Understanding Lipo Batteries

[bweaver]

Last night I noticed a number of students really running their batteries down to get every last moment of flying time out of them. 


Please take the time to refer to the article on lipos found at the following link.  You might find it useful and extend the life of your batteries, or at least not cause them to prematurely deteriorate. http://www.rchelicopterfun.com/rc-lipo-batteries.html


The following extract taken from the article is what I was referring to last night.
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THE 80% RC LIPO BATTERY RULE TO THE RESCUE![/color][/color][/color]A very good rule to follow here is the "80% rule". This simply means that you should never discharge a LiPo pack down past 80% of it's CAPACITY[/b] to be safe. For example, if you have a 2000 mAh LiPo pack, you should never draw more than 1600 mAh out of the pack (80% x 2000). This is assuming a healthy pack as well that has the full 2000 mAh capacity (as packs age, their capacity drops).[/color][/color][/size]
[/size]This again is where computerized chargers pay for themselves many times over so you can see how much capacity the battery takes allowing you to adjust your flight times accordingly to stay within that 80% rule to get the most life out of your pack.[/size][/size]If you don't have a computerized charger to confirm the amount of capacity, another good indicator is to measure the open circuit voltage (no load voltage) of the pack or individual cells right after a flight/drive with a digital volt meter or other similar digital voltage measuring device. An 80% discharged LiPo cell, will give an approximate open circuit voltage of about 3.74 volts. A 3S LiPo pack therefore would show about 11.22 volts after a flight when it's about 80% discharged, a 6S pack would be in the 22.44 volt region. The longer you wait after the flight/drive, the less accurate this voltage method of determining an 80% percent discharge works because as the pack rests after the flight, the resting open circuit voltage recovers slightly, perhaps up to 3.78 volts or so. Remember, states of charge in any battery are based on capacity, not voltage for the simple reason voltage drop in a battery is non-linear.
[/size]I for instance use these little inexpensive LiPo battery monitors after most flights to gauge my flight times to ensure I'm not over discharging my packs much past 80%. These ones I use work with 2S to 6S LiPo packs.[/size][/size]They are also very useful to quickly identify fully charged and discharged packs when you get them mixed up by mistake so you don't accidentally put a discharged pack in your machine thinking it was fully charged. Not sure about you, but when I go out for a full day of flying, I can easily have a couple dozen LiPo's on the go and it doesn't take much more than a simple interruption or memory lapse to get packs mixed up.[/size][/size]You just plug the little rascal into your balance plug on your LiPo battery after the flight (or drive) and it will show the voltage of each individual cell in sequence, followed by the full voltage of the LiPo battery pack.[/size][/size]You can see in the photo above I have plugged the little monitor into this particular 5000 mAh 6S pack's JST-XH balance plug in a Bell 206L scale heli that I just finished building to get an idea of flight times to correctly set my flight timer. All cells in the pack were showing about 3.74V after this 8 minute flight which again is pretty close to an 80% discharged state. I might be able to push it to 8:30 minutes, but to keep things safe, I set the timer to 8:00 minutes and it's working great (confirmed by charging the pack on a computerized charger to see how much capacity it takes). It should take about 4000 mAh of charge (80% x 5000 mAh).
[/size][/size]I have at least half a dozen of these little monitors and take at least two or three out to the flying field to be sure I can always easily place my hands on one. As I said, they are very inexpensive and I personally could not live without them now.[/size][/size]Speaking of timing your flight, I find this much more accurate than having some sort of low voltage monitor or telemetry voltage warning. Under high loading for example, you may get a low voltage warning even when the pack is nearly fully charged. On the flip side, under light loading, the voltage may recover slightly preventing the alarm from sounding even though the pack is being discharged below 80%. The timing method might be old school, but it consistently gives the best results in combination with measuring how much capacity goes back into the battery.[/size][/size]

[sihinch]

Great article and explanation Bruce. Thanks for sharing.

I also read an article recently about running Lipo packs in, before hard use (like racing.) simplified it was :

1st 3 to 5 flights only use a max of 5C current draw  (so on a 2000mAh Lipo that would be 10A) and max 60% of capacity (1200mAh)