Batteries How to measure the fully charged capacity in mAh of a LiPo cell?

[Robair]

Is there a way to measure or determine the total, fully charged capacity in mAh of a LiPo cell?

With my wattmeter and telemetry, I can measure the used capacity after flight, but without knowing what the beginning total capacity was, it makes accurate endurance calculation impossible.

Thanks in advance

 

[Lee]

I don't know of a way of measuring the actual capacity.
I just make sure I only take out 80% of the total capacity when flying.
This can be hard though as the battery gets older, because the capacity reduces, and your 80% no longer matches.
Thats why I rely on the voltage more than the Mahs.

 

[Tony]

I agree with Lee. The only real way to know is to start out at 4 minutes, then charge the battery. The charger will tell you how many mah you are putting back into the battery. As long as it's not over 80%, you are fine. I try to raise my flight times up to 75-80%. Sometimes, I fly a little harder and go over that, but I still never hit the shut off. Keep flying, keep charging and keep records of how many mah you are putting back in.

 

[Robair]

Ok! but this 80% is expressed as a percentage of something. (presumably and correctly, this should be the REAL capacity of the battery)
I can do a calculation based on capacity used as a percentage of the rated capacity and multiply this with the time flown and get a "Rated Capacity endurance" but I know that this is not correct because the rated capacity and actual capacity are 2 different values.
If I have to top up with 2355mAh
Have I just used 2355/3300 or 2355/3000 or 2355/(what value)? to get to the percentage that you guys are talking about.

 

[cml001]

What are ur battery specs?

 

[Robair]

Gens ace
3300mAh
45C
22.2V

 

[cml001]

Then if ur putting more than 2640 back in ur flight time is too long.. I gage that but really use voltage.. I like starting with 4.24 per cell and ending at 3.8.. I use that as my reference.. Typically that's a 5 minute flight.. Honestly I dont nitpick battery figures.. I just keep that voltage in line and don't exceed the 80% rule, don't leave a charge on them when I'm not using them, etc.. There are a few guys on here that are great with the numbers, formulas, etc... Sounds like u have a better grasp on that stuff than I.. But that's how I do it. The actual figures vary from pack to pack I just find this easier.. And with a good quality pack I worry even less... U can get soo wrapped up in calculating all ur packs that's it's just too time consuming.. So unless there is an issue.. This is how I do it.

 

[Lee]

80% of 3300mah pack = 2640
So once you have flown a pack and recharged it, you shouldn't be putting back more than that figure.
If the figure is less than 2640, then a 10,20,30 seconds to your flight, until it hit the 2640 mark.

 

[Robair]

Thank you Lee,
But right there is my real question.
3300mAh is not really the capacity of the battery pack - what with deterioration and all the other factors that age a battery.
I am currently guessing a 10% deterioration and working to 3000mAh. But as I have said, this is just a guess and it is exactly that guess that I want to get rid of.
By your calculation and my logic. The calculation should be 80% of 3000mAh or whatever the REAL capacity of the battery pack is and not what the number on the label indicates.

 

[Lee]

Then you have to got to Voltage as I mentioned in my first post.
Fly to a resting voltage of 3.7V per cell, which is normally around 3.5V under load.
I run a Voltage alarm that goes off at 3.5V. After landing and resting, the batteries are at 3.7V
I also keep an eye on the Internal Resistance, but you can only do that, if you have a good charger with that feature.

 

[Robair]

I want to also add,
Assume the battery pack has in fact deteriorated by 10% and it is in reality a 3000mAh pack and not 3300mAh and you fly it down to 2640mAh needed for a full charge, then you are in fact flying that pack down to 88% and not to the 80% that you think. This might or might not make that much difference but just a few more mAh and I believe you will be in dangerous 'doing damage to your packs' territory.
This is exactly why I want to determine what the real capacity of the battery pack is so that I can make a calculated decision as to what numbers I must fly to.

 

[cml001]

Hummm... Get'n deep! If it's to the point u ? the packs ability to perform... I'd just not use it. How old is the pack? Interesting question tho.

 

[treff]

Just an aside, if I were you I would buy 4000 mAh batteries it makes the math so much easier lol. Because the batteries are all so different, even from the same manufacturer `````i
would add 10% safety margin to any calculations because batteries change so rapidly you would be doing more measuring than flying. Just keep any eye on the IR of the cells.

 

[vimy]

I want to also add,
Assume the battery pack has in fact deteriorated by 10% and it is in reality a 3000mAh pack and not 3300mAh and you fly it down to 2640mAh needed for a full charge, then you are in fact flying that pack down to 88% and not to the 80% that you think. This might or might not make that much difference but just a few more mAh and I believe you will be in dangerous 'doing damage to your packs' territory.
This is exactly why I want to determine what the real capacity of the battery pack is so that I can make a calculated decision as to what numbers I must fly to.

If you want an accurate measurement of your battery capacity then a West Mountain Radio CBA 4 is the way to go:

http://www.westmountainradio.com/product_info.php?products_id=cba4

You can also use it to discharge a battery before disposal and make sure it is fully discharged.

- - - Updated - - -

Here is an example of a test using the CBA 4 on several brands of batteries:

http://www.bigsquidrc.com/lipo-battery-shootout-4-voltage-under-load/

The large unit with two fans is an amplifier used in conjunction with the CBA 4 to increase it's capability, it is not needed for most jobs.