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Mar 11, 2014, 05:46 PM
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Battery Math - Low Voltage Point?


Let me talk myself through this. I didn't get what I was expecting, on my first test. I need to tweak something.

I have an F550 and am using 5000mAh batteries. I was planning to try and use it to the 80% point as a test. I set my cutoff voltage to 14.8 and ran the hexacopter to approximately that point. It ran 16 minutes, which was a little longer than I expected. I didn't actually hit the warning but stopped anyway, just to be safe.

When I connected the battery to the charger, the cells measured 3.3x +/- a couple hundredths. A little lower than I figured but not excessively low. Perhaps they should have been closer to 3.7???

Then, I charged the batteries, expecting that it would be a little over 4000mAh put back in. But my charger stopped when it hit 5000mAh because of the capacity limit. I stopped the charger and restarted it and pumped in another 331mAh, for a total of 5331mAh and my cells were back to 4.2V, at that capacity.

So, Why would 14.8 LVC have consumed so much? Did I set something wrong, is my charger fooling me, or something else.

My first thought is to recheck the cutoff voltage in the Naza assistant SW. If everythign was correct, just increase the cutoff level. Redo the test and see if I'm closer to the magic 80%.

I guess my confusion is what the levels should be for the 1st and 2nd warnings in the assistant software. I thought I was setting it to the 80% point. I had it set for 14.8V for the first warning but never saw the warning lights, for that level. The second level was 13.6 (3.4V/cell).

Anyone see something I must have done wrong?
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Mar 11, 2014, 06:24 PM
qar! vemmoH ghIj crow!
d_wheel's Avatar
Quote:
Originally Posted by MikeMSD
Let me talk myself through this. I didn't get what I was expecting, on my first test. I need to tweak something.

I have an F550 and am using 5000mAh batteries. I was planning to try and use it to the 80% point as a test. I set my cutoff voltage to 14.8 and ran the hexacopter to approximately that point. It ran 16 minutes, which was a little longer than I expected. I didn't actually hit the warning but stopped anyway, just to be safe.

When I connected the battery to the charger, the cells measured 3.3x +/- a couple hundredths. A little lower than I figured but not excessively low. Perhaps they should have been closer to 3.7???

Then, I charged the batteries, expecting that it would be a little over 4000mAh put back in. But my charger stopped when it hit 5000mAh because of the capacity limit. I stopped the charger and restarted it and pumped in another 331mAh, for a total of 5331mAh and my cells were back to 4.2V, at that capacity.

So, Why would 14.8 LVC have consumed so much? Did I set something wrong, is my charger fooling me, or something else.

My first thought is to recheck the cutoff voltage in the Naza assistant SW. If everythign was correct, just increase the cutoff level. Redo the test and see if I'm closer to the magic 80%.

I guess my confusion is what the levels should be for the 1st and 2nd warnings in the assistant software. I thought I was setting it to the 80% point. I had it set for 14.8V for the first warning but never saw the warning lights, for that level. The second level was 13.6 (3.4V/cell).

Anyone see something I must have done wrong?
In my opinion, the 80% rule is something that needs more research. I have a Hyperion EOS SENTRY meter that displays the voltage and % remaining of a lipo pack. I time my flights and the meter displays around 20% at the end of a flight. I also have telemetry on my aircraft that keeps track of current used during the flight. At the end of a flight, the consumed current usually reads 1350 to 1400 mah used from a 2200 mah pack. Current put back into the pack is usually around 1400 to 1450 mah so the in flight reading is pretty close. The resting voltage at the end of these flights is about 3.6 volts per cell. So, how is 3.6 volts 20% of 4.2 volts? How is 1400 mah 20% of 2200 mah? Where does the 80% figure come from? How is it calculated?

Later;

D.W
Mar 11, 2014, 06:33 PM
Registered User
Quote:
Originally Posted by d_wheel
Where does the 80% figure come from? How is it calculated?
From resting voltage, one can easily get a pretty solid estimate of remaining capacity. Here's a recent test that I did on a modern lipoly:



One must stay above ~15% remaining capacity to avoid the voltage dump and associated loss of power that occurs below this level. e.g.- Stay out of 'No Man's Land' if you like your airplanes...:



Mark
Mar 11, 2014, 06:34 PM
ancora imparo
jj604's Avatar
The 80% figure is capacity used not voltage.

So you used 1400mAh from a 2200mAh pack. That is 1400/2200 *100 = 64%. Many packs do not have the full 2200mAh capacity, especially when measured at the current you are pulling so you are probabably getting close to the 80% used.

The relationship between resting (that is no-load) voltage and capacity is only approximate. All of those "Cellmeters" that measure pack voltage and display capacity do a reasonable job (I use one all the time) but they aren't very precise. The relationship between resting voltage and capacity depends on the chemistry of the pack, temperature etc.

Quote:
Originally Posted by d_wheel
In my opinion, the 80% rule is something that needs more research. I have a Hyperion EOS SENTRY meter that displays the voltage and % remaining of a lipo pack. I time my flights and the meter displays around 20% at the end of a flight. I also have telemetry on my aircraft that keeps track of current used during the flight. At the end of a flight, the consumed current usually reads 1350 to 1400 mah used from a 2200 mah pack. Current put back into the pack is usually around 1400 to 1450 mah so the in flight reading is pretty close. The resting voltage at the end of these flights is about 3.6 volts per cell. So, how is 3.6 volts 20% of 4.2 volts? How is 1400 mah 20% of 2200 mah? Where does the 80% figure come from? How is it calculated?

Later;

D.W
Mar 11, 2014, 06:40 PM
Registered User
I guess you're talking about a 4S pack ? If you have set your LVC for 3.7V/cell but the battery only reads 3.3V/cell after a flight then I suggest your LVC is not working correctly. OTOH I know nothing about "Naza assisant SW" works, whatever that might be.

The aim is to read approx 3.7V/cell a few minutes after the flight when the battery voltage has recovered. Generally the LVC is set lower than that but in your case I guess you'll just have to fiddle with the settings until you get the right answer.

Steve
Mar 11, 2014, 09:14 PM
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Thread OP
Quote:
I guess you'll just have to fiddle with the settings until you get the right answer.
I'm in full fiddle mode right now. Just finished another test and set the LVC a little higher. That should lower my capacity put back in. When I hit 4000mAh approx. I guess I'm dialed in. But the math on this seems shakey.

Quote:
The 80% figure is capacity used not voltage.
I understand that. I have a 4s, 5000mAh battery. 80% would be 4000mAh that should be put back in, if I discharged a fresh battery to 80%. But mine charged more.

Anyhow, I'm a bit confused by the instructions for setting the 1st Warning in the assistant s/w.

It says:

1) Measure no load voltage (mine is 16.8), with full charge.
2) Set a "reasonable" 1st limit, with 0 for the loss (I used 15.8V)
3) Fly until the warning goes off. (I did).
4) Quickly measure the voltage. (I did, it was 16.1)
5) The difference between 4 and 1 is the loss (.7V) according to the assistant instructions.

Now, I set the 80% voltage (as a guess, based on what others have said is 14.80). This was 3.7 * 4 = 14.8.

Then, I flew the hexacopter again, until the new 1st warning went off. It should have been 14.8. When the red lights started flashing, I took the battery out and started a new charge. Voltage was reading 14.28 and cells were 3.56. Pretty close (maybe 14.8 - loss is approximately 14.1 so if the LVC number I set is based on the load voltage, that's about right, since I measured it right after a flight. Had I let it sit, it may have risen close to the 14.8. Does that make sense?

Hopefully, this time, when I get the full charge it will be about 4000mAh.

But here's the thing that confuses me. The instructions for determining the loss seem flakey. If I had set a "reasonable" voltage at 14.8, the loss would have been larger. If I set a "reasonable" voltage to 16.00, the loss would have been less. Subtracting the voltage at 1st warning from the full charge no load voltage is not a constant over the full range of battery usage. That's what confuses me.
Mar 11, 2014, 10:54 PM
Registered User
Thread OP
This is wierd. This time, the cells were 3.56 per cell but the battery took nearly the same charge as before. This time 5265mAh. Guess I need to raise the LVC even more. OR, just time the flight for a shorter time (i.e. 12 min instead of 16).


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