than all the claims. I knew the claims were grossly in error, but I'm surprised to find out how far off they really are. I'm surprised that a 7C load is too much for my batteries, which are the no longer available SMC "race spec" 70C 7,200mAh batteries.


Now, I know that these batteries are as good or better than the others, but... I'm a little disappointed right now.

My second conclusion is that it appears that yes, my motor is too big for the batteries.

(Also worthy of noting is that with this 50-80 amp load, my ESC is operating near it's maximum. Data logging shows that it's internal temp gets up to and holds steady at 180 degrees, with the fan running continuously. So the ESC is also over-rated. I did a little research, and what did I find? It turns out that the 220amp (per phase) spec is true but deceptive. What most people don't realize is that there are actually two current measurements for brushless motors, battery current and motor current. Motor current is significantly higher than battery current for some complicated physics reasons I don't understand. So the 220A "per phase" is a trick. Advice I have found suggests that it is relatively safe infer an ESC's battery current capacity of 50% of the rated motor current. This would make the Tekin RX8 a 110 amp ESC... But it does not seem likely that it is designed to run continuously at 110 amps. Maybe the 'fets can handle the current, but the heat sink is too small to dissipate the heat that kind of continuous load would generate....)
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Old 05-08-2015, 07:03 PM
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It appears you may be misunderstanding the data logging.

You wrote:
Originally Posted by fredygump
Back to the "lipo" part of this post..

I purchased a "Watts Up" current meter and did a couple brief tests today before it rained. This is with my dex408T, 4S, Tekin 2250kv
Before that you wrote:
Originally Posted by fredygump
After doing a little testing last week, I found that in a lap of the local track, my motor RPM occasionally spikes to nearly 60K, which is the limit set in the ESC.
Peak voltage on a 4S pack is 16.8V. Even if the pack held that peak voltage, there is no way a 2250KV motor can get to 60K RPM on a 4S pack.

RPM = KV * V = 2250 * 16.8 = 37,800 max RPM of a 2250KV motor on a 4S pack with no voltage drop.
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Old 05-08-2015, 08:11 PM
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You've stated that you broke a rear gear due to your gearing, and you went up to compensate. Since a brushless motor is capable of producing max TQ@0rpm, you have just increased the overall load on the motor. It does sound like you are over geared. Now rather than being able to turn the driveline, you have effectively made the motor have to work harder to drive the wheels. The more the motor has to work to overcome the gearing, the more current it pulls.
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Old 05-08-2015, 08:21 PM
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Originally Posted by ta_man
It appears you may be misunderstanding the data logging.

You wrote:


Before that you wrote:


Peak voltage on a 4S pack is 16.8V. Even if the pack held that peak voltage, there is no way a 2250KV motor can get to 60K RPM on a 4S pack.

RPM = KV * V = 2250 * 16.8 = 37,800 max RPM of a 2250KV motor on a 4S pack with no voltage drop.
I know nothing about RX8s but if it's set-up for two pole and he's using a 4 pole, it will read 2x the rpm. Since that's what your calculations come up with, I wonder if that's what is going on?
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Old 05-08-2015, 09:12 PM
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Originally Posted by ta_man
It appears you may be misunderstanding the data logging.

You wrote:


Before that you wrote:


Peak voltage on a 4S pack is 16.8V. Even if the pack held that peak voltage, there is no way a 2250KV motor can get to 60K RPM on a 4S pack.

RPM = KV * V = 2250 * 16.8 = 37,800 max RPM of a 2250KV motor on a 4S pack with no voltage drop.

Okay, yeah, that is the formula. But in this instance, that kind of math is useless.

The entire point of this thread is acknowledging that common knowledge of RC is not entirely factual. Everything is simplified and generalized to make it less confusing. Randy Pike even said that they are not listing certain specs on their gen2 motors because they have found that it is too confusing to their customers.


My goal is to go beyond the fluffy curtain of "pretend" knowledge and learn the actual facts of what my electronics are doing. And to quote a signature from a guy in a different forum, "False knowledge is worse than ignorance."

So away with the false knowledge, and in with the facts. You are dead wrong about RPM, but so was I. My max rpm is actually 65,000, not 60,000.

But on the race track, my max was ~58,000. I will check it again next time I go to the track, if you need me to confirm this number for you as well.

(Pssst Notice how the ESC temp DROPS when the vehicle is at full throttle? Interesting, huh? Bet you didn't know that would happen.)
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Old 05-08-2015, 09:20 PM
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Temp will drop at top speed because there is less of a load on the electronics. It takes more energy to change speed than it does to maintain speed. Same reason a car gets better mpg cruising on the highway than it does in stop and go traffic. I see what youre trying to do, but you seem to be ignoring facts based on real experience for how you think they should work.
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