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Old Aug 12, 2012, 08:30 AM
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Different voltage readings between PL8 and cell log

Hi all,

I have a 7S pack which i monitor only with a PL8, i get cell voltages like:
3.71 / 3.70 / 3.71 / 3.70 / 3.70 / 3.71 / 3.70

the i disconect the PL8 and immediately check the same cell voltages with the Battery Guard cell log from Graupner and i get diffferent readings,
from 3.69 to 3.74 volts.

This is a little strange to me.

We can challenge the calibration of my cell log, but even with this it's difficult to explain that difference between highest and lowest cell is different between both systems,

any idea ?

could it be that one system displays voltage difference between one cell and it's neighbour, whereas the other displays voltage difference between a cell and the average of cells voltages ? That could explain the lower cell difference on Pl8.

curious.
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Old Aug 12, 2012, 10:32 AM
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First off a voltage spread of 0.1V with cells at nonimal 3.7V is the same as nothing.

Secondly one can debate which is most accurate all day long and unless you have an extremely accurate recently calibraded digital voltage meter you will never know which one is the most accurate.

I feel that most are overly concerned with such minor voltage difference ( inbalance) and as I have posted for years inbalanced at a discharged state does not really matter unless it is extreme. I have seen many decent ( not great) 3S LiPolys with one cell as much as 0.2V lower than the other two.

Question,do you have your PL set to display two or three decimal places? I guess two and likely will not reall have much of an effect on the results spread wise in your case.

Charles
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Old Aug 12, 2012, 12:24 PM
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Thanks for your input Charles.
It's set at 2 decimal places.
I have the same differences around 4.2 and you'll agree with me that 4.3 is not quite the same as 4.2 for LiPo, but that is a little off topic.
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Old Aug 12, 2012, 02:00 PM
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It looks like Graupner is the same or similar re-brand of the Hyperion Sentry, also available under the name ProTek. My personal experience is that these checkers while feature rich are horribly inaccurate. A club member purchased three and we used it check the same pack. Each one reported a different reading. The worst one was reporting one cell at 4.3v. While the other meters reported the same cell as less than 4.2v.

Like all cell checkers these things simply cannot be trusted unless you have followed Charles advice and tested them against a high quality, recently calibrated DVM. I did just that with my Junsi CellLog8S and found it to +/- 3mv when compared against my Fluke. I also noticed while checking the calibration, that since the cell meters are not externally powered, simply connecting them to the pack will cause the cell voltage to reduce slightly. Not by much, but when we start getting picky over 50mv then it matters.





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Old Aug 12, 2012, 03:04 PM
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ok, certainly possible.

In order to eliminate the "loading" effect of the cell log, i'll try to come up with a little dongle with which i can plug in the cell log and also monitor the same with the PL8.

I do have cell 4 and 7 that seem to be higher than others on my 3 packs, i always wondered why, maybe it's a calibration error. But sometimes another cell than 4 and 7 start to show positive or negative variances. Not sure.

i need to find a way to read the same cell voltage on all the input pins of the cell log to see if the readings are the same, in theory they should be.

let's see if i can come up with a test procedure, unless someone has a simple test to propose.
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Old Aug 12, 2012, 03:17 PM
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Also remember that calibration and repeatability are not he same things. The same channel on your cell checker may read different voltages from day to day. Regardless of what ever procedure you come up with, it unless an authoritative method check voltage is involved, then there will always doubt regardless of your results.
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Old Aug 12, 2012, 03:22 PM
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You really need something like this, otherwise you will continue be uncertain and question your equipment.

http://www.amazon.com/Fluke-ESFP-Tru...ords=fluke+197
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Old Aug 12, 2012, 08:50 PM
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Another option is to use a precision voltage reference to check your equipment.

http://www.voltagestandard.com/
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Old Aug 12, 2012, 08:58 PM
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That's a good idea Pete. But it might be hard to use a device like that to check all six or eight channels on typical cell checker.
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Old Aug 12, 2012, 09:12 PM
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It might be. I haven't used one, but have been considering purchasing one.
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Old Aug 14, 2012, 08:14 AM
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Quote:
Originally Posted by Gregor99 View Post
That's a good idea Pete. But it might be hard to use a device like that to check all six or eight channels on typical cell checker.
in fact, yesterday i just wanted to measure the voltage out of a single LiPo cell with my cell log. But I did not manage to do it.

I connected the cell negative to the first pin of my cell log (gnd), then the positive to the second pin. All i got is the screen of the cell log blinking and some noise.

I thought you could measure from 1S to 7S with these cell logs ? I know 4S works, i have one and it reads it.

Am I missing something here ? Do i need to connect aynthing to pins 3 to 8 ?

If I get that to work, then i can at least compare each port of my cell log to see if there are variations between them, which is suspected here.
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Old Aug 14, 2012, 10:54 AM
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It most likely requires at least 4v to power the logic board inside the cell checker
I made a 2s harness so I can check my 1s packs.
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