Hi All,

I have been flying electric stunt now for about a year or so, and my competition model flies with a 4-cell 3300 mAh Desire pack. I have 3 of these packs, and I don't fly that much, but now after around 50 flights on each pack, they no longer last very well. Two of the packs get a little hotter than the last surviving pack, and they seem to be a little "puffy" as well. The last survivor pack has 42 flights on it.

The makes of Lipo's these days are all claiming a couple of hundred flights, so I would like to hear what sort of cycle life is now realistic at this moment in time. Thanks.

Keith R

I think a lot depends on how much overhead you have in your capacity---this coming from someone who is living on the edge of capacity!

I basically draw nearly or sometimes slightly over 80% of my 2100mAHr cells---it is a little hard to say since sometimes I am using the "fuel meter" on my FMA charger which says 18% to 24% of capacity left. However when I charge the battery, I am still putting in less than 1600mAHr. This could be consistent by just saying I am losing capacity over time. However I have had these cells over a year now (two over 1 1/2 years) and they still provide all I need through the flight. At some point I guess I will note a sag in the late pattern maneuvers (like overhead 8 and cloverleaf) so at that point they are goners. On the other hand, I paid "only" $63 each for the packs, so cost per flight shouldn't be too bad. I should go back see how many flights I have on each pack. Last year I tried to put most time on two of the packs to see if I can see a lifetime issue (I thought I was close to 50 full discharges on each of those packs). However I just used all 4 packs (2 at the Windy City Classic on Saturday, and 2 yesterday, and last night all claimed to have a least 20% capacity remaining.

I do note that prior to last nights charging "fest", my packs had been claiming they were being used down to the 15% level. However this weekend finally had temps in the 70-80's, so maybe my packs are telling me that they like those temperatures and not 50's (all degrees F).

However if you are leaving 30-40% in the pack, I would imagine you should get many cycles out of them, as long as you treat them right otherwise (like storing them at ~50% capacity in the fridge over the winter or for long term storage). They do just age too.

From reading posts over on RC Groups, it is pretty clear that people see a lot of variability in the lifetime--even reputable manufacturers seem to have issues time to time. So that's why I have more or less stuck with my FMA packs. Like I said, they aren't the cheapest, or the most expensive (but sure seem to be about the lightest out there--even compared to Thunderpower lites). I don't think I could handle having a $200 pack go bad!

Mark,

Could you give some details on the Heater/Cooler and where it is available?

Best,          Dennis T

I am not sure about cooling----although mine are warmer than I would like. Most others sound like they are barely warm after a flight (everyone can pipe up now if they want!).

If you ran WOT for a CL flight, you would be pulling less than 10C (if you would leave 20% at the end). However all of us are running at partial throttle, so the instantaneous current draw is probably 30-40% higher than our average draw (~10C). This will give you more heating that you would naively think. That why I am getting ready to replace my 880kV motor with a "more efficient 780kV motor, which should reduce the instantaneous current a bit, letting me baby my 2100mAHr cells a little more.

I am having too much fun doing these negligible things!

That is a good question, and one that I have also asked.
I log all my electric flights, and discharge voltage, mah input, and recharge voltage are recorded.
To date, my highest time battery is 130 flights. It was the first battery I bought, and "may" have taken some abuse as I was learning.
It worked fine until one flight it slowed down near the end. I checked, and one cell was bad. With that much time on it, no sense in splicing in a "new" cell from another "damaged" battery.

Thet are Thunder Power 5S2P4200 batteries. I take out ~ 3000 mah a flight +\- 200 mah.

I have now switched to 4000 5S2P Thunder Power "cubes" (weighs 3 ounces less than the 4200) with excellent results.
High time on one of those is about 10 flights so far.

Maybe it was wishful thinking, but I was hoping to nurse 200 flights out of a pack.  I still have many of the 4200's still, but am no longer flying the plane they go in now

Paul,

Just out of curiosity, how is that stacking up against the cost per flight of glow fuel? I haven't bought that stuff in a long time and have lost touch with what it costs. Sounds like LiPos are costing roughly a dollar per 6 minute flight, seems like that might be more than the fuel cost, but I don't have the data.

John Witt

Don't forget the cost of Windex and paper towels!

bugs are necessary, bugs are turbulators 

From all the conversations I have had with my Dark side buddies,, battery cooling isnt as much of an issue as we want to make it. Cooling the speed control, and the motor is in my opinion much more important. the information I have heard is that battteries will give the best performance when they are around 120 to 130 degrees.at temperatures below70 they will show a decline in their ability to deliver enough power.
not the motor on the other hand wants to be COOL.

Speaking of cooling, I've seen the temps rise on batteries, motors, and ESC's after touchdown.  I think that's where the temps begin to soar.

Having one foot firmly in the dark-side (I cringe even as i say it!) Stunters tend to have less well thought-out battery cooling paths than the F3A guys do. We could easily do better, and Rick S' canopy air exit is one great idea.

later,
Dean P.

Previously stated:
"What I mean to say is that the temperature rises, at that point, because the equipment is already hot from the flight, not because cooling stops. After all when the motor quits, you are no longer generating heat anymore. "

In my case, I have a large air intake, and two nice size exits for the air. The Eagle Tree recorder has a probe that is attached to the battery in a place where there is minimal flow past the battery. It is measured throughout the flight. It barely hits 100 deg F just before the motor stops. As soon as it lands, the slope of the temperature increases significantly. At this point there is no more flow by it to help extract the remaining heat in the battery, and thus it heats up. It climbes to 120 deg F and then finally startes dropping.

I have measured the battery with an IR thermometer on as many points as I could reach and found no spot hotter that 120 deg F. I too have been concerned about the "core" volume of the battery, but to date have seen little effect.

Cooling of the battery, motor, and ESC is one on my primary concerns. I took temperatures regurarly in the beginning, but found that the motor was not heating up (to any significant level) , nor was the ESC (it is in the same cooling path as the battery). I too have seen many installations of electric systems that make my skin crawl. No consideration of cooling for the batteries.

Let me ask a question of the folks with some more experience with the LiPo flight packs.  Would it be worthwhile to remove the heatshrink overwrap and separate the cells a little with a thin (say .030 or .040 inch styrene) spacer strips to allow air circulation between the stacked cells? The cells could be strapped back together with tie-wraps or tape or something of that type. This would take some care not to damage the interconnects, and as the car people would say: "void the warranty". Seems to me that the typical heat shrink wrapping is a pretty good thermal barrier.

Probably wouldn't help much for us in Seattle (it's 55 F here currently) as we barely get the batteries up to 120, I'd guess. Might be beneficial to those in Tucson or other less fortunate parts of the country, though. We have beautiful weather, Microsoft and Starbucks. How could it get any better?   

John

Another question. This morning's flights went well, except that I forgot to set in the time limit and ran my 2250 mAh packs down to the low voltage cutoff, which I have set to 3.2V. Took about 2150 mAh to recharge them. This is a pretty deep discharge, what's the potential for damaging the cells?  They got recharged immediately and appeared to charge normally. I'm charging at 2.1 A.

Dopey me, the first flight seemed to go on for a long time and I didn't get the 15 second burp. So naturally I did it again with the second pack before I thought to start checking things and found all the Ztron's dip switches set to "OFF". Lots of nice wingovers, tho...

John

It's hard to know if one discharge is going to hurt a lot--but I am sure it doesn't help matters. A lot of RC'ers use their Low Voltage Cutoff as a timer to land, and of course, done over and over again really will shorten the life of the cells.

One thing I have noted is that the early trimming and setup flights can be dangerous to the equipment since every flight is a little different and it is easy to get distracted.

Stunt ships are classically tight packaging exercises because of aesthetics, but you need  room around the battery for air to flow.  Yes, this requires a small re-think of how we do things. Fortunately, we are modelers and we can solve any problem.

later,
Dean

Yeah, that's for sure, I built the Panther in the first place just because I liked the way it looks.