Battery Life - How many ampere hours exist in a battery discharged to 85%?

Hi - I have a couple of 6S 12000mAH batteries on an octocopter. When I load the copter up with a gimbal and SLR camera I get about 8 minutes useful fight time. The batteries are allegedly capable of 24AH between them and the copter draws ~ 35 amps at hover. I have no reason to suspect the current measuring circuits are faulty and at present I trust these numbers. Going back to Watts = Volts * Amps I would have thought the batteries would provide 35 amps for about .68 of an hour - or about 36 minutes - BUT - I am the battery down to about 85% of charge to avoid damage. Does this mean the 24AH is really only 3.6AH as this is the cell voltage going from 4.2 volts to 3.57 volts - or do I have the wrong understanding here? Is it instead that 4.2 volts to 3.57 volts should provide 24AH between the two 12000mAH batteries? The result seems to me that all I am getting is 3.6AH before the copter performs an RTL. And - 3.5AH would equate back to 7 or 8 minutes. Any suggestions would be greatly appreciated. Hardware setup is 2 x 12000mAH TCBWORTH 6S, 1 x Orange Cube, 8 x Tarot 5008 motors, Storm32 gimbal for SLR, RTK GPS. I have tried 4 sets of batteries and they all behave the same way. Half of them were brand new. They charge to about 25.1 volts on my charger.

It’s 85% consumption of pack capacity. So if you have a pack capacity of 24,000 mah (assuming those batteries are in parallel) you would want to be landing after consuming 20,400mah.

And at 35 A at hover, you should be able to hover for (20.4/35)*60 minutes = 35 minutes.

Given that you are only getting about 1/4 of that (~8mins), then there is something wrong with the numbers.

When you recharge one of the flight batteries, do you see that the charger puts back a little more than 10,200 mA per pack? This would confirm how much the batteries are being discharged during the flight.

What is the all-up weight of your octocopter, and what size props are you using?

Hi @David_Malcom,
Please make sure the following things:

  1. Power module is configured properly(I am talking about mAh, min arming and failsafe voltages)
  2. RTL failsafe for battery is set to correct voltage

Also monitor in the ground control station how much mAh is being consumed with time and current consumptions. Sometimes due to peak loads, if the batteries are of bad C rating, the voltage might fluctuate and drop below failsafe voltage and may accidentally trigger RTL. Hope it helps. Happy flying :slight_smile:

@David_Malcolm, I have had a similar problem where my in-flight reported cell voltages were much lower than expected given the number of mAh expended relative to the pack capacity (which was a hexacopter running on 4S with ~40A at hover). As I understand it, this is not unexpected for larger copters, so you may be safe just using the current sensor.

To continue using the battery low failsafes, have a look at your “BATT_FS_VOLTSRC” parameter - if it’s set to RAW (0), setting it to Sag Compensated (1) may give you better results.

Either you have the low battery fail safe misconfigured, your voltage and or current sensors are not calibrated, or they are malfunctioning.

How much does the aircraft weigh WITH THE BATTERIES INSTALLED?

Post a data flash log file so we can look at it.

Thanks, that’s very helpful. So in effect you’re saying that I am getting 20400mah once my cells drop from 4.2 volts to 3.57 volts? Have I understood this correctly? I suspect my current draw is about right at 35 amps - if I look at the thrust vs. amps for the Tarot 5008 plus the current drawn by other electronics it is about right . . . . .

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Can I ask what the C-rating is of your batteries? I suspect that while under load your batteries are sagging a lot - which results in the voltage being much lower. Is the 3.57 volts the in-flight voltage your presume the battery is dead at - or is it the voltage measured from the battery a couple minutes after you land. Usually the voltage range under LiPo for is between 3.5 to 4.2V (while not under load). So if during normal flight your voltage sags 0.1V (because of internal resistances) - then the in-flight voltage range is 3.4 to 4.1V.

Also, how many mAh of power goes into the battery (while charging) after your 8-ish minute flight?

No, I’m not saying anything about cell voltage. Simply that the 85% value you quoted, which is a good rule of thumb to follow, is 85% consumption of pack capacity in mah not “85% of charge” as you stated. Personally I don’t pay a lot of attention to voltage. I use mah consumed/remaining for flight time.