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Small park flyer batteries; How low can you go?
I'm new to electrics and have been flying a 39" trainer with 3S, 1300mah, 20c-30c batteries. I've been flying the plane for about 5 1/2 to 6 minutes max and I land it with the battery still having aprox 11.50V and about 47% capacity remaining. What is a safe capacity number I can go down to before I should land the plane? (right now I have my timer set at 5 minutes, and bring the plane down within 1 minute of the warning......for a total of 5 1/2 to 6 minute flights). Also, my next 47" trainer with about the same power motor and same 9x6 prop will have a 3s 2200mah battery with 40c. I assume I will get much longer flights (probably 10-12 min), but I need to know if there is a magic lowest number with any capacity battery I'm using, whether it's a 1300 or 2200 mah battery, that I should just simply not go beyond for a safe flight. (??)
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Hi,
The rule of thumb is to have 20% of capacity left in the LiPo battery after landing. If one discharge a LiPo battery to empty or almost empty it can be damaged, at least if is done repeatedly. Oaccasonally it can be OK to go as low as 15-10 % of capacity but the safe and normal level is 20% left of capacity. The storage capacity for LiPo by the way is 50-60% of capacity. /Bo |
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Hi
I have flown electric planes with Electric Speed Control (ESC) that has BEC (Battery Elimination Curcuit) for many years and I have used this universal LiPo 20% capacity rule and it works just fine. You can safely fly so You at landing have 20% capacity left in LiPo battery. So now You have little more flying time to enjoy if you want. Maybe You can have 25% rule if You want to have some extra time for landing approach and possibly go around if first approch is not successful. It works of course to be more conservative but it is not needed for battery to stay healthy, 20% is enough. The BEC will give rather constant voltage to the receiver+servos and will not drop just because You are at 20% capacity left, and should not give any voltage drops so the receiver temporarily loose voltage. I use in my larger electric planes where I use BEC a small Scorpion Backup Guard (a small 38 gram 2 cell 500 mAh LiPo with special circuitry) attached to receiver and it kicks in if voltage from BEC gets below 5 volt (or BEC fail completely). OPTIPOWER ULTRA Guard has same task. The aim is to give power to receiver and servos so You can at least land safely with control if BEC for some reason can not supply power to receiver+servos. I fly alot F3A aerobatics (competing) and use 10 cell 5000 mAh Lipo in the plane and I have my telemetry alarm set to 4000 mAh (20% rule) so if I'm still in the air when the alarm (sound) tell me 4000 mAh is consumed it is time for landing (but I seldom reach the threshold, sometimes in very windy conditions where I use more power). I suppose You know what telemetry is? If not, most newer mid-high end radios has telemetry today and a bunch of telemetry sensors one can mount in the plane to measure various things in the plane (for example current sensor to measure capacity consumed, voltage, current and there are many other sensors also like speed-, vario-, height, gps-, rpm and so on). I use Futaba radio myself. /Bo |
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Last edited by bossee; Oct 04, 2015 at 01:15 PM.
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The 80% recommendation is based on 80% of the packs capacity at the time of discharge rather than the new packs stated capacity.
See how that affects % left in pack using mah used to determine 80% used.. New pack - 5000mah, 80% = 4000mah used, 20% left. Pack that has lost 10% of original capacity. Now 4500mah, 4000mah used, 11% left. Pack that has lost 20% of original capacity. Now 4000mah, 4000mah used, 0% left. Using voltage as the 80% indicator is always correct. 3.7V resting = 80% or measure actual capacity to use mah as an indicator. |
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Bossee,
Thank you for the in depth explanation. Between your input and that of hoppy, I now know the facts and have more confidence with my batteries now knowing the numbers and what to expect. (thank you too hoppy) I have only gotten back into RC a few months ago after dropping out 8 years ago. My 39" and 47" Park Flyer trainers I built in the last 4 months are my entry into electric flight. For the time being I am satisfied with my old JR8103 72mhz radio. Perhaps later if and when I get into larger and more expensive and complicated electric planes I will switch to the newer 2.4 telemetry radios. And yes, your information here has just added another 20% longer flight for me, as now that I feel I can go down safely to the 30% or so capacity, instead of quitting at about 50% capacity. Being I only started doing solo flights on my own just now too (I was on a buddy box 8 years ago when I quit) the 5 minute flights on my two batteries have just been extended to 6 minutes each. That is 12 minutes of flight, instead of 10 minutes with the two different batteries, which actually feels like a long time when you are as nervous as I am. LOL . . . . Thanks for your help. -Randall ........ ..... .. By the way, your English and grammar are quite good. I'm always impressed how people around the world learn English as a second language, when so many of my own fellow Americans born and raised here can't properly speak or write their own English language! But we have been in decline for a long, long time, (I've observed this since the mid 1960's) and the education system has lowered the standards so Johnny can still feel good about himself and be promoted out of grade school though he can't hardly add and subtract basic numbers, or read and write his own language properly. Good thing he can get by with texting, and carry a dollar store calculator to add 2+2. Topic for another day I guess! |
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Thanks for that good info! |
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Quite agree with you about that, and I'd never want to test anywhere near zero or purposely below 20% capacity. What James brought to my attention was that I'll still have some margin of error if on that rare ocassion I happen to make the wrong estimate of how much flying time I have..... before the plane really gets so low in capacity that it's in trouble.
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Yes I would never suggest that you actually fly down to 0%, just that you won't have a brownout if you do. I have accidentally taken a pack below 0% a couple of times though, 0% is not completely flat, I just discharged it lower than I meant to and it took more than the rated mAh to charge it back up. I didn't lose power to the motor but it probably took a chunk of life off the battery.
I often fly down to around 40% just so the pack is at a reasonable storage charge when I finish. On planes that already have a reasonably long flight time it's worth the tradeoff to have the packs already at a storage charge when I'm done. |
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