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When the batteries say to stop flying.

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When the batteries say to stop flying.

Old 04-06-2008, 06:12 PM
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poppy2
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Default When the batteries say to stop flying.

I have a question about when to stop flying due to battery condition. I use nicad batteries. When I get a new receiver pack (700ma) I charge them from a discharged state using an overnight charger (55ma). The time I charge is 700/55x1.4=18 hours. I then discharge them using an expanded scale voltmeter and plot the voltage and time. Usually a pack will charge up to around 5.5 and then discharge rapidly to around 4.8 and stay there for some time then drop off rapidly. A recent new receiver pack checked out as follows every half hour: 5.60 5.20 5.08 5.05 5.00 4.93 4.85 4.71 4.30.

My question is this. Just when do you quit flying? Do you fly for so many hours/minutes based on your knowledge of the voltale/hour graph or do you fly up to a certain voltage reading. I might add that the plane is a trainer and the the flying is nothing but normal flight. Just wondering how everyone attacts this problem. Thanks for any insights.

Poppy2
Old 04-06-2008, 07:46 PM
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gboulton
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Default RE: When the batteries say to stop flying.

For me, it's simple, really.

When the voltage UNDER LOAD gets down to nominal (4.8V in this case) or less, that batt's done until I recharge it.

As a "safety margin", I usually won't fly if it's close. On your numbers above, I'd be fine with any of those voltages 5.6-4.93 (again...UNDER LOAD), but would probably make the 4.93 flight my last until a recharge. I MIGHT go ahead and fly at 4.85, depending upon the plane, situation, type of flying, etc...but that would certainly be the exception rather than the rule.

As you have discovered...when a batt starts falling below nominal, it does so in a hurry. I'm just not willing to risk an airplane wondering "Hrmm...how long has this thing been hanging out around 4.8?"
Old 04-06-2008, 07:49 PM
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krayzc-RCU
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Default RE: When the batteries say to stop flying.

when my loaded voltage hits what the manf says is to stop i do
Old 04-06-2008, 08:00 PM
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pcm
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Default RE: When the batteries say to stop flying.

I agree strongly with gboulton, the 4.9's under a load would be my last flight then I would recharge. I have been sticking to this rule for over 20 years and I have never had a battery issue yet. When cycling if I get lower then 75% of the rated capacity the batteries are promptly replaced.
Old 04-07-2008, 08:09 AM
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gboulton
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Default RE: When the batteries say to stop flying.


ORIGINAL: pcm
When cycling if I get lower then 75% of the rated capacity the batteries are promptly replaced.
Excellent point, pcm. One should absolutely have SOME sort of plan for battery replacement.

What that plan "should be"...well, you'll get as many answers for that as you have people in this hobby. *heh*

But, absolutely, have some sort of "schedule" or "criteria" for battery replacement. Battery packs are (certainly as a percentage of airplane cost) too inexpensive to risk an airplane on questionnable or unknown packs.
Old 04-07-2008, 09:06 AM
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poppy2
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Default RE: When the batteries say to stop flying.

I want to thank everyone for taking the time and thought to answer my question. First thing I had to do was go back and re-read my post and see where I might have left some pertinent information out. I was tired when I wrote that post and to make matters worse, I wrote it once and tryed to submit it only to find out that I had not mentioned a subject matter and went back to correct that error and found out that my first attempt to post got lost in cyber space some where for what ever reason.

So here goes a few corrections. First, the expanded scale voltmeter I use is made by Hobbico. It was my understanding that when I discharge the batteries after being fully charged and plot my graph that it was UNDER LOAD and simulated a load my plane would have on it , just as if I was flying. I pulled out the instructions that came with it last night and it does in fact say that is the case. Now using my graph as a guide, it seems that I could use it in two ways. (1)so many hour/minutes to fly up to a cut off point, or(2)periodically check the voltage with the expanded scale voltmeter. I tend to initially lean toward the first method but definitely with a built in fudge factor, but surely not ignoring the the second method of actually checking the voltage.

Also, I think pcm is right on the money with the 75% rule. I also ment to say something about that and my rule has been if the batteries lose 20% capicity, it is time to throw them in the trash can and do yourself a favor. I think I have filled in all the left out information. Thanks again everyone for all the insights. I welcome all the help I can get.

poppy2

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