voltage vs mah

[abc123]

If you have 2 7 cell's one with 2000mah and other with 4000mah will it be the same performance?Or the 4000mah one will perform better?Because of more mah.

[vonclod]

4000 should give more amps + runtime

[DVO]

The higher MaH pack will also, sometimes, have a higher discharge rate... Meaning it can handle being pounded on better or bigger loads like high kV motors or multiple servo's...

[Dukk]

A sub-C has a discharge rate of something like 15C I think.  This means a 2000mAh pack can discharge up to 30amps while the 4000mAh pack can discharge up to 60amps.

If you're ESC/motor combo demands, say 45 amps, what happens is the 2000mAh pack delivers it's 30amps and then the voltage starts to drop off fast.  Consequences are the car won't go as fast as the 4000mAh pack because of the lower voltage, the ESC will heat up, you may get a low voltage glitch out of your receiver (like it won't steer on hard throttle), and the battery will really heat up as it is seeing essentially a dead short when the current demand is more than 30amps.

Always buy as many amps as you can afford.  

My personal guidelines  for 1/10 are:

Stock motors = at least 50amp capability
SuperStock motors (17.5 brushless, 15turn and up brushed, etc) = 60amp capabilty
Mild Brushless (Velineon, 4000kv and less) = 80amps or better.  Should really use only LiPo.
Brushless 4000kv+ = 100amps or more.  Use kv x 15 = mAh as a rough guideline.

For 1/8th scale, you need 100amps minimum IMO or there's gonna be trouble sooner or later.

[abc123]

also lets say a 7cell thats has 5000mah and a 8 cell with 2000mah will the 7 cell be better?

[Dukk]

The 8cell has more voltage and therefore should have more top end as long as it can supply the current.

The 7 cell will last WAY longer though and handle a heavier load.

Overal the 7cell is superior.  Change that 8cell to 3800mAh or better and I would say it is better overall.

[fsracer]

good topic and nice explaination.

[abc123]

also if you have a mamba max 7700kv on 2s and a mamba max 5700kv on 3s will the 7700kv be faster or 5700kv?

[Dukk]

7700 x 7.4v = 56,980rpm

5700 x 11.1v = 63,270rpm

One reason the 5700 is so popular...

[abc123]

oh so thats how it works i just times KV with battery voltage?

[DVO]

Yep that how it works with kV ratings... The Novak and other x.5's I don't quite get yet, but, from what I can gather, just double the # for a closely similar brushed turn equivalent......

kV is how many RPM per volt it will turn... A 5700kV motor will do 5700 RPM at 1 volt, 11,400 at 2 and so on up to where Dukk got... Like that with all kV type ratings, too... The lower the kV, the more you can run thru them; in most cases... More efficiency, too... Some guys run rather low turn motors with 3-4-5S LiPo's as where some will run high kV motors and 2-3S...

The final # you come up with is an unloaded RPM, btw.. Meaning no load on the motor at all... Once you put it into something, it takes away RPM like in a fullsized where adding to the drivetrain,  IE 4x4, it horsepower... So don't expect 63kRPM out of a 5700 with 3S in a 4WD offroader.. But you'll still need to hold on...

[Dukk]

^ All true.  And further to that, the lower the KV rating often the more torque the motor has.  It is the same tradeoff of RPM and torque that all motors face.  You should select the correct motor for the application, not just the 'fastest' one.

[SlyFox]

The final # you come up with is an unloaded RPM, btw.. Meaning no load on the motor at all... Once you put it into something, it takes away RPM like in a fullsized where adding to the drivetrain,  IE 4x4, it horsepower... So don't expect 63kRPM out of a 5700 with 3S in a 4WD offroader.. But you'll still need to hold on...

yup very true. its just like in full sized cars, hp at the flywheel and hp at the wheels. you will always end up with different numbers at the wheels due to drivetrain loss. usually u minus %15 due to drivetrian loss and for 4x4 more like 20-25%. so in other words if its 63k at the motor, at the wheels it would be more like 53550rpm on a 2wd and on a 4x4 it would be 47250rpm. big difference between the two.