Help understanding 80% charge rule with Lipo batteries

[esrazor]

Question 1: I'm confused on the 80% charge rule for LiPo batteries. I have a 3S 2100mah battery. At full charge this should be about 12.6 volts. 80% of 12.6 is 10.08 volts. So according to this rule I shouldn't be running my batteries down below this value? I thought I could have the cells as low as 3 volts/cell, or 9 volts for this battery, and still safely charge and maintain a healthy battery?

Question 2: My ESC has a battery cut-off at 60%, 65%, and 70%. All of these values are well below the 80% rule. Even the highest, at 70%, would still leave my battery at 8.8 volts...

Should I even use the battery cut-off feature? Seems like I'd be below safe voltage when it would finally alert me.

Can anyone clarify these issues for me?

Thanks in advance for your help!

[Coolice]

Hey,

I think you may be reading the 80% rule wrong. Basically this relates to how much of the LiPo's capacity you can safely use on each flight, which is 80% of the overall capacity.

As an example lets look at a 1000mah LiPo pack.

80% of 1000mah = 800mah, this means that with each flight you must not use more than 800mah to prolong the life span of the pack.

Yes, you can run the LiPo to 3 volts per cell, so a 3s pack will need to stop flying @ 9volts. There are many lipo guards, eg. small volt meters which monitor the pack voltage and signal this 9volts via a light or buzzer which work very well.
This will not harm your pack as long as you land when the pack reaches 9volts. A safe guard is that as soon as you power down the packs voltage will rise again slightly meaning you are being kinder on the pack. Just dont be tempted to take off again.

I have found that when the time to land arrives you will see and hear a change in the rotor rpm, with time you will notice this to I am sure.

Question 2 : I think those figures possibly relate to how much you use from your pack in terms of a percentage figure. Eg. fly 60/65 or 70% of the packs capacity.
You haven't mentioned what ESC yoiu are using and so I cannot comment further but that is what I think it means.

Personally I dont use the ESC's battery cutoff facility, I'd rather have the say as to whether I land or not and not the ESC killing my main drive motor at the wrong time. However this does mean you have to be careful not to cause damage to your LiPo's by flying them to long. But with a little trial and error you can work out a time limit for your flying and use a timer to time your flights and then land when the buzzer goes off. You can work this out be monitoring how much charge you put back into the pack after a flight.
Start off low, say 4~5 minutes depending on how hard you fly and go from there.

Hope this helps a little?
.

[esrazor]

Thanks - The ESC is an Align 25amp on my T-Rex.

OK so it sounds like the 80% rule is 80% of the rated milliamp hours, and not 80% of the volt capacity...

Thanks!

[Pinecone]

And I disagree that you will recognize when to land. If you fly the pack to the point where the performance drops off noticeably, you will be drawing more than 80% of the capacity.

I use two methods.

1) Timer - I fly a 5 minute flight and measure the amount put back into the pack upon charging. Divide by the 5 minutes and get teh mAH per minute. Divide 80% of pack capacity by the mAH/minute number to get the number of mintues of safe flying. Set timer to 30 seconds less.

2) Low voltage alarm - in case you forget timer, timer doesn't work, of various reasons the amp draw goes up (binding bearings, tight tail belt, motor going bad, etc), or you take off with anot fully charged pack and you hit a low voltage before your timer goes off.

[Coolice]

Hey,

With voltage detectors fitted to Swifts, Trex's etc. you can audioably hear the head speed drop slightly just before the lipo alarm or LED lights. You can see it to. I have less experienced mambers in my club which have all picked up on the signs.

The good thing is as soon as you land and the main motor is switched off the load is removed and so the lipo voltage rises slightly, back into a more safe zone.

A lot of it will depend on your surroundings, if your out with an IC machine buzzing at the same time then yeah you may not notice anything.

Either way a LiPo monitor of some sort is a must for battery safety.
.

[Efliernz]

The alarm will also spot a pack that suddenly starts to loose a cell. Stopping at 80% current draw will really help the batteries future. They don't like being taken too far down.

Failing to start your stopwatch also creates issues - I was sent a photo from someone who forgot to set the stopwatch and liked to do stationary flips at 3 feet. When the low-volt cutoff from the esc dropped in, it got messy... he flys with an alarm now :glasses2:

[Pinecone]

I would say then you lipo alarms are set too low.

WIth my helis and alarms from Efliernz (super products BTW) there is no head speed change at alarm activation. And I put back in just right at 80% when the alarm goes off.

[xircom]

I have tried many things, ESC cut-off, alarms ... whatever, but at the end of the day the recommended method is the timer, and adjusting the timer after you see how much the charger is putting back. Of course after some weeks or months, the capacity of the battery will go down a bit and then I use 80% of that one, for this (and only this) purpose I might use the dis-charging function down to 3V/cell and see how much I've got still.