2S vs 3S run times?

[GO-RIDE.com]

I have a crt.5 brushless 2S that I'm going to convert to a 3S system.

The 2S battery that is recommended is a 4200. It is about the right size and weight for good vehicle balance. If I want to keep similar run times to a 2S 4200 what size of 3S battery will I need if I gear for the same 35mph top speed?

[BrianG]

When using the same motor and all around setup, going to a higher voltage will keep the same runtime but be faster. Actually, it may be a little higher because you won't be into the throttle as much.

The parameter solver is perfect for "what-if" scenarios like this. Just enter all your current data for the 2s setup, except the speed. The answer will be the speed. Then, clear everything and enter everything the same except change 2s to 3s and leave the pinion entry black. The answer will be the pinion you need.

Since you are keeping the same overall speed, runtime will be about the same since the motor is doing the same amount of work.

[GO-RIDE.com]

Here's a more detailed example:

Givens:
CRT.5
MambaMax
3500 kv motor (undecided on model)
3.8" truck tires
Driven on an indoor clay track

Setup 1:
Maxamps 4200 "2s" (220gm) (W 34mm x L 98mm x H 32mm)
18T pinion = 35mph

Setup 2:
Maxamps 3100 "3s" (215gm) (W 30mm x L 93mm x H 38mm)
12T pinion = 35mph

My questions are:
1. If both of these systems are driven at the same pace (ie: same lap times) would the voltage advantage of the 3100 3s allow it to last nearly as long as the amperage advantage of the 4200 2s?
2. Would the 3s voltage advantage give the car more torque to clear jumps out of corners even though it is geared for the same top speed as the 2s?

[Hickoryhead]

I am going out limb here so correct me if I am wrong Brian but the way I see it if you are running the same motor and ESC and the difference is gearing and batteries then no your run time will not be similar. Batteries are all made the same. The number of cells matters but a 2 cell vs a 3 cell 3500 mah battery will last the same amount of time. The difference will be in the voltage which can change your handling factor. ie jumps, cornering...

With this being said then:
1: no it wont. You changed the gearing factor which took away the voltage advantage you gained with the 3 cell. But it should run cooler. (I think)

2: No. Same as above.

Hope this makes since. I have been up for 38 hours so far.

[zeropointbug]

Correct ME, if I am wrong. But your runtime will go DOWN when running 3s over 2s setup... higher voltage will allow more current, and thus exponentially more power (volts x amps= watts). This is according to how anyone would drive a BL truck (handfuls of throttle)... however, if you hold a constant speed (say, to hold same speed for both 3s and 2s), or drive it the same as 2s, the 3s setup will have a longer runtime.

[GO-RIDE.com]

Originally Posted by zeropointbug

Correct ME, if I am wrong. But your runtime will go DOWN when running 3s over 2s setup... higher voltage will allow more current, and thus exponentially more power (volts x amps= watts). This is according to how anyone would drive a BL truck (handfuls of throttle)... however, if you hold a constant speed (say, to hold same speed for both 3s and 2s), or drive it the same as 2s, the 3s setup will have a longer runtime.

That was my theory, but the only indication I have is that the packs I quoted are nearly the same size and weight. I'm guessing they have the same # of lipo cells, they are just wired differently. One is a 3100 3s and the other is a 4200 2s.

[GO-RIDE.com]

For conversations sake, let's say I used my above example except that:

1. I optimized each motor for the voltage of the battery and thus used the same pinion for each setup to reach my 35mph top speed.
2. As stated before I would drive each car the same pace around the track.

Would this allow the 3100 3s to have as long, or nearly as long, of a run time as the 4200 2s?

[Hickoryhead]

I think it would come down to how you drive the car.

[BrianG]

Originally Posted by Hickoryhead

I am going out limb here so correct me if I am wrong Brian but the way I see it if you are running the same motor and ESC and the difference is gearing and batteries then no your run time will not be similar. Batteries are all made the same. The number of cells matters but a 2 cell vs a 3 cell 3500 mah battery will last the same amount of time. The difference will be in the voltage which can change your handling factor. ie jumps, cornering...

With this being said then:
1: no it wont. You changed the gearing factor which took away the voltage advantage you gained with the 3 cell. But it should run cooler. (I think)

2: No. Same as above.

Hope this makes since. I have been up for 38 hours so far.

Maybe I should have been more clear.

Yes, if you go from 2s to 3s on the same exact setup, runtime will be lower. Of course, the car will go faster as a result. But, if you only apply the throttle so it is as fast as the 2s setup, runtime will be about the same, minues any difference due to the added battery weight.

If you changed the motor so you get the same total rpm with 3s as you did with 2s, then runtime will go up. If you keep the same motor, go to 3s, but reduce gearing, runtime should go up. It's all a matter of the amount of work the motor is forced to do.

also you have to remember that the 3100mah 3s battery has 34.41 watt/hours of power and the 2s 4200 has only 31.08.

which to me means if you were to gear down so that the top speed was the same with the 3s pack and you were not running the motor at too high an rpm as to the extent of not effecting the efficiency of the motor significantly then i suggest that you would get an increase in runtime with the 3s pack.

[aqwut]

BrianG & Chilledoutuk... that's exactly what I was thinking....

[GO-RIDE.com]

Originally Posted by chilledoutuk

also you have to remember that the 3100mah 3s battery has 34.41 watt/hours of power and the 2s 4200 has only 31.08.

which to me means if you were to gear down so that the top speed was the same with the 3s pack and you were not running the motor at too high an rpm as to the extent of not effecting the efficiency of the motor significantly then i suggest that you would get an increase in runtime with the 3s pack.

This is what I was wondering. Essentially, the 3100mah x 11.1 volt battery does store a slight bit more energy (123,876 joules) vs the 4200mah x 7.4 volt (111,888 joules).

So, if you took two equally efficient "S" can motors that had the appropriate KV rating to top out at around 35,000 rpm (4730kv = 2S and 3153kv = 3S), and the same pinion then you should have very similar run times between the two systems?

I'm then guessing that if you choose to go for more punch and use an "L" can motor with a larger pinion on the 3S system you will start to see reduced run times?

[lincpimp]

Load will be the important variable in going from a S can to a L can. If you can gain effecientcy by using the higher torque of the L can you would see a difference between the two. If you vehicle is only going to load the motor to a specific output, finding a motor that can produce that amount with the highest effecientcy will be the only method of increasing runtime.

From personal experience, the higher voltage systems appear to produce more power when under heavy loads. This is most likely from the battery being able to cope with the power needs better due to less amp draw with higher voltage. Voltage does sag under load, and a lower voltage battery will require a large capacity to be able to produce the necessary power and hold the voltage level.

Vehicle weight and application will determine the best motor choice, however a cheaply made motor will continue to heat up during use and lose effecientcy during that time. Plus a higher end motor will produce more power and torque.

You will find that a higher kv motor such as the feigao s can will heat up faster on lower voltage than a lower kv motor on higher voltage. If geared for the same top speed and both motors running at the same rpm the higher voltage system will react better. I say this with the current battery technology, as even the best batteries operate better at lower current draws.

In order to maintain the same weight the number of cells must be balanced with the weight of each cell. This usually leads to the use of lighter, lower capacity cells to make up the higher voltage pack while retaining the same weight.

The main concern I see with Brushless is heat. And higher voltages just seem to produce less heat.

[Finnster]

One other consideration is that running a cheap motor on partial throttle kills the efficiency. You may not gain much runtime as you are just burning it up as heat. Misgearing a cheap motor is also bad.

Don't be stuck on the same motor. Pick what voltage you want to run on (2s or 3S) and then match the motor to the battery. Ie if you want have 2S/3500kv combo (24500rpms) then a 3S batt w/ a 2300kv motor would be about the same spd, but will run cooler and longer due to higher efficiency.