3s lipo vs 2s lipo run times?

[Sapes]

Tonight I put a 3s battery in my Vaterra Kemora and it's just way out of control. I had to dial the throttle back to 25% to make the car manageable. Now in theory, I should get longer run times with a 3s run at 25% versus a 2s run at 75% of the same mah capacity, correct? The larger voltage causes a lower amp demand. So to go a set speed the 3s would use less amps? Also would I have to worry about higher temps since I'm only asking 25% of what it's capable of? I need to order batteries and was wondering if it was beneficial to run 3s vs 2s?

[6sharky9]

If you go 3s power be prepared to replace a lot of your drivetrain as they will not hold up long to the power.

If you go 3s you don't need to gear down as much for torque..but again you may break parts.

Not sure about if a 3s with the same Mah lasts longer compared to a 2S but ive always though capacity and how much throttle you used during the duration of use would determine that.

I ran a 4000mah 2s and got about an hour if not slightly over just playing around my house or a park...maybe someone else can answer that one for you.

[wings_of_fire]

To put the problem in a mathematical form we will fix some variables like cell voltage, power usage and time.
1] Cell voltage = 3.2
2] power draw of rig = 50 watts continiously
3] Time = 1 hour
4] Battery mAh = 5000


So to find out the power in watts inside each lipo pack we will have to multiply total voltage by battery capacity....
For 2 cell lipo
Total pack voltage 3.2 volt x 2 = 6.4 volt
Converting milliamps to Ampere - 5000mAh = 5000/1000= 5 Ampere
Now we need to find power - 5 Ampere x 6.4 volts = 32 watts

For 3 cell lipo
Total pack voltage 3.2 volt x 3 = 9.6 volt
Converting milliamps to Ampere - 5000mAh = 5000/1000= 5 Ampere
Now we need to find power - 5 Ampere x 9.6 volts = 48 watts

Now we will consider a situation where the rig is pulling a continous power of 50 watts for 1 hour

2 cell lipo - 32watts /50 watts = 0.64 hour = 38.4 mins

3 cell lipo - 48watts/50watts = 0.96 hour = 57.6 mins

Hence a 3 cell lipo with the same mAh capacity will have more run time if the power draw is exactly the same while comparing to a 2 cell lipo.

[Sapes]

Originally Posted by wings_of_fire

To put the problem in a mathematical form we will fix some variables like cell voltage, power usage and time.
1] Cell voltage = 3.2
2] power draw of rig = 50 watts continiously
3] Time = 1 hour
4] Battery mAh = 5000


So to find out the power in watts inside each lipo pack we will have to multiply total voltage by battery capacity....
For 2 cell lipo
Total pack voltage 3.2 volt x 2 = 6.4 volt
Converting milliamps to Ampere - 5000mAh = 5000/1000= 5 Ampere
Now we need to find power - 5 Ampere x 6.4 volts = 32 watts

For 3 cell lipo
Total pack voltage 3.2 volt x 3 = 9.6 volt
Converting milliamps to Ampere - 5000mAh = 5000/1000= 5 Ampere
Now we need to find power - 5 Ampere x 9.6 volts = 48 watts

Now we will consider a situation where the rig is pulling a continous power of 50 watts for 1 hour

2 cell lipo - 32watts /50 watts = 0.64 hour = 38.4 mins

3 cell lipo - 48watts/50watts = 0.96 hour = 57.6 mins

Hence a 3 cell lipo with the same mAh capacity will have more run time if the power draw is exactly the same while comparing to a 2 cell lipo.

I understand what you are saying. P=ExI, Ohms law. So I guess my question is would the motor have a lower continuious watt demand if the motor output was limited to 25%(by adjusting the throttle trim) as opposed to 75%? Does adjusting throttle trim effect the continious watt demand? Or does a motor have a given power draw depend on number turns and wire size of the internal windings and it doesn't care about throttle imput?

[Espeefan]

The math only tells half the story. The above example used fixed numbers for the ease of calculating and comparing the statistics, but the real world will have more variables to account for.

I say if you do not require a 3S LiPo pack, then you are better off running a 2S LiPo instead. It will be a more efficient system.

[shakemandril]

Originally Posted by Espeefan

I say if you do not require a 3S LiPo pack, then you are better off running a 2S LiPo instead. It will be a more efficient system.

Efficiency is going to be dependent on the motor windings and gearing just as much as voltage, right?

[6sharky9]

Originally Posted by Espeefan

The math only tells half the story. The above example used fixed numbers for the ease of calculating and comparing the statistics, but the real world will have more variables to account for.

I say if you do not require a 3S LiPo pack, then you are better off running a 2S LiPo instead. It will be a more efficient system.

I agree with using a 2s...If your pocket book will allow all the upgrades you need to run a 3s then go for it..Even if it does it never made sense to me personally to over power something ...Too much is not necessarily a good thing.
I run an SCX10 and im confident enough in saying I think I could keep up with any SCX10 using a 3s and a more powerful motor with all those expensive bling upgrades. And I run a stock ESC and a 27T mod motor using a 2s lipo.

They may out pull me with traction in a tug o war or pull a sled farther but that's not a concern to me as that's not what I built the truck for...If I did I would consider that added power of a 3s and better motor and hundreds of dollars in upgrades I would need to handle it somewhat reliably.

We all get what we like or want in the end no matter what most say but its what you will be happy with that counts..

[Johnnysplits]

wings of fire...lipo's are 3.7 nominal...where are you getting 3.2 from?

[wings_of_fire]

Originally Posted by Sapes

I understand what you are saying. P=ExI, Ohms law. So I guess my question is would the motor have a lower continuious watt demand if the motor output was limited to 25%(by adjusting the throttle trim) as opposed to 75%? Does adjusting throttle trim effect the continious watt demand? Or does a motor have a given power draw depend on number turns and wire size of the internal windings and it doesn't care about throttle imput?

Throttle trim regulates only voltage given to the motor and voltage automatically controls speed of motor. The other part is current. But then who controls current draw?...its Load that controls current draw.
If in a situation a single motor demands 50 watts of power for work to be done, then there can be a multiple of possibilities.
1] You give the motor a voltage of 6.4 volts and it will automatically draw 50watts/6.4 volts = 7.8125 Ampere current to balance out with the required power of 50 watt to get the work done.
2] You give the motor a voltage of 1.6 volt (25%) and it will automatically draw 50watts/1.6 volt = 31.25 Amphere current to balance out with the required power of 50 watt to get the work done.

What ever throttle trim you keep the power requirement to do a certain work will have to be given no matter what. If you are trimming down the throttle, the Speed of work will be reduced (time will increase), but it will eventually take the same amount of power to do that work if using full throttle.
-------------------------------------------------------------------------
Adjusting throttle trim will effect the continious watt demand. But Time will increase or decrease.

watt demand = energy needed to do work and this energy has to be given no matter what. Either you give this energy slowly or you give it fast and get the work done. At the end the power demand in watt remains same to do that particular work.

------------------------------------------------------------------------
Above I have said load controls current draw and there is more complexity to it.
Yes. Motors have a given power draw dependent on magnetic flux which is created by magnet, number of turns and wire size of internal windings. But then this magnetic flux determines at what point the motor will stall. When a motor approaches a stalling point, only then it starts drawing more ampere.

Take two differently constructed motors doing the same work. First motor stalls at say 6.4 volts @ 5 Amps and the other stalls at 6.4 volts @ 6 Amps. Here you have kept the throttle constant giving it 6.4 volts. The rate of time at which these two motors will do work will differ even though your throttle is constant.

[wings_of_fire]

Originally Posted by Johnnysplits

wings of fire...lipo's are 3.7 nominal...where are you getting 3.2 from?

Considering least working voltage.

[dentonmac]

Wow, I actually understood that.

I noticed that I got an extra 15min out of my 4s 850 pack compared to 3s 850 pack (same c rating/brand/type and driving style). I was curious why...now I know.

Thanks for the explanation in a way this numbskull could understand.

[crazy nick]

I was always told to run the highest cell count to lowest kv motor to match your driving needs. A 1900kv on 4s will be way more efficient than a 2650kv on 3s. The 4s combo will run much cooler temps and longer runtimes. Now I have learned this from 1/8 scale buggy racing. Whenever we were allowed (which wasn't often) we would run 1700kv on 5s 6000mah and make a balls out 20 minute main. I couldn't with a 6400mah 4s. I think I made 18 minutes once.

[Johnnysplits]

A lot of BL motors have voltage limits though

[JesterSpec]

I'll continue to run 3 cell in my 13t crawlmaster. I love the torque it makes with the Top speed that is always there when you need it. I've ran 2 cell, and to me its the difference between a corvette to a Geo Metro.

[Meatwad]

When the voltage is changed, gearing must change accordingly. The gearing change could potentially cause less current draw, potentially giving slightly longer run times. However, mah is mah...

[6sharky9]

Originally Posted by JesterSpec

I'll continue to run 3 cell in my 13t crawlmaster. I love the torque it makes with the Top speed that is always there when you need it. I've ran 2 cell, and to me its the difference between a corvette to a Geo Metro.

What Mah do you use? and how much run time out of it would you say get on an average bash.?

[JesterSpec]

2200 mah and I ran for 2 hrs flat at our end of year event. That was straight driving with some winching. Keep in mind these were brand new batteries also... value hobbies 30c 2200 (450 class heli batteries)

[idaho]

Originally Posted by dentonmac

Wow, I actually understood that.

I noticed that I got an extra 15min out of my 4s 850 pack compared to 3s 850 pack (same c rating/brand/type and driving style). I was curious why...now I know.

Thanks for the explanation in a way this numbskull could understand.

I'm guessing he's either a teacher or professor, I asked question once got a million different answers and here comes wings_of_fire with a perfectly understandable yet precise answer and a calculator to do the work for me.

[6sharky9]

Originally Posted by JesterSpec

2200 mah and I ran for 2 hrs flat at our end of year event. That was straight driving with some winching. Keep in mind these were brand new batteries also... value hobbies 30c 2200 (450 class heli batteries)

Im guessing a huge difference between types of vehicles, weights and gearing, tire sizes.

Ive run a 4000mah 2s in my 6lb scx10 and cant get longer than basicly an hour running a less powerful motor (27T) with 16/87 gearing and 1.9 tires

Would be hard to swallow thinking all things the same but one battery being a 3s will make that big of a difference though.

I just don't run a 3s because I don't personally feel I need more power than I already have to do the things a trail truck is capable of and don't want it to become a money pit replacing the drivetrain to handle the added power.

But most impressive to get 2hrs from 2200mah non the less.

[Johnnysplits]

Originally Posted by 6sharky9

Would be hard to swallow thinking all things the same but one battery being a 3s will make that big of a difference though

Oh but it does my friend. I started with a 2s in my wraith. I'm now running it on 4s. Now the only drawback to this was dead slow speed smoothness. I run a 13.5T sensored BL and I will say that it was very smooth on 2s compared to 4s but I wanted the top end of the 4s so I sacrifice just a little of the low end. I also have it geared way down.