Help needed. Why the battery is struggling with 3.8volts left of charge? - RC Groups
Jan 28, 2013, 01:49 AM
Vote F5J Gracia for FPV
Discussion

# Help needed. Why the battery is struggling with 3.8volts left of charge?

I am facing a problem I cannot solve.
I am using a 42 NTM motor on a big flying wing I have.
Max current draw at WOT is 55A.
I have two zippy HK 5.000mah 4s lipos. One has a discharge rate of 30 and the other of 40.
My esc is 80A.
The strange thing is that when the lipos are fully charged (4.2V) they give a full 55 A to the motor. While flying the performance drops significantly. When I land and check the lipos have 3.8v left in them. When I try to give WOT they only give 25A.
This happens when I am using only one of them connected. If i connect them both parallel the give a combined 55 A to the motor.
What is happening?
Is there any logical explanation?
ESC is not the problem because when I connect a fully charged 4s it works like a charm. Maybe is some ''soft cut off'' to protect lipos? But at 3.8V? Strange.
The weather is cold at 5 degrees Celsious. Is it possible to affect discharge? Shouldn't this happen below zero degrees?
Any help is appreciated.
Thank you
Sakis
 Jan 28, 2013, 02:24 AM Registered User When you measure 3.8V after flying the battery has had some time to auto recover. However this means the at the end of the flight and wot the battery has dropped under 3.3V. So the bottom line is you are discharging the battery as low as it can go. And yes the cold weather is contributing. If you are going to fly in colder temperatures you need to keep the batteries warm. Keep them in your pocket before flying. You might also consider insulating then in the plane. If a single battery only gives you 25 amps then it is logical that two in parallel can give you 50 amps. I am not about to split hairs over the additional 5 amps. Glen
Jan 28, 2013, 08:04 AM
Registered User
Quote:
 Originally Posted by sgrouts I am facing a problem I cannot solve. I am using a 42 NTM motor on a big flying wing I have. Max current draw at WOT is 55A. I have two zippy HK 5.000mah 4s lipos. One has a discharge rate of 30 and the other of 40. My esc is 80A. The strange thing is that when the lipos are fully charged (4.2V) they give a full 55 A to the motor. While flying the performance drops significantly. When I land and check the lipos have 3.8v left in them. When I try to give WOT they only give 25A. This happens when I am using only one of them connected. If i connect them both parallel the give a combined 55 A to the motor. What is happening? Is there any logical explanation? ESC is not the problem because when I connect a fully charged 4s it works like a charm. Maybe is some ''soft cut off'' to protect lipos? But at 3.8V? Strange. The weather is cold at 5 degrees Celsious. Is it possible to affect discharge? Shouldn't this happen below zero degrees? Any help is appreciated. Thank you Sakis
How long does it take before the power sags and is this something that just started happening? As Glen said the fact that the battery was at 3.8V doesn't mean much since it's an unloaded voltage and the battery has had a chance to recover somewhat.

Mike
Jan 28, 2013, 08:57 AM
Space Coast USA
The reasonable explanation.

1- Paralleling the packs increased the capacity.
If you have a 5500mah battery and a 55A current, that is a 10C draw.
If you parallel 2, 5500mah packs together, that is now a 11000mah battery and a 55A draw is now only 5C.
The paralled packs will provide the 55A for about twice as long as a single pack.

2-
Temperature and Lipo performance.
From the Battery University. http://batteryuniversity.com/learn/a...w_temperatures
Quote:
 Cold temperature increases the internal resistance and diminishes the capacity. Batteries that would provide 100 percent capacity at 27°C (80°F) will typically deliver only 50 percent at –18°C (0°F). The capacity decrease is linear with temperature.
Looks like 5C would reduce the capacity by about 10%. The low temperature will also reduce the C-Rate available.

3-
How old are the batteries?
Were they stored at full voltage?
Last edited by hoppy; Jan 28, 2013 at 09:06 AM.
 Jan 28, 2013, 11:23 AM Vote F5J Gracia for FPV ggcrandall1, Mikecr and Hoppy, thank you so much for taking the time reading my Q and posting a helpfull reply. So if the decrease is linear, at -15 I get 2500mah from a 5000 mah at 0 C I get 70%, so about 3.500 mah. Right? Regarding parallel lipos, I flew using one lipo at a time. I just mentioned that I connected them in parallel to troubleshoot and exclude the ESC as problematic. Yes I kept them fully charged for about ten days due to cold and rain. Can this lead to such problems? Regarding age of lipos. One has about 20 cycles and the other about 40 cycles. I surely keep them in my pocket to be warm before take off. But during flight they are exposed to cold. Mikecr, this a new heavy wing I built for fpv usage, so this problem is new. My 5.000 4s lipos were used until now in a smaller plane with a top amperage draw of 20 Amps the most. This new config draws a max of 55 A at WOT. But I only WOT for a few secs during handlaunch take off , and then I was cruising with less than 10 amps just to estimate how much time do I have for fpv flying with one 5.000 mah lipo. So after about 10 min I got the strange dying of my lipos. Maybe they froze during flight, I felt 5 C on ground, but they were flying top side exposed on a big flying wing. I cannot estimate how low the temp dropped. Thank you again and if you think of anything else, pls share. Sakis
 Jan 28, 2013, 02:07 PM Space Coast USA Ok, thanks for the feedback. If you fly for 10min till the battery died, that's a 6C draw. 6C x 5000mah = 30A ave draw. Are you sure it was a 10A draw that you observed? That would be a 2C draw. The 2 weeks at full charge wouldn't explain the loss in power/capacity
 Jan 28, 2013, 02:34 PM Frankenstein recycled packs When in doubt I always put a simple cell checker on the balance taps. Run the motor at WOT and watch the voltage of each cell. I expect above 3.5V per cell under WOT or the battery can't deliver as designed or as I need. Also, being you have 2 packs doing this store one in the house for >4 hours. Wrap it in anything that can insulate it from the prop wash. Repeat test with both packs. 5C is very cold and is probably the cause. Rick
 Jan 29, 2013, 03:15 AM Vote F5J Gracia for FPV Hoppy, Maybe I am pulling more amps without knowing. If only I had my fpv gear onboard I would knew exactly what is going on through my current sensor. But I don't want to put expensive stuff on the wing before I am completely certain that it is reliable. Rick, I will retry using a lipo checker in the balancer cable of my 5.000 4s bat. I never believed that winter cold has such an adverse effect on lipos. I have read about it but I always hoped that the winter dense air will make everything better regarding performance. Guys thank you both for helping. Sakis
 Jan 29, 2013, 10:23 AM Frankenstein recycled packs True for your motor and ESC but not for any battery that I know of. Remember the old TV commercials showing a Die Hard battery starting a car at xx below zero? Let us know the findings when you have time to do this test. Rick
 Jan 29, 2013, 11:48 AM Space Coast USA Sakis, Open this document and look at the 3rd graph showing discharge at 0.5C at various temperatures. Even at this low of discharge rate notice how the voltage drops with decreasing temperature. I imagine the drop is worse at higher current draw. Heat the pack up to 30C and wrap it in a foam blanket.
Jan 29, 2013, 11:51 AM
Vote F5J Gracia for FPV
Quote:
 Originally Posted by rampman True for your motor and ESC but not for any battery that I know of. Remember the old TV commercials showing a Die Hard battery starting a car at xx below zero? Let us know the findings when you have time to do this test. Rick
Will do, sir!

Jan 29, 2013, 11:55 AM
Vote F5J Gracia for FPV
Quote:
 Originally Posted by hoppy Sakis, Open this document and look at the 3rd graph showing discharge at 0.5C at various temperatures. Even at this low of discharge rate notice how the voltage drops with decreasing temperature. I imagine the drop is worse at higher current draw. Heat the pack up to 30C and wrap it in a foam blanket.
Impressive! Thanks.
I must cover them somehow. And heat them.
( using speed controllers heat dissipation maybe)
I dont know... I'll think of something.
Sakis
 Jan 29, 2013, 05:42 PM That's a funny word This trouble can show up because you are at the tail end of the discharge "knee" of the packs. There is capacity at 3.8 volts , but not much. Certain cells are probably falling on their face for voltage because they are at the end of their capacity , not because they are cold. I can think of two ways to distinguish between the two problems (end of capacity or cold temp) and the temp problem is a very real scenario. One way is to do what rampman said and put the cell checker on during the discharge. Watch it at full charge and then watch it again at 3.8 resting. You'll even be able to spot your problem cell(s) at the full mark , but they will be glaring (low voltage) at the other end where you're having the trouble. The other way is to simply measure the temps before and after. ps. I question the resting voltage of 3.8 at the end of your problem flight. Are you sure that all cells are the same voltage? (3.8 or so) Watch the balance of these pack closely as cell degradation sometimes isn't a slow process once it shows up like this.
Jan 29, 2013, 06:08 PM
Registered User
Quote:
 Originally Posted by rampman True for your motor and ESC but not for any battery that I know of. Remember the old TV commercials showing a Die Hard battery starting a car at xx below zero? Let us know the findings when you have time to do this test. Rick
Hey Rick, how would you know about cold affecting batteries living there in the land of perpetual summer.
Glen
Jan 29, 2013, 07:23 PM
Vote F5J Gracia for FPV
Testing results:
I got my two lipos without charging them after the test flights that I already told you about.
Room temp= 24 C
I connected a lipo checker to the balance port.
I also connected my HK wattmeter that usually measures a top Amperage draw of 55A with lipos fully charged.
Both of the lipos started with 3,85 each cell with some cells at 3,84 and gave at most 25-27A at WOT.
During that WOT the individual cells dropped at 3,55V the minimum. To be honest they were holding at around 3,65 but after a few secs I saw 3,55 at some cells.
So no below 3,30V dropping as I was expecting.
Time to check other things.
I reprogrammed my moxie 80A esc (low pricerc) using my programmer card and changed two settings:
Gas pedal set to high from medium
Model start set to low from high.
Repeat the tests using both lipos:
They both gave almost 45 Amps each!
And without recharging after the problematic flights...
As I said this is a new setup on a new wing that I built.
I have used my esc at the past with 3s lipos on smaller motors,
But now I am using an NTM 42-38 motor with a 14x8 folding prop.
To be honest I do not even know what that esc settings are doing exactly.
Thank you for helping.
Sakis.