Reliability of voltage reading from different sources.

[MelkerSkogstam]

i know all devices have different sensitivity when reading voltage but it seems a little to off, i tested one of my 4 cell lipos on various devices and this is what i got.

IMAX B6AC charger: 15.22v
KK2.1 board: 16.2v
KK2.0 board 15.6V
HK voltage alarm 1: 15.2V
HK voltage alarm 2: 15.4v

Not quite comforting not knowing your batteries exact voltage, what if the reading was dangurously off without even knowing it.
so most of the readings are around 15.2 - 15.6 except the kk2.1 suggesting that the lipo packs true voltage are around 15.4? is it normal the readings are this different? feel like i can't trust my kk2.1 board now..
which of the devices are the most reliable, prob the charger right?

 

[xuzme720]

Seems like I read something about the voltage being off on the KK2.1. If I come across it again I'll post it here...
In the meantime, I would get a multimeter or wattmeter and test with that to verify. Most chargers and alarms do have some variance, but like you see, it's usually only a tenth of a volt or so.

 

[Craftydan]

I'd ignore the voltage number read from the kk2 and instead adjust the alarm number up/down so when the alarm goes off in hover, after you land it is *around* 3.7v/cell.

Again don't take that 3.7v/cell reading from the kk2. You don't care what it calls that "time to land voltage", so long as it's calibrated to "I'm almost empty". I typically use my HK-010 power meter for my baseline voltage measurements. If your IMAX charger has a measurement mode, that would work well too.

BTW, there is a bug in the stock kk2.1 firmware that gives poor voltage readings. look into re-flashing it with one of Steveis's modded firmwares. It fixes far more than that bug, so I expect you'll be happier with one of his latest firmwares.

http://www.rcgroups.com/forums/member.php?u=367321

 

[rockets4kids]

Most devices use a resistor voltage divider to reduce the voltage down to a range that the microcontroller inside can deal with. Even if 1% resistors are used, this can still lead to error. Calibration is an expensive step, so it is often not performed on cheap chinese items. I know that all of the accucell-style chargers allow for user-calibration against a multimeter. You should check the manuals on those other boards to see if they have a calibration option as well.

 

[lonewolf7717]

see attached calibration vid. Many of the Accucel "knockoffs" do not spell out calibration instructions. Had my GTPower X-charger quite some time before checking calibration. It was reading VDC substantially higher than it really was....meaning it wasnt really topping off my lipos.

 

[MelkerSkogstam]

ok so i checked the charger with a multimeter and it was off by .02v it is now calibrated, thanks i didn't know it could do that! the voltage alarms.. guess you get what you pay for, accurate enough. but the kk2.1 board is off by 1v?? thats crazy! yes i'm gonna uppdate it now, iv'e just got my usbasp programmer. as for the kk2.0 can you not you calibrate it to? would be nice to have it accurate but i guess i can compensate with the voltage alarm

thanks for the help guys.

 

[jhitesma]

Even with calibration there's errors. I calibrated both my accucell6 and my HK watt meter to match my volt meter (which is nicer than harbor freight quality but no fluke) and even so they still both give different readings.

The issue is that they're just not designed for the level of accuracy they can display. Between variance in the resistors used in the circuitry and rounding in the software they're just not accurate to v0.01 even though they can display to that level.

Plus the calibration routine on the accuccel isn't that great. Mine is too high on one setting and then too low on the next, to get "just right" I need a setting in between but that's not possible.

 

[El_AMPo]

Voltage reading precision in cheap or simple MCU integrated systems comes in hand with the in-circuit voltage regulation.
This is because they get the voltage reference for the ADC from the 5V or 3.3V regulated lines and use them as the baseline for the battery voltage reading. (5V = 1024(max) on 10Bit ADC -> 0.048V per ADC step)

A variation of 0.1-0,01 V in the regulated supply breaks havoc in the battery voltage reading specially in the decimal places.
Sum that to the fact that a 0,1V variation is usual on a linear regulator under load (LCD backlight on or off?) and that they use a resistor voltage divider to read bigger voltages (5V = 1024 -> /4 (for 4S packsl) -> 20V = 1024 -> 0.192V per ADC step) and you get so, so readings.

For precise readings trust a voltmeter, or your battery charger (in monitor mode) if you don't have a voltmeter.
Probably they get closer to the real reading.

Anyway for monitoring purposes an error of 0,2V is still over 3,0V per cell if you use 3,3V as the limit and over 3,3V per cell if you use 3,5V as the limit. so don't worry.

 

[rockets4kids]

Most microcontrollers with ADCs also have internal voltage references. Supply regulation shouldn't be an issue in most cases.

 

[El_AMPo]

Usually they make the proper choice with voltage regulation and reference voltage, but for the sake of being cheap sometimes they just go on VDD trying to reduce the noise floor in a high current environment.

Only one way of finding out if there is the culprit, open the charger and check the vref pins of the MCU maybe they will get you to something.