Electric Stunt > Gettin all AMP'ed up!

Testing capacity of an older battery pack?

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Dennis Toth:
Getting ready to start flying my electric Ringmaster and have to select some battery packs. I have several Thunder Power 4S-2200 packs that I was using in my Stuka but switched out to 2450 packs to be able to fly the full PA pattern as well as the OTS pattern (needed about 35 sec more for comfort). The packs were fine when I stopped using them and when I gave them a charge took a full charge. My question for the group is can we load test them on the ground somehow to have confidence they won't quite in the middle of the vertical eight when they get under stress?

Best,    DennisT

John Rist:
Testing a LiPo on the ground is tricky.  Obviously if it is puffey  it is probably ready to retire.  Reading some articles it is suggested that measuring internal resistance is a way to check the health of a battery.  Low resistance is good high resistance is bad.   Supposedly some battery chargers can measure internal resistance.  At best it is difficult to measure internal resistance.  It is a very low value and takes good equipment to do it accurately.  What I am saying it that it can be done but it is not easy.   What I would suggests is fly the battery in the airplane.  Keep the flight safe.  Inside loops, wingovers, etc.   Stuff that will not kill the aircraft if the motor quits.  Things to look for is does the battery voltage get so low that the motor slows down.  IE the voltage gets so low that all of the voltage head room needed to maintain RPM regulation is gone.  If the motor felt strong for the hole flight make note of put back current it takes to fully recharge the battery.  It should be no more than 80% of battery capacity.  All this does not answer your question. Others may have an easy way to test battery capacity on the bench. But I don't know of any.

However I am using an KR timer.  If an airplane starts to slow down before the waring burp at end of flight I go to level flight.  When the RPMs drop a little more the KR timer turns off and I land safely.  The KR timer measures RPMs and when the RPMs falls a certain percentage below the set point the KR timer turns off.  For me there has always been plenty of time between motor slow down and KR timer shutdown to get the aircraft ready to land.  I have used plenty of crappy batteries that did not have enough capacity to finish a flight however because the way the KR timer handles thing I have never lost an Aircraft because of this problem.   D>K

Crist Rigotti:
Be sure the ESC is set on REDUCING the load an NOT on SHUT OFF.  Then if the battery starts to go out, the plane will just slow down and the motor will not stop.

Fred Underwood:
I watched a plane go to slow down mode because of low voltage in overhead flight toward the end of a flight.  It doesn't take much off peak rpm to lose line tension, and can be an unpleasant surprise.  I have "tested" batteries by by saving safe maneuvers toward the end of the flight, but that isn't a fool proof test as the load may not be as high as with overhead maneuvers.  IR is not a great way to follow the cells unless you have been watching the number of that battery on your equipment to see if it changes.  It can help if you find one cell significantly different.  That bad cell may low volt the pack.  I use a discharger with about 10 amp load and keep a cell checker on the pack to see if the cells are even under load.  If that seems ok, I fly the pack safely with recoverable maneuvers toward the end.  If the pack had normal capacity, even cell voltage under load, and normal IR for it, then it is likely safe.

When I data logged, I found that the average amp draw was about 20 amp for a 6S pack, and can be about double that in a hard draw maneuver, so that maneuver can discover low volt when you don't want to discover it.

Put the next size bigger motor on the plane and all your problems will disappear.


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