Help with what battery charge pecentages are reflecting.

[jw423]

So I have been doing some research on Lipo care and I am a bit stumped on these pecentages and what they mean exactly.
Here is a chart I found.


Battery Voltage per cell at resting charge amount.
4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%
Do not go below 3.0V

3s Values
12.60 = 100%
12.09 = 76%
11.58 = 52%
11.49 = 42%
11.37 = 30%
11.10 = 11%
10.80 = 00%
Do not go below 9V

How does this work? 50% of 12.6V is not 11.5 something...

Also is it better to keep track of battery use with Volts or with amp/hrs used?

I hope I'm not just being stupid here but it's bothering me.

Thank you.

 

[rcspaceflight]

I think it's because you go by amps for how much battery is left, but volts is a lot easier to check than amps. As the charge of the battery gets used, the volts drop by a certain percent. Once all the amps are used, then the volts drop down to 0 really, really quickly. It's like if you have a five gallon bucket full of water with a hole on the bottom of the bucket. The volts are the force that the water leaves and the amps is how much water is running out. The force of the water leaving the bucket slowly gets less as there gets less water in the bucket, but once the water is gone, the volts drop down to 0 really, really quickly, if not instantly.

You want to go by how many amps are added to the battery when you charge it. That is the most accurate number. But if you simply have a battery laying around and want to check to see if it's close to full, then you go by volts. It's really the only way to check how full it is with out charging the battery.

By the way, thanks for the chart. I recently put a 3cell LiPo in my Tx and I have a voltmeter to check the volts and I was unsure of how low to let the battery get before I have to charge it.

 

[xuzme720]

With lipo's, because of their chemistry, they are considered dead at 3v. Dead in this case just means that they have no more capacity to give. Not dead in voltage. Damage can be caused by taking the per cell voltage below 3v. Resting stable voltage should be around 3.8v per cell for storage if you aren't planning on using the pack for awhile.


Keep in mind most voltages given between 3 and 4.2 volts are going to be "per cell", so if you see someone recommending storage or stay above, etc. in that range, it's not for the whole pack but per cell.

 

[Craftydan]

I disagree with using mAh. mAh consumed is only reliable when the battery is new (not trying to be disagreeable, but mAh consumed is unreliable).

As the pack ages and it looses capacity, using mAh to time the battery will encourage you to draw a larger percentage of the pack each time, accelerating the decay of the cells. Voltage level, however, is based on the chemestry and the remaing % capacity based on resting voltage doesn't change with age and wear.

For the direct answer to the initial question, the voltage/mAh discharge curves for batteries are not linear -- they have a shallow slope between resting 4 and 3.7v, but around 3.7v it bends sharply down and an increasingly small draw in mAh will drive the voltage down. In short, we stop early at 3.7v, because it drops to "too low" too to bring the airframe in to land without damaging the pack.

 

[jw423]

For the direct answer to the initial question, the voltage/mAh discharge curves for batteries are not linear -- they have a shallow slope between resting 4 and 3.7v, but around 3.7v it bends sharply down and an increasingly small draw in mAh will drive the voltage down. In short, we stop early at 3.7v, because it drops to "too low" too to bring the airframe in to land without damaging the pack.

Great! I think I'm getting closer. So with respect to a single cell and my above found chart. What is 3.86V or 52% represent? Is it 52% of useful Voltage compared with time, or mAh or something completely different? Because the math doesn't work for useful voltage. 52% of the .5V that we I guess use is not even close to 3.86V

Or maybe I need to look at this from a different perspective. Volts are not really something we can consume right. So is the resting voltage some representation of what amount of mAh should be left in the pack? That would make cense of the odd relationship of percentage. Does a pack have about have of it's mAh's left at 3.86V? I think this is somewhat what rcspaceflight was saying.

Thanks everyone for being patient and helping out! All comments are very appreciated.

 

[jw423]

By the way, thanks for the chart. I recently put a 3cell LiPo in my Tx and I have a voltmeter to check the volts and I was unsure of how low to let the battery get before I have to charge it.

Any time I'm glad I could help someone else with a question. That doesn't happen very often.

 

[Craftydan]

Or maybe I need to look at this from a different perspective. Volts are not really something we can consume right. So is the resting voltage some representation of what amount of mAh should be left in the pack?

That's pretty much it, and if the draw is roughly constant (I'm always flying plane A at 1/2 throttle criuse) you can corrilate an "in use" time-to land-voltage with the "resting" almost-empty-voltage, but that might take a flight or two to bracket that in.

In practice I've used an adjustable voltage alarm, and I set it too high. It goes off in a cruise, and I land and measure %. If I've got battery left, I adjust the voltage down a touch, and fly a little more. I usually bracket around 20-30%, so if I'm in the teens, I bump it up a notch and test again on the next pack. There are many of these out there, most are identical -- look for adjustable voltage and a loud alarm. IMO, they're useless without these, and any other features are fluff.

As for how these tables come about, my understanding is the percentages are ususally measured using mAh off of a slow (1/10C rate or slower) draw between fully (and safely) charged at 4.2 resting and 3.3v resting (yes, voltage drops that much between 20-0% that quickly). The percent is then calculated by what was drawn at each voltage divided by the total draw top to bottom. I have no idea of the petegree of the chart you found but the numbers don't look that far off -- Most I've seen have 3.7v as 20% capacity, but even in yours I expect they'd set 20% in the very low 3.7s -- the curve starts to bend sharply there.

So the little argumentitve voice in my head starts screaming, "if mAh is unreliable, then aren't you just changing mAh to percents? same thing isn't it?" (I hate that guy)

In measuring the percents, mAh is used to measure the fraction of the whole and not as an ablsolute amount -- as the battery ages (or you pick a different size battery), the fraction will stay the same because both the top *and* bottom of the percent fraction scale up/down together. The voltage to percent capacity remaining is only related by the discharge chemestry and not the health or size of the pack.

 

[jw423]

Very good! Thank you Crafty Dan. I think I got it now. To be honest I guess it just doesn't matter but it was something that has been driving me nuts. At first I just took all the recommended voltages and percentages and just went with it. But then that silly old What is going on here part of me decided to interrupt the fun.

Thanks again, I feel much better and can now go back to just following all the good advice. Except now I don't have the nagging feeling.