Voltage discrepancy

[Matt Neumann]

I have literally just started using LiIon batteries instead of LiPos.  I have had a gizmo that dad got me when I first started electric where I would plug in the balance plug and on the display it would show the voltage of each of the cells.  I don't know the make or model of it but I have seen many guys use something similar.  Anyway, on LiPos when fully charged it would show 4.2V.  On these LiIons it is showing 4.1 V.  When I use my actual voltmeter it shows 4.2V.  Why would it be accurate on LiPos and not LiIons?  I know many of the gizmos used to tell percent charge left is not accurate because of the difference in resistance.  But this is voltage.  I would think voltage is voltage.  Any ideas?  And yes my charger is set to 4.2V.

[Brent Williams]

Will DeMauro can probably describe it best, but many chargers don't give a full 4.2 charge apparently.  My powerlab8 didn't give me a full 4.2v in factory settings when measured separately. It would only give me 4.18v or so.  To get a full 4.2v I had to cheat it by requesting 4.23 or so. 

[Matt Neumann]

Will DeMauro can probably describe it best, but many chargers don't give a full 4.2 charge apparently.  My powerlab8 didn't give me a full 4.2v in factory settings when measured separately. It would only give me 4.18v or so.  To get a full 4.2v I had to cheat it by requesting 4.23 or so.

This does not have anything to do with the charger.  I have mine set up the same as yours.  The thing is the separate voltmeter that I have been using for LiPos is now giving an error voltage reading with the LiIons.  I am just curious if anyone has had this same issue with voltage?  My voltmeter itself is reading correctly.  Again separate from the charger itself.

[Motorman]

Maybe the lippo is within the margine of error on both meters and the LiIon is not.


MM

[Matt Neumann]

I just checked with another voltmeter and it says 4.1V.  Sounds like something is out of calibration.  One of the guys here at work has another voltmeter and another gizmo like I have to check batteries.  I will check them tomorrow.  Still weird why I am getting different voltages though.

[Howard Rush]

My guess was that the LiIon has a lot more internal resistance than the LiPo, and the gizmo draws a lot of current: enough to cause the .1V drop in measured LiIon voltage.  I sacrificed a balance extension cable for Science.  I tested three such gizmos: a Hyperion, which I thought was the good one; a cheap one from Temu; and an even cheaper, wee deafening one.  I tested with a battery of six Molicel P30Bs at 3.5 V/cell, although I just used it as a voltage source with a convenient connector.  I assumed that the "ground" wire in the connector carried the current from the whole battery to the gizmo.  The pictures show the measurements in milliamps. 

The worst gizmo, the Hyperion, drew 29 mA.  According to Molicel, https://www.molicel.com/inr-18650-p30b/ , a fully charged (if that's what "0 SOC%" means) P28A cell has .028 ohms internal resistance.  That would only account for a .8 mV drop.  Multiplying that by 4.2V/3.5V would be just shy of 1 mV.  It's a mystery.  Maybe your gizmo sucks.

[John Rist]

With Digital devices the LSB is usually +- 1 bit.  Ie to get an accurate reading at the 0.00 level you need an device that reads and displays 0.000.  This is electronics 101

[Brett Buck]

With Digital devices the LSB is usually +- 1 bit.  Ie to get an accurate reading at the 0.00 level you need an device that reads and displays 0.000.  This is electronics 101

    That assumes that the only issue is the display accuracy. Given consumer-quality components from China, you could easily have scale factor errors of 100,000 ,200,000, 300,000 PPM and for it to be highly temperature-sensitive. 100,000 PPM on a 4.2 volt signal is .42 volts.

    I would suggest attaching your gadget with a breakout cable or something that allows you to measure it with a quality dedicated voltmeter at the same time, you can pick out the errors easily doing that.

[Matt Neumann]

I figured out my actual voltmeter is off calibration wise on a certain setting.  I checked the battery with a voltmeter from here at work and it said 4.1 V per cell as well.  Diagnostics 101.  Got a discrepancy?  Get a third opinion.  In this case the third voltmeter agreed with my gizmo.  So then my question was why was my batteries only charging to 4.1 when it was set to 4.2?  I charged an old LiPo battery and got 4.1V as well.  OK, what is going on here?  Well, dummy here must have changed a setting by accident.  Instead of LiPo it was LiLo whatever those are.  Pays to read carefully.  But the cutoff voltage for LiLo must be 4.1V.  So that is what my charger did.  Stopped at 4.1V.  No wonder I was running out of "fuel" at the end of a flight.  I switched back to the LiPo setting and got 4.2V and 250 mA more per battery.  This morning no issues of running out of "fuel".

So this goes to show as Howard Rush tested and found out not all gizmos are created equal.  In most cases, you get what you pay for.  My Voltmeter was a freebee from somewhere.  Close enough for most stuff for the price but in this case I caused me to scratch my head.