My brushless motor looses power gradually over 3 minutes?

[rcdangar3]

Hi y’all, I recently purchased a rc plane from eBay, on my maiden flight I crashed and bent the motor so I replaced it with one I had. The issue comes when I test the motor. It happens as so:
Step 1 - Turn transmitter on and connect battery, esc beeps as should to say the motor is all ok.
Step 2 - I check the servos to make sure they function and they do.
Step 3 - Throttle on to max.

Now the motor works and operates at full throttle for one minute, but then slowly over 2 minutes the power decreases until the motor stops. The servos still operate. I am using a 2 cell, 7.4v 2200mah battery. Any idea why this is happening?

 

[quorneng]

My first suggestion is the motor is depleting the battery to the point that the low voltage cut off (LVC) in the electronic speed controller (ESC) is being activated. This is a safety feature so that although the motor stops the radio & servos keep working so you can land the plane under control.
I sincerely hope the battery was fully charged to start with.
Without details of the motor or the size of the prop it is turning it is difficult to know why it is draining the battery so quickly.
Was the motor hot after the run?
If the motor really is draining a fully charged 2200mAh in 2 minutes it will likely not be doing the battery much good either. LiPo are powerful but delicate batteries that do not like to be discharged too low too quickly. 6 minutes at full power from a fully charged battery without the LVC cutting in would be good.
Always a sound investment is to get a Watt meter so you can measure the amps the battery is providing with the motor running at full power. Remember the amps go from the battery through the ESC to the motor. They all have limits and can be wrecked by too many amps. In the case of the ESC, being entirely electronic, any amp over load damage will be instantaneous and permanent!

 

[rcdangar3]

My first suggestion is the motor is depleting the battery to the point that the low voltage cut off (LVC) in the electronic speed controller (ESC) is being activated. This is a safety feature so that although the motor stops the radio & servos keep working so you can land the plane under control.
I sincerely hope the battery was fully charged to start with.
Without details of the motor or the size of the prop it is turning it is difficult to know why it is draining the battery so quickly.
Was the motor hot after the run?
If the motor really is draining a fully charged 2200mAh in 2 minutes it will likely not be doing the battery much good either. LiPo are powerful but delicate batteries that do not like to be discharged too low too quickly. 6 minutes at full power from a fully charged battery without the LVC cutting in would be good.
Always a sound investment is to get a Watt meter so you can measure the amps the battery is providing with the motor running at full power. Remember the amps go from the battery through the ESC to the motor. They all have limits and can be wrecked by too many amps. In the case of the ESC, being entirely electronic, any amp over load damage will be instantaneous and permanent!

Prop size is 8x4. Haven’t got details of motor as they aren’t on it. Motor not hot after running. Battery was at around 6.9v at time of running

 

[Merv]

...Battery was at around 6.9v at time of running

How old is the battery?
As batteries age, they loose the ability to deliver power under load. It could also be a failing ESC.

 

[rcdangar3]

How old is the battery?
As batteries age, they loose the ability to deliver power under load. It could also be a failing ESC.

The battery is two months old. Only used twice

 

[Bricks]

Battery or charger junk,, fully charged battery should be 7.4 volts.

 

[NickRehm]

Prop size is 8x4. Haven’t got details of motor as they aren’t on it. Motor not hot after running. Battery was at around 6.9v at time of running

That battery was dead dead when you started. Should be 8.4v or 4.2v per cell for full charge. You had it at 3.45v/cell which is approaching damaged cell territory

 

[rcdangar3]

Battery or charger junk,, fully charged battery should be 7.4 volts.

I had not charged it to full on that test

 

[quorneng]

rcdanger3
Your problem is solved. A low battery charge.
Before you use it on such a test balance charge it to 8.4V.
What charger are you using?