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BQ2050h C-rating

Chakras
Prodigy 110 points
Other Parts Discussed in Thread: BQ2050H

Hello,

I would like to know how does the BQ2050h determine the actual C-rating of a discharge.

I have a 4000mAh lead acid battery that has aged and is now yielding around 1000mAh of capacity. Observing the contents of FLGS2 register, it gives a DRATE result of less than 0.5C even when i discharge with a current of 2.4A.

I am not sure if the C-rating is determined from the current sensed at the SR input or from the LMD value.

Look forward to your replies

 

 

  • 0
    TI__Genius 10225 points

    The C-rating is determined from both LMD and the current sensed at the SR input.  The C-rate = Current / LMD.  The FLGS2 register gives a rough indication of the C-rate of the battery. Depending on what this register is set to, DRATE can be <0.5C, between 0.5C and 2C, or greater than 2C.  So, if your LMD is 1000mAh, a current that is anywhere from 0.5A to 2A should give you a DRATE value corresponding to the 0.5C to 2C range.

  • 0 in reply to Chase7071949597
    Prodigy 110 points

    Chase

    Thanks for the reply. Your answer is as I suspected. A capacity learning phase for the connected battery needs to be performed first to ensure LMD is up to date and C-rating is correct. I wasn't sure if the PPU/PPD values were used in any way or if instaneous VSB and VSR were used to derive a C-rating. Just to make it clear, our particular application has the BQ2050h separate to the battery and connectable to different batteries.

    I wonder if you can offer any thoughts on my other posted question about using CACD for capacity calculation during 1A or 4A current discharges. I presume now that CACD will only differ from NACH/NACL when DRATE is greater than 0.5C? Also during some investigations yesterday, I observed that when DRATE was >0.5C, as reported by FLGS2, CACD would report a value almost half that of NACH. For instance when NACH*256+NACL reported a capacity of 800, CACD*256+NACL was reporting 400. Slightly puzzling. Also I noticed that the BQ2050h datasheet refers to CACH and CACL in the resetting section of command description. As this is what I am looking to use, is the command table (table 8) incorrect as it has this missing?  

  • 0 in reply to Chakras
    TI__Genius 10225 points

    Chakras,

    I will have to consult with one of our bq2050h expert on your other question.  Both of them are out of office this week, though. I will get an answer for you early next week.

  • 0 in reply to Chase7071949597
    TI__Intellectual 940 points

    Hello Chakras,

    If the discharge rate is greater than C/2, the CACD register will be set to either LMD*13/256 or LMD*26/256, depending on whether the pin programming is set up for graphite or coke anode respectively.  It is not a function of NACL or NACH.  So, if NAC is very low compared to LMD, it would be easy to have a large percentage reduction in NAC.  I don't know your LMD value, but if the value is 144, for example, a NAC reduction of 144*13/256 for coke would be a NACH reduction of 7 (7*256 = 1792 NAC counts).  Once CACD is reduced from NACH, it will remain reduced even if the discharge rate drops below C/2, until NACH decrements to equal CACD, or a charge current is applied.

    The reference to CACH/L is in error and should have been CACD/CACT.

    Bill

  • 0 in reply to Bill J10799
    Prodigy 110 points

    Hello Bill,

    I am not sure if I have completely grasped what you are saying. I presumed that CACD and NACH/L were interchangable.

    You seem to be saying that CACD defines the amount that NACH needs to be reduced by to give a discharge compensated. So the software code needs to calculate (((NACH-CACD)*256)+NACL) to yeild the remaining capacity? I didn't understand this from the description in the datasheet and was calculating ((CACD*256)+NACL). For my application LMD varies according to the particular battery connected to BQ2050h and is only correct after a full learning cycle. If CACD isn't used this way, please inform me how it is used to provide discharge compenasted capacity readings during discharge?

    Chakras

  • 0 in reply to Bill J10799
    TI__Intellectual 940 points

    Hello Chakras,

    There has been some misunderstanding.  If load is greater than C/2 and a 10% reduction is applied, the reduction is computed as 10% of LMD.  Suppose LMD was 144, then the compensation reduction would be 10% of LMD or 14.  So if NACH was 30, CACD would become 30-14 = 16.  This example points out that a 10% compensation value can result in a much larger percentage change of CACD as compared with NACH.  If the battery were fully charged and NACH-LMD, then this 10% compensation would only reduce NACH by 10%, but the reduction in available capacity at higher loads is basically due to the voltage drop of the load current across the internal battery impedance.  It's clear that if the battery did not have much charge left when a heavy load was applied, that the battery voltage could immediately drop below the EDV1 threshold and so the available capacity would drop to zero immediately, rather than alway dropping by some percentage of the available capacity.   As NACH decrements with continued discharge, CACD would also decrement, staying a constant 14 counts lower than NACH.  You could use CACD*256+NACL if you wish to have the resolution of a 16-bit word.  CACT takes the compensation a bit further and will equal the CACD value minus any compensation due to cold temperature.

    Bill