G'day DIY Drones,

I have a question about batteries, yes, I know this has been done to death, but I would like some real world experiences rather than the math.  

I have used the search bar but I guess I am not using the right keywords, hence the reason for this post.

I am considering using 1 10000mah 4s instead of 2 5000mah 4s batteries to get more flight time from my 690mm photography rig.

A bigger battery of the same C rating or close to it.

Here is the math (feel free to correct me if I'm wrong)

2 x 5000mah 4s at 25 C = 125 max amp draw and has 10000mah. Weight, approx 1300g

1 x 10000mah 4s at 25 C = 250 max amp draw and has 10000mah. Weight, approx 800-850g

I only need about 115 max amps so both will do the job.

My question is about flight time and battery voltage sag.

I don't want theory but real world experience please.

Will the 10A, lighter battery give me more flight time because of the weight saving or will the voltage drop off sooner because it is only a single battery?

Another way to put it would be, would the 2 5A batteries hold the voltage longer than the 1 10A battery given the same flight situations?

Another way to put it is, are two batteries that equal the one in MAh, better?

I hope I am making sense here 

Cheers

Aussie

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Harry, if you want to have precise idea of what is going on with your battery I suggest you use EagleTree data acquisition , it has also 2 inputs for temperature sensors that you might use to monitor the Esc temperatures or battery temperature.

It is a device that helped me to understand a lot of what is happening with copters.

Ottopilot ? 

Did you mean Attopilot ?

No, you would not have enough current to sustain flight as most current would be going into the failed battery, unless the cell failed open. You would need isolation circuitry to save the flight.

Harry,
I don't understand what you mean,

"I have calibrated Amps per volt to result in a current reading that is consistent with motors listed no load current".

What do you mean by the motor's no load current?

Yes and no, the battery or cell very rarely fail as a short circuit in which case you are correct, the usual failure is that the cell goes high resistance and so that pack now supplies much less of the required power, the remaining good pack now provides the flight power for a shorter time until the low battery failsafe operates and hopefully saves the aircraft, if the same happens with a single battery the result is a crash as the battery cannot provide enough power for any flight.

Luc, thank you I will check out the eagletree product.

and..

Yes, Attopilot

OK, I will not make the case that my reasoning is valid, but it seems to be working .  Tell me what you think.

By manipulating the parameter below, the current will show what you want as long as its proportional to the voltage.  The motor specs tell me the max current load at full power, and so as long as we trust the specs, this can be used as a baseline. I adjusted the below parameter until it reported the current at full power with no props/no load, that the motor specs tell me I should be pulling.  With that setting the reported current at hover is very near what Ecalc predicted.

Amps per volt (BATT_AMP_PERVOLT)

Number of amps that a 1V reading on the current sensor corresponds to. On the APM2 or Pixhawk using the 3DR Power brick this should be set to 17. For the Pixhawk with the 3DR 4in1 ESC this should be 17.

Harry, what did you mean when you wrote :
"..One battery has a 180A Ottopilot battery monitor" ?
The 3DR Attopilot battery monitor should measure the overal current and so need to be placed just before the distribution board , if it is place after one battery it will measure only 1/3 of total Ampere

About amps per volt...
Do not trust manufacturer specs, never.
Running a motor at full power with no prop is not a good idea
Ajusting a real world parameter to match Ecalc is not a good idea.

If you want to measure Ampere load I suggest you to use the setup for "compassmot" testing , props upside down and props exchanged.
Then you can use a multimeter, a battery monitor as HK's or EagleTree.

The 3DR Attopilot is quite good even if it does not seems so the reason is that during flight in Loiter or any other flight modes where throttle is controlled by Pixhawk/Apm the Ampere value you will see in Mission Planner will vary in decimals while in Stabilized mode is much more constant because of very rapid changes in motor speed command.

Yes and no, during normal operation, both battery packs resistance increases as they discharge, failure modes are another story, from an informative battery site;

"There are several possible failure modes associated with the complete breakdown of the cell, but it is not always possible to predict which one will occur. It depends very much on the circumstances.

  • Open circuit - This is a fail safe mode for the cell but may be not for the application. Once the current path is cut and the battery is isolated, the possibility of further damage to the battery is limited. This may not suit the user however. If one cell of a multicell battery goes open circuit then the whole battery will be out of commission.
  • Short circuit - If one cell of a battery chain fails because of a short circuit, the rest of the cells may be slightly overloaded but the battery will continue to provide power to its load. This may be important in emergency situations.

Short circuits may be external to the cell or internal within the cell. The battery management system (BMS) should be able to protect the cell from external shorts but there's not much the BMS can do to protect the cell from an internal short circuit.

 

Within the cell there are different degrees of failure.

  • Hard Short

    Solid connection between electrodes causes extremely high current flow and complete discharge resulting in permanent damage to the cell.

  • Soft Short or Micro Short

    Small localized contact between electrodes. Possibly self correcting due to melting of the small regions in contact caused by the high current flow which in turn interrupts the current path as in a fuse.

The existence of a soft short could possibly be indicated by an increase in the self discharge of the cell or by a cell with a higher self discharge than the rest of the population. This indicator is unfortunately less pronounced in larger cells where it is most needed."

"as being wired in parallel both packs have the same terminal voltage"

Correct, and that voltage will be controlled by the strongest source, i.e. the good battery, and current will flow into the bad battery, from the good, just like it would from a charger, and there is nothing to limit the current, as there is in a charger, except the bad batteries ability to accept charge.

I agree, but if you do go parallel, make sure you use a battery monitor/alarm on both batteries.

If you need redundancy, go with a coax-quad, with one battery driving the upper motors and one driving the lower motors, as an example.

Luc,

Thank you for your input, it sounds like you have a grasp of these power issues.  I am aware that my Attopilot setup is strange, and my use of the amps per volt parameter was probably never anticipated.  But the results seem to suggest this can produce useful information.

My battery monitor is on only one battery as I have separate (parallel) feeds from each battery to the distribution board.  As such, the voltage will be correct and the current will report 1/3 of the actual current.  By manipulating the amps per volt I can make the current report 3 times the actual reading.

I do strongly agree that trusting manufactures specs alone, or ecalc results alone, is a bad idea, however, in the current settings, each of these values where used separately and resulted to close the the same value.

What would you think if I got real time data from the Eagle Tree Data Acquisition product and used that to calibrate the amps per volt value?

I'm sorry Martin, which "basic circuit knowledge" are you referring to?

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