3.7V or 80% for LiPos

[ProPilotWannaBe]

Two rules of thumb I've read around are: 1) Don't run the cells below 3.7V, and be 2) Don't use more than 80% of the battery capacity.
These two rules don't seem to jive with each other, unless I'm just misunderstanding them. I started using voyage alarms a few weeks ago and set them to 3.7V. Now when I charge them it seems like I am consistently putting back only about 65%.
Is the 3.7V rule under load? Should I be lowering the alarms to say 3.6 or something so that I am putting in more like 75 or 80%? What am missing? Looking forward to hearing your thoughts and reasoning for them.

 

[Bowley]

I set my alarm to 3.5V so it sounds at 3.5V under load, time I've recovered, Lipo sits at 3.7-3.8V no load, which is convenient as thats storage level. obviously dont get as much flight time, I set them to 3.4 if I want more flight time, still give me 0.1V to recover quickly without risking lipo damage at under 3.3V.

 

[Silverburn]

I use spektrum telemetry set to 10v (3S) under load. This is about 10.8 at rest - about 3.6v per cell. I try and get back to home early on, so I'm normally landing at the first beep - which ends up being 3.7 per cell.

 

[Electro 2]

After months at 9.7V (3Cells) on the Spektrum telmetry, I reset the voltage alarm to 9.6V. This seems to work out very well. The bottom for LiPo is officially 3.0 VPC, loaded or otherwise. That said, I shoot for 3.2VPC to allow for some cell mismatch slop.

 

[bruce365]

Electro how much value do you place in your multi rortor? at 3v the battery is empty empty empty at 3.2 volts its also literally empty how much room do you give yourself to land before the tank is empty? very very poor practice, not to be encouraged.

 

[CrashMaster]

My cells Zippy and Nano-Tech both are set to 3.3v to 3.5v under load so the alarm sounds when any cell falls to 3.3v. At rest they are back upto 3.5v which is exactly 80%.
Note that the only way to know how much is left is to re-charge them and note the mAh put back in - 5000mAh Lipo takes 4000mAh to full charge so was 80% discharged before re-charging. However, I am also told that the Nano-Tech can be taken lower, even to 90%, without problem as they are a higher quality composition.
The only problem is that different manufacturers, individual cells and even different Lipo packs from the same manufacturers have differing results so it is a suck and see. I would say 3.3v is the lowest you ever want to go and land immediately. To give a bit of space set it to 3.4v or 3.5v. However, it is a test and check situation for every pack. Start at 3.5v and work down if you find you have 30%, 25% etc... then document each pack and write the voltage on the pack so you can alter the alarm for each pack. One of my packs do have set to 3.5v. Also remember that packs will be subject to deterioration with use so the value may vary over time.

 

[ProPilotWannaBe]

Thanks guys for your input. I know I wouldn't be confortable going below 3.3V, especially when I got camera gear on it.
CrashMaster, you pretty much described what I was thinking of doing which is to aim for 80%, and monitor each battery to see what voltage gets me there and continue to adjust as the battery ages. I hadn't heard about the higher usable percentage on the nano-techs. Don't plan to push it on the S800, but maybe I'll try that on the 450 to see how they handle that.

 

[Dewster]

I ran into the same problem, but my lipos were...old I guess. My volts would drop before hitting the 80% threshold. You should track the flight times of you batteries. They are also affected by temperature. Lipos lose flight time with age. It is a good idea to use a lipo checker. My video transmitter has a microphone that picks up audio from the craft. I can hear the lipo alarm in flight and know that the battery is getting low. My EZOSD also shows the voltage level.

Be careful stretching flight times, especially with old lipos and DJI's voltage protection. You'll end up like this YouTube video

http://youtu.be/hGZWIQRBVE8

 

[Electro 2]

"At 3v the battery is empty empty empty at 3.2 volts its also literally empty how much room do you give yourself to land before the tank is empty?"

Mere seconds to land, nada issue, special flight conditions. Front yard, learning to fly, max altitude 5 feet. When we start to fly "off", somewhere, it will need to be raised for an early warning. Based on current/voltage trends from current flight data loggings, I'm guessing to about 10 volts or so for a 3 cell is a good place to start.

 

[ProPilotWannaBe]

Yeah, I've been tracking each battery's flight times and voltage and capacity filling pretty close for the last 3 weeks and plan to continue doing so now that I use RC Flight Log app and carry it with me everywhere.
I also have the voltage on my OSD, as well as audible voltage monitors on my aircraft. I haven't turned off the voltage protection on the WKM yet, but plan to do so as soon as I get some free time (been traveling a lot the last 10 days).
Thanks guys!

 

[ProPilotWannaBe]

When we start to fly "off", somewhere, it will need to be raised for an early warning. Based on current/voltage trends from current flight data loggings, I'm guessing to about 10 volts or so for a 3 cell is a good place to start.

Electro, just as a data point for you, I have friends that return to home and land as soon as the 3.5 voltage alarm kicks off, maybe allowing themselves another 30 seconds to finish what they are shooting at the time. The idea is to preserve the batteries as much as possible, and more importantly, the 10K aircraft they are flying. At 3.2V per cell, I'll bet you those batteries are pretty warm which indicates the battery isn't really happy. Just keep that in mind on any expensive batteries or aircraft you'll be flying.

 

[CrashMaster]

Another thing I forgot to mention above is to note the voltage in hover and at rest.
If you get the voltage alarm with the led you can see the voltage for each cell and total voltage. Mount it so you can see it and hover 4-5 feet off the ground to remove ground effect and note the voltage - land and give it a couple of seconds and note the voltage again, you will notice a difference. Do this several times during the flight and compare the differences. My xaircraft X650 has a voltage difference at the beginning of the flight of only 0.3v but as it gets to within the last minute of flight it goes to 0.4v or 0.5v dependent on the lipo pack. This is the one you are interested in because this is how you know if you have killed your battery or not. also note that voltage drop near the end of flight is also the fastest. If the machine is flying and the voltage drops to 3.3v and the alarm goes off and you land you find that your pack voltage at rest (in my case) is still 3.7v or 3.8v. this means that my cells will never die from being over discharged. You can take them down lower so I could set my alarm to 3.00v per cell and still have 3.4v a cell when at rest. This, in theory would not kill the pack.

It is up to individuals but the lower you take them then the shorter the life of the pack.

I am now beginning to think I should disable the lipo protection in the WKM because when it cuts in it really does drop the machine out of the sky at an alarming rate. I don't know why because the NAZA doesn't - The NAZA does a graceful decent and touches the ground bounces twice and lands.

 

[Electro 2]

"Return to home and land as soon as the 3.5 voltage alarm kicks off"

10.5V I'll keep this in mind when setting a for-real flying alarm. Really, all the logging isn't necessary, just do a total pack impedance check every few flights and you'll know immedately which packs kick and which are "iffy".

"At 3.2V per cell, I'll bet you those batteries are pretty warm"

Actually, no, only comfortably warm, not hot. If it gets genuinely hot, pack is a sinker. Source imepdance will show this in a heartbeat.

 

[ProPilotWannaBe]

Good info. Thanks for sharing!