Voltage measurements

131 views

Zoltán Nagy

Nov 1, 2013, 2:50:39 PM11/1/13
After uploading 0.31 firmware to my B6 clone. After calibrating with 6s LiPo I have done some measurements. I used XL830L multimeter.

Here are the results.
http://sdrv.ms/1ght2F0

As you can see, it needs some correction on lower voltage.

Paweł Si

Nov 1, 2013, 8:04:42 PM11/1/13
2013/11/1 Zoltán Nagy
How did you manage to measure 3 decimal places with an 1999 display (measuring > 3V)? ;)
(maybe I'm reading the wrong multimeter documentation)

You should also calculate the "relative error" to complete your table
and compare it with the multimeter accuracy.
(if I'm not mistaken Your biggest error is within the error limits of your multimeter)

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But it's clearly visible that the biggest error is on the first two cells,
Unfortunately, this is due to the construction of the charger
(the first two cells don't have an op-amp to the input)
I think you can still improve the error on the second cell,
(try to calibrate it more accurate)
but after all it's an 20\$ charger.

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My thoughts about LiPo battery charging:
(Yet another meaningless opinion ;) - I'm a software developer so my opinion doesn't count)

The truth is that the only thing that matters when charging LiPos is that You never exceed 4.20V per cell.
When you go above this value you can physically damage the battery
(actually the truly upper bound is 4.23V so there is some space for errors).

So You probably should calibrate Your charger near the 4.20V value.

In my opinion it's not so important to reach the 4.20V, for example:
I have charged a 3-cell batter to: 4.20V, 4.20V, 4.17V
assuming that a cell is discharged when it reaches: 3.20V
(and the stored energy is linear proportional to the voltage - which is untrue ;) - but I need some simplifications)
I will end up with: 3.23V, 3.23V, 3.20V on each cell after discharge.
so I started with a battery that was electively charged to 97% of its capacity, which I consider as not so bad.

But like I said I'm not an expert.

----

Are there any other opinions?

Best Regards,
stawel

yt60343

Nov 2, 2013, 5:45:16 AM11/2/13

Hi testers ,

Highest cell against lowest cell in the battery pack wil give the real delta.

Difference between MM and charger is normal , but should be as less as possible .

I compared the MM i use with theXL830Ll  ;

-          Xl830l - € 6,35 incl shipping on ebay ;

 DC Voltage 200mV/2V/20V/200V/600V +/- (0.5%)
-          Fluke 110  - € 112,- at the  fluke dealer ;

Voltage DC

 Accuracy* ± (0.7%+2) Max. Resolution 1 mV

So , according to the specs the xl830l should even be better than the Fluke 110 ( and a lot cheaper !! ) . à i am not going to discuss mm here ;-)

I  have calibrated my charger with a little device i made , mainly consisting of a reference temp compensated voltage reference and a 0,1 % resistor voltagedevider .

The benefit of that is , it allows me to set every voltage needed so i can do a low voltage calibrations as well ( fe. 2 volt on all 6 ).

It is also possible to use 6 voltage regulators like the TL431 ( as skyrc does in production )  but that is a lot more expensive.

-- IF you ever revert the b6 to normal firmware ( for testing )  , mind that calibration needs to be 25,2 V and 6 x 4,20 V exactly !!

According to the info , what mostly counts is how much energy you can get IN the battery preferably with a safety marging towards the 4,23 V max limit.

Most of the times i have measured LESS than the capacity on the label could be charged INTO the battery.

A good easy and cheap way of testing a whole battery is the BVM-8S ( chinese cellog 8 knockoff ).

techy

b6-calibrator.jpg

Paweł Si

Nov 3, 2013, 11:42:10 AM11/3/13
Actually, the only thing I wanted to say is that measuring 4V with 1mV accuracy
is a very difficult task, we should not expect that a cheap charger can do this.
(After all 1mV is only 0.025% of 4V)

The only reason why I display mV is that the Thevenin method I'm using needs
to know the internal resistance of a battery which should be > 0.
The mV shouldn't be taken too seriously
(same applies to the internal resistance).

The resolution of each channel is (approximately):
b1,b3,b4,b5,b6: 5mV
b2: 10mV (for Imax B6)
b2:  5mV (for GT.Power)
Vbat: 30mV

I don't know what the errors are but I'm sure they are 2,3  times greater then that.

But like I said, there is hope :)
a 10mV undercharged battery has only approximately 1% less stored energy.

Best regards,
Paweł

2013/11/2 yt60343

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