|Jan 03, 2007, 07:53 PM|
Lipo Burst Values & Long Life?
I have been using lipos for about 3 years now and follow the input of EveryDayFlyer (Charles), RD, Dr. Kiwi, and Rod, and I have developed my own protocols for using lipos in my "normal" planes by taking their advise. By normal planes I mean planes that are under some form of power almost all of the time - say 40%-75% throttle with limited bursts of WOT. I do all the standard stuff to take care of my packs: prop only up to 80% C max continuous discharge rate, charge only when the cells are at ambient, balance on each charge, LVC 3.1V/cell, etc.
I have now added a powered sail plane to my hanger, where the load profile on the cells is very different from my other planes. My latest addition (Thank you Santa) is a Hyper 2 meter sailplane at about 22oz AUW (inc pack) with a Hacker B20-15L in a 4:1 gb and a Graupner 10x8 folding prop.
The pack suggested by NES is a 3S1P TP-1320 PL. With the power package above it draws about 12A - 18A in flight for 20 seconds or less with at least 5 - 10 minutes between bursts - plenty of time to cool down. After the last glide down I have checked the pack temp and it is at ambient or just a few degrees warmer. This pack is advertised by Thunder Power as capable of 13C (17A) continuous and 20C (27A) burst, but we all know about manufactures' ratings.
Am I stressing this pack too much and how do I tell? How does one size a pack for this kind of use to get reasonably long pack life?
|Jan 03, 2007, 08:34 PM|
According to RD's 15C and 20C burst here
it would appear that the temp. should stay below 130F.
Based on my experience the temp. will rise more as the capacity drops.
|Jan 04, 2007, 12:55 AM|
Thanks for the reply. I looked at those before posting my question. This is what I got from them:
The first of RD's graphs is of the temp at a burst rate of one burst of 15 to 20 seconds every two minutes at different discharge currents, with his highest being 13C, about the same current I will be running. So far so good.
While my bursts will be of about the same duration, there will be much longer between bursts and should give the pack longer to cool, and that should help keep the final pack temp lower. What is a concern is that the 13C burst run ended with a temp of 175*F; clearly too high to be safe. I can postulate that a longer rest between bursts will keep the pack below 160*F, but I don't know that the data allows that conclusion to be drawn.
His single burst 15C and 20C discharge graph shows the pack holding its voltage fairly well over the duration of each burst - even at 20C -, but there is no temperature correlation other than the delta T per that single burst. That shows the cells are capable of producing the current without a single burst causing a thermal runaway and melting down the pack, but it says nothing about what this is doing to the pack's longevity.
His final graph shows only minor loss of capacity after all this abuse. However, since he exceeded 175*F in the 13C run, how would we know if it was that single run or the pattern of intermittent high discharge that caused the loss?
I am not criticizing his data or testing, but I want to learn the "why" of how you came to your conclusion.
Are you saying that because the pack showed only a minor loss, even after one "hot run", if I keep the pack temp at less than the standard 140*F admonition, I should be fine?
I guess the only real way to tell is to put the BNB digital in-flight recorder on the plane and check both the current and the pack temp to see if it is staying within the limits. If it is not, than at least RD showed that one hot run will not kill the pack.
|Jan 04, 2007, 06:53 AM|
I have often pulled short (3-5 sec. ) 16C burst from 10C rated LiPolys. I am not saying it is good for them but they still lasted close to 100 cycles.
Four of the cont. discharges were in excess of 140F however at the end of the second burst (20C) temp. was 91F 7F Beta
If you really want something to ponder on.
Many cllearly state that a battery is subjected to the max. system load at all times but in extremely short burst.
They state that if the power system is capable of drawing 50 Amps. then the LiPoly must be sized to provide 50A Cont. Their logic is that since the ESC is merely a high speed switch that each time it is on the battery see max. current load. . Your Watt meter ,etc. only records the average .
Read page 4 here.
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