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
Replies
One battery simply is not a good thing for my drone since I placed the GPS in the center (I designed it for 2 batteries anyway), Having 2 batteries means I have more options as to where to place them.
2 batteries solution is not good
since you need 2 chargers or charging time is extended twice.
There is no smart balancer manufactured to control 2 battries at once, so you need
to install 2 balancers.
You need telemetry to control 2 batteries in separate in your OSD.
Price of 2 batteries of the same mah as 1 single battery is much higher today.
Multi-battery solution has been rejected by manufacturers of eCars.
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.
"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'm sorry Martin, which "basic circuit knowledge" are you referring to?
If you lose one cell in parallel batteries, the good battery is going to try to charge the bad battery, the overall voltage will drop to whatever the the lowest charge voltage the bad bad battery can develop under that charge, in any case the voltage will be lower and the good battery will discharge quickly,
In the case of series batteries the result is the max current will be limited to whatever the dead cell can pass, and overall voltage will be down by 1 cell.
So when using 2 batteries, you would have some redundancy and be able to get home in the case of a cell failure. With only one battery, it would drop like a stone. Correct?
Aussie
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.
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.
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.
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."
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