Lipo Voltage Questions

[alienx]

I have two new 3S lipos that I have not charged yet, but have been using them to center my servos and test my motor and whatnot. I am getting a reading of about 11.55 volts (11.1 volt batteries).

I was wondering if someone could explain why I see a higher voltage than 11.1, and what I should actually expect to see on a full charge and a fully discharged battery. I would think the discharged voltage will be either 9v or 9.9v (I'm a little confused on the 3v versus 3.3 volt cut-off that I've seen described in different places).

I guess I am also curious about if you use your battery for a short flight and then check the voltage, how can you decide if you have enough juice left to go back up?

Thanks. Andy

[Geoff_Gino]

Hi Andy

A fully charged 3S pack should read 21,6 volts (4,2 volts per cell).

Do NOT EVER let the individual cells get below 3 volts per cell or 9,0 volts for a 3S pack.

This table will give you the remaining charge in a 3S pack:

100%=12,60
90% = 12,45
80% = 12,30
60% = 12,00
40% = 11,70
20% = 11,40
10% = 11,10

Hope this helps.

[Unbalanced prop]

No load fully charged 3S pack should read around 12.6 volts. A pack at 50% charge should have a no load around 11.55 volts. An empty pack is around 11 volts. These are resting voltages. A healthy lithium pack bounces back up after a LVC under load is reached.
Most "experts" are saying to set the LVC to 3.3 volts per cell and it should bounce up to 3.7 volts after resting a few minutes. They say this will help the packs last longer.
I personally time flights and rarely hit LVC. I have had pretty good luck doing this. If a pack is lower then 11.6 resting volts, I probably would charge before flying it again.

Doug

[alienx]

This helps a lot. Thanks fellas. One more question. Do batteries fall off at a constant rate or exponential? That is, Say it took ten minutes of flight to get to a 60% voltage indication. Would it be at 20% if I flew for another 10 minutes? Obviously my example isn't realistic.

Thanks.

[Geoff_Gino]

Andy

No they don't. There is a large initial drop off which then evens off. Closing the throttle and decreasing the load on the battery allows it recover somewhat.

Look throught the forum or do a search, some guys have left great graphs of the discharge curves on LiPo's.

[Unbalanced prop]

Here is the chart I use for no load voltage with lipols. Very similar to Geoff's!

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%


Doug

[alienx]

Thanks! Hopefully this weekend it won't just be theory anymore. I have a Cessna 182 ready to maiden and I am minutes from finishing my P-47D too!

[Geoff_Gino]

Hi Andy

I have this table taped to my flite box. Find it is the only thing that works for me. Measure the cell voltage and you will have capacity left in the pack at a glance.

1S1P 2S1P 3S1P 4S1P 5S1P 6S1P
100% 4.20 8.40 12.60 16.80 21.00 25.20
90% 4.15 8.3 12.45 16.60 20.75 24.90
80% 4.10 8.20 12.30 16.40 20.50 24.60
60% 4.00 8.00 12.00 16.00 20.00 24.00
40% 3.90 7.80 11.70 15.60 19.50 23.40
20% 3.80 7.60 11.40 15.20 19.00 22.80
10% 3.70 7.40 11.10 14.80 18.50 22.20

[Unbalanced prop]

Originally Posted by alienx

Thanks! Hopefully this weekend it won't just be theory anymore. I have a Cessna 182 ready to maiden and I am minutes from finishing my P-47D too!

Good luck AlienX!!!

Doug

[Geoff_Gino]

Andy
I can't get the columns quite right. Just move the column heading over to the right and you'll see how it goes.

[alienx]

That's a good chart, I'll work it out.

Thanks for the help guys!!

[JRBeaman]