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Lipo balance: how much difference is acceptable?

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Lipo balance: how much difference is acceptable?

Old 05-19-2022, 09:23 PM
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baerster
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Default Lipo balance: how much difference is acceptable?

Hi all, when I plug a tester into any of my charged lipos, the cells are a few hundredths of a volt off from each other, meaning less than a one-percent difference of charge between cells. When I have seen videos of people demonstrating how to use a tester on a lipo, their cell voltages are a little off also. They are close but not exact.

How much can the voltage between lipo cells vary before it could be a problem? .05 volts from the average voltage?

By the way, I tested one of my 4S batteries and here is what I got:
Cell voltages: 4.18 / 4.16 / 4.19 / 4.22 (Total 16.75 volts)
Mean cell voltage: 4.19v
Std. Deviation: .025v
Range (lowest to highest): .06v

Thanks for any information based on experience that you have. And/or, thanks for any definitive information from experts in battery design.

- Tom

Last edited by baerster; 05-20-2022 at 06:44 PM.
Old 05-20-2022, 09:58 PM
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I am no Lipo battery expert by a long shot but I figure since I am learning about electric power and see a question I am not sure about, I need to know the answer also .

If you are talking about batteries which have been balance charged ( do you know that not all charge options balance the cells ?, not everyone does... ), I can tell you that most of my Lipos ( totaling about a dozen at the moment ), see a final voltage difference between cells of no more than 0.001 V ( after balance charging ). I have one battery that has a 0.005 V difference.

I then did some reading to get other people's ideas ( because I am interested too ), and found that previous respondents to a question like yours ( and they have thousands of posts to their names, for what it's worth ) generally gave the figure for max. V difference between cells after balance charging as 0.02 V ( 0.03 V and 0.04 V were also mentioned ). These people seem to have been using their batteries without issue with their given values.

Bear in mind there may also be inaccuracies in the voltage measuring equipment and also in the battery charger's balancing function, that could account for some or all of the seen voltage differences between cells. .

I can't really speak as to the implications of higher voltage differences than these, other than possibly reduced battery capacity. ( Incidentally, just be sure you never run a Lipo cell below 3.0 V and for maximum battery life, 3.7 V is not a bad minimum to observe when actually using your packs. I once left a battery in my rc plane and thought it was safe because the switch was off. Nope. There must have been some minute current draw somewhere and over the days the battery cell voltages fell below 1 V. That was an expensive lesson. In retrospect also, that switch on the side of the plane was not in the battery lead path so I should have known better... )



Last edited by Tphage; 05-21-2022 at 04:07 AM.
Old 05-21-2022, 05:07 AM
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Originally Posted by Tphage View Post
I once left a battery in my rc plane and thought it was safe because the switch was off. Nope. There must have been some minute current draw somewhere and over the days the battery cell voltages fell below 1 V. That was an expensive lesson. In retrospect also, that switch on the side of the plane was not in the battery lead path so I should have known better... )
If you are talking about an electric powered plane with the standard LIPO/ESC/Brushless motor setup, and there is a small switch connected to the ESC to ensure the motor won't start, I can see where you'd have a problem leaving the battery connected for a number of days. While that tiny switch does disable the ESC's BEC, the capacitors on the ESC's battery side are still connected, and while in a perfect world capacitors aren't supposed to have any measurable internal current leakage (resistance), it's all too common for there to be a tiny amount of it, especially on cheaper made parts. Not enough, of course, to affect it's operation as a capacitor (In this case being used as a "ripple filter") , but just enough to put an ever so slight draw on the battery in your situation, draining the battery over a period of days if left connected.
Old 05-21-2022, 03:08 PM
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Originally Posted by Tphage View Post
I am no Lipo battery expert by a long shot but I figure since I am learning about electric power and see a question I am not sure about, I need to know the answer also .

If you are talking about batteries which have been balance charged ( do you know that not all charge options balance the cells ?, not everyone does... ), I can tell you that most of my Lipos ( totaling about a dozen at the moment ), see a final voltage difference between cells of no more than 0.001 V ( after balance charging ). I have one battery that has a 0.005 V difference.

I then did some reading to get other people's ideas ( because I am interested too ), and found that previous respondents to a question like yours ( and they have thousands of posts to their names, for what it's worth ) generally gave the figure for max. V difference between cells after balance charging as 0.02 V ( 0.03 V and 0.04 V were also mentioned ). These people seem to have been using their batteries without issue with their given values.

Bear in mind there may also be inaccuracies in the voltage measuring equipment and also in the battery charger's balancing function, that could account for some or all of the seen voltage differences between cells. .

I can't really speak as to the implications of higher voltage differences than these, other than possibly reduced battery capacity. ( Incidentally, just be sure you never run a Lipo cell below 3.0 V and for maximum battery life, 3.7 V is not a bad minimum to observe when actually using your packs. I once left a battery in my rc plane and thought it was safe because the switch was off. Nope. There must have been some minute current draw somewhere and over the days the battery cell voltages fell below 1 V. That was an expensive lesson. In retrospect also, that switch on the side of the plane was not in the battery lead path so I should have known better... )
Thank you Tphage! If you are seeing V differences in the thousandths instead of the hundredths that I see, then I wonder if you either have better batteries, a better charger, better tester, or a combination.

When you came across mentions of max. V difference between cells in your reading, were they taking about the max difference between the lowest and highest cell, or max difference from the mean voltage?

Thanks,

-Tom
Old 05-21-2022, 05:58 PM
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Originally Posted by init4fun View Post
If you are talking about an electric powered plane with the standard LIPO/ESC/Brushless motor setup, and there is a small switch connected to the ESC to ensure the motor won't start, I can see where you'd have a problem leaving the battery connected for a number of days. While that tiny switch does disable the ESC's BEC, the capacitors on the ESC's battery side are still connected, and while in a perfect world capacitors aren't supposed to have any measurable internal current leakage (resistance), it's all too common for there to be a tiny amount of it, especially on cheaper made parts. Not enough, of course, to affect it's operation as a capacitor (In this case being used as a "ripple filter") , but just enough to put an ever so slight draw on the battery in your situation, draining the battery over a period of days if left connected.
Ah ha ! Thankyou. It's nice to know why it happened. ( Yes it was the switch you are talking about, on the side of an Apprentice STS 1.5 ).

Last edited by Tphage; 05-21-2022 at 07:18 PM.
Old 05-21-2022, 06:08 PM
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Originally Posted by baerster View Post
Thank you Tphage! If you are seeing V differences in the thousandths instead of the hundredths that I see, then I wonder if you either have better batteries, a better charger, better tester, or a combination.

When you came across mentions of max. V difference between cells in your reading, were they taking about the max difference between the lowest and highest cell, or max difference from the mean voltage?

Thanks,

-Tom
Hey Tom

They were definitely talking about the difference between the max. and min. voltages of cells, in the same battery ( and those figures I gave were for the same ).

Are you able to check battery cell voltages in at least one other way, preferably two ? That way, you will get some indication on the accuracy of the measurements.

My batteries are all the Turnigy brand ( e.g. Zippy Compact ) from Hobbyking and my charger was an Imax B6 ( genuine ) so not especially high $ stuff. That said, I have just added to my equipment so will re-measure some battery voltages in a variety of ways to look for variation between "voltmeters".

Check back here in a few days and I should have posted some results.

Old 05-23-2022, 01:05 PM
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Thank you Tphage !
Btw, I got myself a new charger (ISDT D2) and I think it has a way of displaying cell voltage. I will figure that out to see it if gives me values that are different from my hand-held battery checker, which is
this one this one
. Using the new charger and checking the voltages with the little tester, I now get cell voltages that are within .03 of each other.

Also, I asked Horizon Hobby support what sort of spread is OK, and they said usually .05 or less is fine. HOWEVER, the person who helped me did not have a definitive answer from the battery manufacturer. I think it's interesting that battery manufacturers will be up front with information such as the voltage for full charge and the voltage for storage charge, but not on the acceptable maximum difference between cells, even though that is an important thing to pay attention to from a safety and battery life standpoint.


Last edited by baerster; 05-23-2022 at 01:12 PM.
Old 05-23-2022, 01:07 PM
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I want to thank everyone for being so helpful. Yesterday on Amazon I got a reply from someone that made me further appreciate the people in the RC plane community who have been helpful. I asked one of the Lipo battery vendors how much voltage difference between cells they consider acceptable, and some random person answered with simply "Please do your own research on this type of battery before you hurt yourself." What a helpful answer LOL.
Old 05-27-2022, 02:26 AM
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Battery Cell Voltage Measurement : "Accuracy" of Various Measuring Devices.

I checked a single battery with various voltmeters or voltmeter functions of e.g. battery chargers. Not that it matters particularly but the battery was a Zippy Compact 3S 40C 3300 mAh from Hobbyking.

The voltage of the three cells was measured on each device and I did two trials, measuring the battery on all the devices once ( Trial 1 ), then again in the same order ( Trial 2 ). I am not going to list the results of Trial 2 as it becomes too unwieldy to view and to the hundredth of a volt, they were identical anyway.

I don't have a calibrated voltmeter so I interpret the results by looking for closeness of readings between the various devices. And I'm not going into accuracy and precision because it makes my brain hurt. It's not a valid scientific test of each device in general because it is based on only one or two examples of each - that's all I had - but it still provides some indication. ( And incidentally, for anyone wanting a lowish-cost battery checker for the field, I can recommend the dlux Turnigy "Auto Checker" Cell Checker Auto Balancer - from HobbyKing. You simply plug in the battery balance lead and it shows all the cell voltages instantly on the one screen as well as the remaining battery capacity. )

The first column is the device used and the next three numbers are the measured voltages of cells 1, 2 and 3.

1) Imax B6 AC V2 battery charger ...................................3.79, 3.80,3.80
2) Imax B6 AC V2 battery charger No. 2 ...........................3.80,3.80,3,80
3) Icharger 308 Duo battery charger Channel 1 .................3.794,3.794,3.792
4) Icharger 308 Duo battery charger Channel 2 .................3.795,3.793,3.793
5) HobbyWing Multifunction LCD Program Box ...................3.74, 3.78, 3.78
6) dlux Turnigy "Auto Checker" ........................................3.79,3.79, 3.80
7) dlux Turnigy "Auto Checker" No. 2 ................................3.79,3.80,3.79
8) Aokoda Low Voltage Buzzer Alarm.................................3.78, 3.80,3.80
9) HobbyKing Cell Checker ..............................................3.79 , 3.79, 3.79
10) HobbyKing Cell Checker No.2 .....................................3.79, 3.79, 3.78

Last edited by Tphage; 05-27-2022 at 03:00 AM. Reason: Improve formatting
Old 05-27-2022, 02:32 AM
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Originally Posted by baerster View Post
Thank you Tphage !
Btw, I got myself a new charger (ISDT D2) and I think it has a way of displaying cell voltage. I will figure that out to see it if gives me values that are different from my hand-held battery checker, which is this one. Using the new charger and checking the voltages with the little tester, I now get cell voltages that are within .03 of each other.

Also, I asked Horizon Hobby support what sort of spread is OK, and they said usually .05 or less is fine. HOWEVER, the person who helped me did not have a definitive answer from the battery manufacturer. I think it's interesting that battery manufacturers will be up front with information such as the voltage for full charge and the voltage for storage charge, but not on the acceptable maximum difference between cells, even though that is an important thing to pay attention to from a safety and battery life standpoint.
That's a great battery charger. I've read good things about it.
Old 05-27-2022, 02:33 AM
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Originally Posted by baerster View Post
I want to thank everyone for being so helpful. Yesterday on Amazon I got a reply from someone that made me further appreciate the people in the RC plane community who have been helpful. I asked one of the Lipo battery vendors how much voltage difference between cells they consider acceptable, and some random person answered with simply "Please do your own research on this type of battery before you hurt yourself." What a helpful answer LOL.
That's his ( impolite ) way of saying "I haven't got a clue what you are talking about"...
Old 05-28-2022, 05:54 AM
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I usually find the cell with the highest internal resistance will reach end voltage the quickest, and that same cell will drain the quickest. When I find one cell dropping too low at ESC's low voltage cutoff, that's when I start to closely monitor a pack's cell health.

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