topic Re: Counting problem with Coulomb Count in BMS MC33771B in Power Management

Counting problem with Coulomb Count in BMS MC33771B

WJC — Fri, 07 Oct 2022 09:13:48 GMT

Dear Sir,

Using MC33771B driver in our BMS board calculated using coulomb count method.

Case 1, constant current source, I = 0.1A

average Isense value = 241510817 / 28443 / 0.1ohm = 84.9 mA

"ISENSE : 101 mA" & "average Isense : 84.9 mA " , the data does not match !

WJC_0-1665133825243.png

Case 2, constant current source, I = 1A

average Isense value = 134598805 / 1609 / 0.1ohm = 836 mA

WJC_1-1665133839292.png

Best regards.

Re: Counting problem with Coulomb Count in BMS MC33771B

JozefKozon — Fri, 07 Oct 2022 11:07:52 GMT

Dear William,

please try to use the formula from the section 9.15 in the MC33771B datasheet.

JozefKozon_0-1665140669846.png

JozefKozon_1-1665140685401.png

If the result is still incorrect, please conduct the Current measurement diagnostics described in the section 9.14 in the MC33771 datasheet.

With Best Regards,

Jozef

Re: Counting problem with Coulomb Count in BMS MC33771B

WJC — Tue, 11 Oct 2022 11:33:12 GMT

Hi Jozef,

Thanks for the reply.

We are using "sw499711 MC33771BMC33772B-Battery-Cell-Controllers" for "Monitoring_Diagnostics Examples".

We are observing log for this output like image below.

WJC_0-1665487866576.png

Multimeter (1.01A) and MC33771 (ISENSE : 1003mA) is consistency, but i am getting Coulomb Count measurement result as 134598805.

What is the relationship between COULOMB_CNT with ISENSE?

How to calculate values?

Average Isense = COULOMB_CNT * V2RES / CC_NB_SAMPLES / RSHUNT !!!

Thanks,

Re: Counting problem with Coulomb Count in BMS MC33771B

JozefKozon — Wed, 12 Oct 2022 07:19:56 GMT

Hi William,

please check if the FAULT pin is activated. The coulomb count value is impacted if the overflow occurs. I suspect this is the reason for incorrect current result.

In the event an overflow occurs in either COULOMB_CNT or CC_NB_SAMPLES, the
CC_OVR_FLT bit is set and, when unmasked, the FAULT pin is activated. The coulomb
count value is impacted by conversions performed during diagnosis of the current
measurement chain.

The COULOMB_CNT is an integer whose associated resolution is V2RES, therefore,
COULOMB_CNT·V2RES gives μV. If the shunt resistance RSHUNT is expressed in μΩ,
then COULOMB_CNT·V2RES / RSHUNT gives μA.

JozefKozon_0-1665559167604.png

With Best Regards,

Jozef