E-Flight Power Sources Ask questions or read about power sources as they specifically relate to e-flight including Lithium-ion, Li-Poly, Nimh and Nicad battery packs.

Cell readings

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Old 10-06-2010, 06:03 PM
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callsignMERLIN
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Default Cell readings

So the guy at the LHS said that I had to have this little tool that reads the individual cells and the total voltage of the pack or else I am charging and discharging blind..I believed him but I have really no idea what I'm looking at. After a charge the cells read 4.1 4.2 4.2 which is a total of 12.5.. Post flight(6-8 min flight) they read 3.85 3.87 3.86 total11.6. The packs are 3 cell 11.1V 1800mAh packs. Does this sound right? What is a safe level to run them down to? and at what rate is the best to charge these lipo's at? and what voltage should they be stored at if used on a weekly basis?

I also have a 2200 mAh pack that after charge is 4.11 4.13 4.12 = 12.4

My other question is what does the the mAh have to do with the performance of the motor and flight time of the plane? I really dont notice any difference between the two..

Thanks guys just trying to figure it all out.

Merlin
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Old 10-06-2010, 06:07 PM
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Default RE: Cell readings

The lhs guy is correct.

The "mah" capacity is directly responsible for runtime. It is like "gallons" of fuel.

For the voltages: Per cell. 3.5-3.7 is considered "empty" Here's two charts.

3.7=0%
3.8=20
3.9=40
4.0=60
4.1=80
4.2=100%




100.00% 4.2
90 4.13
80 4.06
70 3.99
60 3.92
50 3.85
40 3.78
30 3.71
20 3.64
10 3.57
0.00% 3.5

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Old 10-06-2010, 06:10 PM
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Default RE: Cell readings

I forget storage voltage is about 3.85 per cell. Charge rate depends on the packs themselves, most are 1C-6C. Older ones maybe 1C , newer higher discharge packs are 5C.
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Old 10-07-2010, 07:13 AM
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Default RE: Cell readings

So basicaly the volt reading on the pack is the minimum V the pack will reach.. Does the mAh also determine the motors performance along with its run time or is the performance/speed of the motor regulated by the esc? What does the max C discharge determine, for example the one pack says 15C while the other pack says 20-30C...

The chart makes sense.. but is 4.2 the max charge for all lipo cells? How do you know what max charge is if (11.1)/3 is the min or a dead battery how do you find the max?

Angain thanks

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Old 10-07-2010, 04:37 PM
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Default RE: Cell readings

Yep, pretty much the voltage listed on the pack is the nominal (or very close to the minimum) It is actually 3.7 volts/cell. A 3S is 11.1 and fully charged a 3S is 3x4.2 = 12.6 volts. Your first charge listed is full, but the second one you listed lacks a bit , but is almost full. 4.2 is full for all know lipo cells.

The capacity doesn't directly affect any performance , only the voltage of the batery (under load) does. (when the esc is at full throttle) The capacity "C" does enter into the equation for figuring out max currents. It is multplied by the number in front of it. The 15 or 20 is a mutiplier and the "C" stands for capacity so in two examples :

A 20C 1800 pack = 36 amps
A 15C 2200 pack = 33 amps
A 30 C 2500 pack = 75 amps


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Old 10-07-2010, 06:41 PM
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Default RE: Cell readings

so likein the third example a 30C 2200 = 66amps but that would be limited to 30A max if you have a 30A esc?

and 15C is really 1.5

Is it normal to see a decrease in the cells v over time or with proper charge discharge should you be able to get to 4.2 all the time?

agains thanks

Merlin

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Old 10-07-2010, 06:47 PM
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Default RE: Cell readings

If a vendors says 30 C continous as in the 3rd example it means that the batery has the max capability of a continual discharge current of 66 amps. (don't always trust them though) The esc has a rating of 30 amps , but it is the motor/set-up that will determine the actual load. In this chain the esc is the weak link so a load of 30 amps would be the max. In a general sense then if we got a 60 amp esc then the weak link is still the esc and we can pull 60 amps max.

15 C is really "15 times the batery's capacity"

A charger should alway shoot for 4.2 at the ending voltage (full mark)
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Old 10-07-2010, 09:04 PM
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Default RE: Cell readings



ok that makes sense..  does the amount of restriction that an esc puts on a battery affect its life/performance..it seems like a waste to have a 3cell 2500mAh battery paired up with a 15-30A esc. but I guess if you went with less cells with same or less mAh the battery would draw a higher current(drain faster) resulting in less flight time? am I learning anything? 

this is kind of a broad question but what does the Kv rating of the motor determine compared to the amp output of the battery?  how do motors of different sizes have the same kv rating?

If I increase the esc can it increase the performance of the motor, or am i possibly pushing it to the point of meltdown and would the flight time be so short it wouldnt be woth while?

again thanks

Merlin

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Old 10-08-2010, 05:43 PM
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Default RE: Cell readings

Higher current does indeed = shorter flight. Increasing the esc size will only "protect it" from a motor/prop that pulls high current. The motor/gear/weight/prop will determione the power or thrust , and the esc and battery just have to be size "bigger than that"
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