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LiPo Cell Voltage & Balance Checker True Values?


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I recently acquired a ISDT "BatteryGO  BG8S" battery checker.

(It has other functions too but that is the primary reason I bought it).

The instruction manual quotes a Voltage Measurement accuracy of +/- 0.005mv @ 4.20V and a Balancing Cells accuracy of <0.005V

 

Over the years I have accumulated a variety of other LiPo battery checkers, from various sources, some of which include cell balancers.

Also, a few small stand-alone battery cell balancers.

Add to which, the digital readouts on several LiPo battery chargers incorporating balance charge/discharge settings as well.

To add further to the measuring mix, a fairly expensive digital voltmeter.

 

When checking, ALL the above give different readings of individual LiPo cells, some of which are noticeably at odds with each other.

OK, I appreciate they are not science laboratory or industrial quality and calibrated items but the typical devices 'Joe Bloggs' electric flight operator would buy and regularly use.

 

With safety uppermost, (as well as getting long life and best performance) from our LiPo, we may inadvertently be relying on incorrect readings.

Perhaps the best we can do is stick with one measuring device and then at least the variances/errors are consistent across our flight battery management.

 

What thoughts others? 

 

 

 

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Measuring voltage this accurately, at this level of precision is incredibly difficult. You won't get any such info from any product from bangood/aliexpress/hk, or any model suppliers that sell these items with their own sticker rebadged over the top. Despite how many decimal places they claim.

Don't sweat it, your battery is not going to burst into flame or fail catastrophically because one battery checker said it was 0.005v different than another.

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CK, Its a matter of opinion, but I would say most are accurate for their intended purpose as they are "indicators". With Lipos the voltage from fully charged to flat is just about predictable, but for say LiFe its a different story as they have a very flat curve for most of their capacity.

 

So here is the twist, say your lipo is sitting there with a terminal voltage of 3.8V and you measure that voltage with a super expensive voltage instrument (with very high internal resistance), the load it imposes is next to nothing and so everything looks good, but that may not be totally representative of its true stage of charge. Batteries tend to sit high voltage so its best to add a little load now of course if each tester applies a different load then you will get a different voltage hence capacity, it won't be much different as the battery has a very low internal resistance so the more load (for testing) the less the voltage drops.

 

image.png.047892d7aafe4ea27090ebcb36ff7631.png

 

 

Are you really pushing Lipo's down to sub 30% capacity on a regular basis? I have had models that cane lipos at their C rating and discharged a full lipos in 3 minutes (sub 10% capacity), pushing them this hard results in reducing their life considerably (no surprise there). and results in a dead stick landing (was pylon racing at the time).

 

If you keep them between 30% and 100%, don't discharge them at heir C rating, compare their capacity and the charge put back in then you won't be far off.

 

I notice the top end power drops off and the capacity (about 10%) reduces towards the end of their life, still useable, but not in model that needs 100% of everything!

 

PS For LiFe type cells you can see the problem around voltage and current drawn, see how flat the curves are + the difference between load voltages. Great as they don't voltage sag under load, just harder to determine capacity from voltage alone. 

image.png.017399f6734390dbefa835405e4ba7a7.png

 

 

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1 hour ago, Chris Walby said:

...........

So here is the twist, say your lipo is sitting there with a terminal voltage of 3.8V and you measure that voltage with a super expensive voltage instrument (with very high internal resistance), the load it imposes is next to nothing and so everything looks good, but that may not be totally representative of its true stage of charge. Batteries tend to sit high voltage so its best to add a little load now of course if each tester applies a different load then you will get a different voltage hence capacity, it won't be much different as the battery has a very low internal resistance so the more load (for testing) the less the voltage drops. .................................

 

 

Chris, I may be miss-understanding but I am a little confused by your suggestion of adding load to determine the charge state/capacity from the voltage.

For Lipos, the charge state can be determined by off load voltage reasonably well.

If you add a little load the voltage will drop due to the cell's internal resistance but the internal resistance will vary with size of battery, condition of battery, and temperature. Adding load would surely require you have different "voltage calibration curves" for different loads, battery capacities, and temperatures.

 

What am I miss-understanding?

 

Dick

 

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It is perhaps worth noting that at a Voltage accuracy of 3 decimal places even contact resistance makes a difference.

The meter might have suitable contacts to read the voltage that accurately but will the charger read them as well. The charger sets the charge voltage limit to what it 'sees' as the cell voltage.

 

As an example I balance charged a 3 cell yesterday but the charger seems to struggle at the end of the charge with one of the cell voltages alternating between 4.18 and 4.20 whilst the others were stable at 4.20. Possible bad cell?

A tiny 'jiggle' of the balance connector at the charger and in less than 10 seconds all the cells showed 4.20 and the charge stopped indicating 'full'.

  

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Dick,

 

I think you are missing the point of the OP, the accuracy of the meters and their respective internal resistance will have a bearing on the measured value. I was just pointing out that various different designs may have very minor but different internal resistances which leads to differing values displayed.

 

I have a set of DMM leads that for safety reasons have fuses, okay for general electrical work, but a pain in the backside for anything electronic because they indicate 4.6ohms when shorted together (just swap to the unfused ones!). Part of the problem is that the DMM has a very high internal resistance so that it does not load electronic circuits, but that works against trying to measure very low resistances.

 

It will also read 280V ac across the mains, when in fact the supply is closer to 240V ac (know why?) and as for measuring within <0.005 V I don't think so.

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It's also worth bearing in mind the difference between resolution and accuracy.  Just because the meter is capable of reading a voltage difference of 1mv it doesn't mean it's accurate to 1mv.

 

As Chris Walby writes, most (all?) of the LiPo checkers sold for model use are adequate for our purposes.

 

Geoff

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2 hours ago, Chris Walby said:

Dick,

 

I think you are missing the point of the OP, the accuracy of the meters and their respective internal resistance will have a bearing on the measured value. I was just pointing out that various different designs may have very minor but different internal resistances which leads to differing values displayed.

 

I have a set of DMM leads that for safety reasons have fuses, okay for general electrical work, but a pain in the backside for anything electronic because they indicate 4.6ohms when shorted together (just swap to the unfused ones!). Part of the problem is that the DMM has a very high internal resistance so that it does not load electronic circuits, but that works against trying to measure very low resistances.

 

It will also read 280V ac across the mains, when in fact the supply is closer to 240V ac (know why?) and as for measuring within <0.005 V I don't think so.

 

Now I am even more confused. I thought the discussion was about measuring voltages accurately not resistance.

If measuring voltages accurately is the issue, then a high internal resistance for the DMM is a benefit as it makes your 4.6ohm test lead almost irrelevant.

If you want to assess the state of charge of a Lipo you need an accurate voltage measurement without putting any load on it.  It is voltage being measured here not resistance.

 

However I agree that the so called capacity checkers, and many chargers etc. are of variable quality and the best you can do is compare batteries with the same device and avoid comparing results from different devices.

 

I am intrigued by your 280v ac mains measurement -  care to explain why?

 

Dick

 

 

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I have two chargers that measure cell voltage, one with four channels, and a battery checker. They all read differently, by about 0.02v at worst. The battery checker seems to be particularly bad on cells 3 and 4.

A multimeter can measure each cell in sequence with the same meter, so if it is wrong, all measurements are similarly wrong so cells that measure equal would be actually equal.

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43 minutes ago, Andy Gates said:

I suspect Chris's DVM may be reading pure peak to peak voltage rather than the normal RMS voltage hence the 280/230v discrepancy.

A bit more complex than that and hence an interesting problem.

If the correct rms value is close to 240v as he said, the peak should be about 340v not 280v.

If his meter is reading average instead of rms it would be reading lower than 240v.

 

Dick

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