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lipo run time


Daviejp

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Ok here goes. Got an 8th buggy a 2250kv motor and a 1900kv motor. Im running mechanical brakes also. The 1900 has a 1800w output the 2250 has a 1250w output. Lipo is a 4500mah 4s. Does the 1900 draw more power than the 2250 meaning less run time?

asking cause when I first started with the 2250 my run time was around 16 - 18 minutes when I added the mechanical brakes (to help cool the motor) that dropped to 12 minutes. I thought if I got the lower kv motor I could raise that but it has got even lower.

Also when I set the lvc to 3.4 it comes back as 3.8 on the battery even under load I've even gone down to 2.6 and it still comes out as 3.4 under load.

Esc is a sc8 motors are 1900kv turnigy and 2250kv hobbywing. Anyone got any thoughts on any of my incoherent ramblings cheers

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each motor is different so it could be a wide range of things like efficiency of the motor, gearing and how you drove the car.

was the gearing the same?

Edited by alex97
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Lower Kv does in theory consume less current giving extra runtimes. Its not quite as simple as that though, there is an amount of efficiency to be taken into the equation along with gearing. 

 

The HW escs' are renound for being over cautious with the LVD so as long as the end voltage its about right, the setting on the ESC doesnt really matter. 

 

I have had bothe Turnigy and HW motors and TBH neother were that great and did tend to consume a lot of power, the HW one actually cooked a 150a esc. 

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If you geared the same for both motors one could be over geared or the other could be under geared. One causing a big current draw while the other could be causing a motor to spin way too fast. 

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For efficiency, run the 1900 on the highest voltage you can. Then gear it up slightly to compensate for the small drop in kv.

I have two HW Xerun ESCs, both are set to 3.0 volts per cell to achieve a perfect 3.4 volt cut-off. If your cells are new and taking less than 4000mah to recharge, then your LVC is cutting early.

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My Lipo limit is 4s both ran 19/48 gearing for both motors highest temp I got was 50? so I don't think they are geared poorly. Might just bite the bullet and buy 6000mah 2s in series. Hobbyking here comes the credit card lol

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  • 2 weeks later...

Best way to explain is how far you cover in the same time!

higher KV motor just means it gives more RPM per Volt of power! How much power you lay down and how fast you go will be the test of run time! 

When we race the higher KV will last less on the track but you may well cover more laps! its all about gearing and driving style, a little between manufacturers too!

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lower KV means more torque, in quadcopters we go with lower kv to throw bigger props at a slower speed.

we dont need speed we need the torque. 

 

a 1900kv on a 3s lipo will be spinning at around 19,000 to 21,000 rpm 

a 2250 on 3s will be spinning at around 23,000 to 25,000 rpm

 

good motors will come with a data sheet or you will find one online.

what your looking for is the magical power to rpm number. a motor is made to run say 2s and be most effiecent

at 2s, but its tollerance is such it can be run on 3s.

 

now thats all good and the numbers above i have adjusted for some loss.

 

high kv is more on spindle speed, an 8000kv on 3s is going to push 88,800rpm

this means its fast but it lacks low end torque, its going to preform at high spindle speeds.

 

on the other hand you 1900 has plenty of torque, but its missing out on top end rpm you max out at under 25,000rpm.

 

now your car has a gear ratio, the one you really want to know is the final drive output something like 4.2:1, every 4.2 turns you gain one full rotation on wheel.

if you take the roll out of the wheel with tyre on your given another number, with maths you can devolve a rough rpm of the wheel and how fast your machine in theory can go.

BUT rotation drag, air drag, body drag and other factors come into play so we take around 15-20% off for these other factors.

 

High kv is more for long sweeping race tracks,

low kv for offroad torque and short winding race tracks

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you added mech brake and the runtime went down. 

 

this shows your now taking more juice from the system, every time you come to the brake your using juice.

so adding a second servo to control brake means your going to cut down runtime. 

 

if you unplug the mech brake you will go back to 16 min runtimes. 

 

cheap lipo buzzers are just that, calibrated in a different country and possible on a none conformed and calibrated meter either.

they are a rough guide, to get full pure readings you need an expensive volt/amp meter thats able to handle the currents and voltages we use.

so treat them as that just a guide to what the battery is like not a pure 100% it is this number.

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