Voltage Problems

[reuben bennett]

I have a 250 size quad with Multistar 1704 motors and running 1300mAh 3s battery and 12 amp esc. Mobious and Immersion videoTX. Total weight is 485 grams with battery) I have a voltage alarm that goes off when ever one cell drops below 3.3 volts. My problem is that with a fresh fully charged battery , giving any throttle(More than a quarter stick) the voltage alarm will sound. All I can do is hover about 2 inches off the ground, try to go any higher and the alarm sounds. What do I have wrong? My first guess was the quad with camera and receiver are too heavy for the motors I have, but not sure. Can anybody give me some direction on what to do or try?

 

[Mike]

What charger are you using for your battery? Is it a balance charger? Some chargers don't balance the cells.

Update: I was looking at some other 250 builds and I'm seeing motors in the 2300-2400kv range with your same setup (FPV + Mobius), so you could be right. If you draw more amps than your battery can handle, your voltage will dip to compensate. Does your low voltage sensor read out the voltage of each cell? Or does your charger? How is the voltage after you cut the throttle?

 

[reuben bennett]

Yes I always balance charge my lipo's, voltage seems to be fine when I cut the throttle. I can hover around 2 to 3 inches off the ground no problem, If I try to throttle up then the alarm goes off. Throttle back down and the alarm stops. After seeing what you posted about the KV, I went looking too and I agree. I think your right, my motors just cant handle the load.

 

[Mike]

Sounds about right - if your voltage goes back to normal when you let off the throttle, you're drawing too much current from your batteries when you throttle up.

It could be that the all up weight is too much for your motors - but also if your props are too big, they could be drawing too much power. What size props are you running? 5x3 should be right for a 250 build. Also, is your vtx on your flight battery, or a separate one? What is the discharge (C) rating on your battery?

 

[holtneil]

H mate MM is spot on as you throttle up there is a big pull on the battery this is the c rate ,the alarm will go off , then as you stop the pull and hover the battery pull stops too and alarm stops , if you using alarm as for flight times your better off using a timer
To work out flight time fly as hard as you can for 3/4 mins land and check cell if above 3.7 amps a cell fly some more and recheck keep making note of times and you will soon work out a flight time ,say its 10 mins on a still day take off 1 min for when its windy so set timer for 9 mins
I dont know what FCB you have but the kk as a built in voltage alarm and works well , most of the battery alarms work on amps when reading so any power pull will set it off
your battery is small my friend i would go 2200mah min on that set up , or a small battery just to run fpv and the main for the quad, the mamba mini quad that does not use FPV runs on a 1500mah battery

 

[Ken l]

H mate MM is spot on as you throttle up there is a big pull on the battery this is the c rate ,the alarm will go off , then as you stop the pull and hover the battery pull stops too and alarm stops , if you using alarm as for flight times your better off using a timer
To work out flight time fly as hard as you can for 3/4 mins land and check cell if above 3.7 amps a cell fly some more and recheck keep making note of times and you will soon work out a flight time ,say its 10 mins on a still day take off 1 min for when its windy so set timer for 9 mins
I dont know what FCB you have but the kk as a built in voltage alarm and works well , most of the battery alarms work on amps when reading so any power pull will set it off
your battery is small my friend i would go 2200mah min on that set up , or a small battery just to run fpv and the main for the quad, the mamba mini quad that does not use FPV runs on a 1500mah battery

You guys are right but let me explain this another way. If you try to start your car with too small a battery, to the battery the circuit looks like a short. It can't supply the necessary amps and the voltage across the terminals will drop sharply. In effect the battery's resistance to flow increases with the demand. A bigger battery might be all that's needed.
I am brand new to this so I can only talk theory and not always that but there is a direct relationship between current draw and battery size. From experience you know what it is at least roughly.