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Battery sag, not battery fault, answers on a postcard please

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martin collins 122/06/2019 22:28:22
421 forum posts
199 photos

I have a plane that is running a 35-42 propdrive motor on a 12x6 prop and a 60amp controller. the motor is pulling 47amps at WOT. The battery is a 5s a123 pack. The pack sags at full throttle so i tried a 3s lipo pack that i have run at 75amps without sagging but this too sags at full throttle. So options are ESC (60amp Turnigy plush) or the motor, both are new. Anyone have similar problems before?............Martin

Willyuk23/06/2019 07:55:09
176 forum posts
22 photos

Battery voltage sag is not unusual especially with well used batteries. I pull 85 amps from 10s lipos and on long up lines with my older battery packs, I get sag down to 35 volts on occasion it really depends on how much I try to pull from them and how old the packs are.

Frank Skilbeck23/06/2019 08:10:37
4730 forum posts
101 photos

Can you describe the problem a little more, where are you measuring the battery volts? if it is between the connector and ESC then it could be the connector is faulty and providing an inline resistance.

martin collins 123/06/2019 08:27:45
421 forum posts
199 photos

No the problem is not just with the Watt meter connected, i had to make forced landing on the maiden flight as the power was falling off on climb out. It is something that is affecting more than one pack, as i said i have pulled 75 amps on one of these packs moments before trying it in this plane.

Simon Chaddock23/06/2019 09:43:08
5716 forum posts
3034 photos

ESCs can reduce power if they overheat...

You may have to set up and test the motor/ESC/battery combination together on the bench to identify where the problem lies.

If it will run on the bench for say even 30 seconds all full power then you know it has something to do the actual installation in the plane.

martin collins 123/06/2019 09:49:57
421 forum posts
199 photos

Neither the motor or ESC feel even warm, the plane has a radial cowl and plenty of space and cooling holes round the esc, Will change the motor first and see what happens..........

Dickw23/06/2019 10:11:09
720 forum posts
99 photos

I don't know that ESC, but some ESCs have a low voltage cutoff that reduces the throttle slowly if the battery volts drop too far, Does your ESC have that, and could it be set too high?

Other than that, it does sound like an overheating connector somewhere in the circuit,


onetenor23/06/2019 11:38:42
1901 forum posts

Maybe an under insulation break too.

Geoff S23/06/2019 12:15:12
3666 forum posts
26 photos

Have you actually measured the battery terminal voltage and the current draw so you know how much it drops under load? A battery can be described as an ideal battery in series with a resistor. The resistor is the effective series resistance (ESR) and increases with age. A good LiPo (I have no experiences with a123) will be around 2 milliohms and a tired one greater than 10 milliohms per cell. Current draw will result in the terminal voltage falling as well as power dissipation (current squared x resistance) in the battery itself which is what makes it warm.

In your case it would seem that the battery is not at fault (though it could be. Have you measured the ESR?). I would be surprised if it were the motor (they usually either work or totally fail and are usually very reliable if not over stressed). The leaves the ESC. I suspect the set up rather than a fault but it might be worth swapping it.


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