• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

Load voltage cutoff before full throttle ?

#1
I was running some amp tests on my EMAX CF2822 1200v motor using 20A esc and amp/power meter

Why would the battery go so low in voltage under load ? Same thing happened with the 10x4.7SF on the Zop battery

The Zop batteries are rated 25C and the Nano Tech 25C/50C burst so should be able to go to 25A for Zop and about 21A for Nano Tech.

1000mah 2S Zop1 8.1v 8x6

2.73A | 20W | 7.34v

5.8A | 37W | 6.35v

1000mah 2S Zop2 8.3v 8x6

2.10A | 16W | 7.70v

6.1A | 40W | 6.53v

850mah 2S Nano Tech 8.2v 8x6

2.62A | 20W | 7.54v

6.1A | 40W | 6.66

850mah 2S Nano Tech 8.1v 8040

3.17A | 23W | 7.43V

6.8A | 43W | 6.27v

850mah 2S Nano Tech 8.1v 9050

2.92A | 22W | 7.38v

6.5A | 41W | 6.30v

850mah 2S Nano Tech 8.0v 9x4.7SF

3.01A | 22W | 7.23v

7.4A| 46W | 6.00v

1000mah 2S Zop2 8.2v 9x4.7 SF

2.93A | 22W | 7.37v

7.2A | 44W | 6.05v

850mah 2S Nano Tech 8.1v 10x4.7 SF

3.80A | 27W | 7.18v

Battery goes below 6v under load and esc cutoff


(Recharged the Nano Tech battery back to 8.1v before doing the 10x4.7SF test)

First, do those number seem correct ? first time using the power meter.
Second, I'm trying to understand why the battery goes from 8.1v to below 6v under load that so that I can select the right combination of Motor/Prop/ESC/Battery for my planes.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
Firstly my fully charged 2S batteries read 8.4V not 8.1V so perhaps the charger is not giving a full charge. Or you are not using a balance charge and one cell is actually quite low in charge level.

You do not mention the age of the batteries, (as batteries get older they tend to get higher in internal resistance).

Finally you do not mention if you are measuring average current or peak, (I suspect only average which can be much lower than the actual peak on a overpropped motor).

The reason the battery is dropping in terminal voltage is due to resistance under load. Without knowing the battery history, its age, its actual charge/balance state, and your actual test setup I cannot give a definitive location as to where the voltage loss is occuring though it is either in your test setup or the internal resistance of the battery.

Would require more information to give the exact reason for your measured results.

Have fun!
 

ElectriSean

Eternal Student
Mentor
#3
You have to take battery C ratings with a grain of salt, they are pretty much always exaggerated. All batteries sag under load, the bigger the load the more they sag. Higher quality higher (theoretical) C rating batteries sag less.
 

Bricks

Active member
#4
To add for each test did you recharge your battery before running the next test? If not then it can skew your results.
 
#5
@ElectriSean I figured as much but I figured maybe 15a max out of the 21A C rating. It goes from 8.1v (current charge during test) to below 6v on load at around 7-8amps??

@Bricks
no I didn't recharge the battery, but this wasn't really an in dept test, I knew it would be off a bit, just wanted an idea of the amps draw at this point.

I was actually very surprised, I bought a CF2805 from headsuphobby because it had prop test data, and it listed it to pull 13oz at 9amps for a 10x4.7SF prop on 2S, but after running the test, I get this:

 

Half Throttle

Full Throttle

850mah 2S Nano Tech 8.2v 10x4.7SF

3.29A | 25W | 7.46v

4.65A | 33W | 7.04v



I just don't get why my Nano Tech charged at 8.1v can't draw over 8 amps because the load voltage drops below 6v when it's rated at 21 amps draw. I thought the Turnigy Nano Tech were a good brand of battery, but my cheap bangood Zop battery do almost as good.
 
#7
I didn't know it did, but after looking closer it does have a resistance check:

Turnigy Nano Tech 2S 850mah 25C/50C: 9 mΩ on first cell and 11 mΩ on the 2nd cell for a total of 20 mΩ

That's tested after balance charging it. I always balanced charged it with my Accucel S60 and stop discharging around 3.7-3.8v per cell

Edit:
I just charged and tested the resistance on my cheap 1000mah 25C ZOP batteries, first has 31-24 and second battery 31-23 lol

I got a few batteries a week ago, those are storage charge values, haven't charged or used them yet.

Turnigy Nano Tech 2S 1000mah 25C/50C: 7 - 9 (8-8 after balance charging it)
Turnigy Nano Tech 3S 950mah 25C/50C: 13 - 14 - 14
Rhino (Zippy) 2S 2150mah 20C: 5 - 4

However, if I run the resistance test again, it seems the values jump around a bit, is it because they're not fully charged or is that a normal thing, in which case, how do you figure the right resistance ? highest values ?
 

Attachments

Last edited: