IMAX B6 SkyRC - incorrect voltage when discharging

[Harris Michakis]

Hello all!

I originally considered flashing my charger because I was always getting incorrect voltage when discharging, around 80-100mV less than the actual battery's voltage. I have just finished installing and calibrating the 0.33 firmware for a genuine IMAX B6 and I still have the same problem.

Is there a way to introduce a discharging voltage offset? In settings -> options there is an entry called "disch +" with a value in mV, I have set it to 100mV but it does not affect the discharge voltage.

Any ideas on how to fix this? I use my charger to test the capacity of various LiPo batteries so an accurate discharging voltage is very important for correct cycle termination.

[Paweł Si]

I did a  drawing to show what is going on during discharge:

info:

1. I - discharge current

2. U - a negative voltage (relative to GND) *this is the problem

3. Ubat+,Ubat-,Vb0_pin,Vb1_pin,Vb2_pin - voltages cheali-charge is measuring (relative to GND)

4. Vb3, (Vb4,Vb5,Vb6) - voltages cheali-charge is measuring on cells 3, (4,5,6)

Unfortunately the chargers ADC sees only positive voltages.

the voltage on Ubat- and Vb0_pin during discharge is negative (relative to GND),

and the charger sees them as 0V.

so It's impossible po measure the correct battery voltage by design.

But this shouldn't be a big problem for you, because the charger thinks that

the voltage on the battery is lower then it actually is (there is no battery damage risk).

Additionally when the charger reaches the desired voltage it progressively decreases the discharge current 

(when "options"->"settings"->"dis agres:" is set to "Yes") which translates into reduced voltage U

(the error gets smaller and smaller).

[Harris Michakis]

Thank you for the detailed technical explanation, much appreciated.

I was under the impression that the ADC was measuring the voltage drop across the resistors that do the discharge (or maybe the current flow through them) so as long as you could program the actual resistance into the mCU then you could get accurate discharging profiles. Would it still be possible to program a dV offset so that we can measure and correct the discharge voltage in order to match the actual battery voltage? It is much worse when I discharge NiMH cells, I think the IMAX B6 with the original firmware had a dV of nearly 200mV when discharging an Eneloop at 1A, I'll test and see how the cheali FW performs. What does the disch+ option do actually?

[Paweł Si]

2014-06-27 17:53 GMT+02:00 Harris Michakis <hmic...@gmail.com>:

Thank you for the detailed technical explanation, much appreciated.

I was under the impression that the ADC was measuring the voltage drop across the resistors that do the discharge

This is true for cells 3,4,5 and for all cells on  200W chargers.

 

(or maybe the current flow through them) so as long as you could program the actual resistance into the mCU then you could get accurate discharging profiles.

Yes, someone has already suggested this, but the resistance depends also on the batteries main leads,

and (most important) I don't have time to implement this.

 

Would it still be possible to program a dV offset so that we can measure and correct the discharge voltage in order to match the actual battery voltage?

 

the dV depends on the discharge current and the total resistance, so a simple dV wouldn't be accurate.

but Yes, it's possible.

 

It is much worse when I discharge NiMH cells, I think the IMAX B6 with the original firmware had a dV of nearly 200mV when discharging an Eneloop at 1A, I'll test and see how the cheali FW performs. What does the disch+ option do actually?

 

 "disch+" adds to the discharge end voltage per cell. The default end voltage is 3V for LiPo.

if you set "disch+" = 200mA the end voltage will be 3.2V.

[Harris Michakis]

Once again thanks for the prompt and detailed reply. Why does this hold true only for the 3,4,5 cells? What is different for cells 1,2 and 6? Also another question perhaps not very relevant, I don't have a 6S battery to calibrate the charger with, can I just calibrate Vin, V1, Ic and Id since I only plan to charge single cell batteries?

[Paweł Si]

2014-06-27 18:43 GMT+02:00 Harris Michakis <hmic...@gmail.com>:

Once again thanks for the prompt and detailed reply. Why does this hold true only for the 3,4,5 cells? What is different for cells 1,2 and 6? 

My opinion: they just wanted to save some money 

For measuring the voltage on cell 1,2 they would need an additional IC (2 x op-amp)

http://elektrotanya.com/rc-power_bc6_imax_b6_hobbyking_eco06_digital_charger_voltcraft_b6_digital_balanced_charger.pdf/download.html

 

Also another question perhaps not very relevant, I don't have a 6S battery to calibrate the charger with, can I just calibrate Vin, V1, Ic and Id since I only plan to charge single cell batteries?

Yes you can, but for the calibration you need to connect the cell also to the balance port.