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A battery pack false reading / BQ27210

TKay
Prodigy 30 points
Other Parts Discussed in Thread: BQ27210

Hi,

I've got a ASUS Eee PC 701SD laptop with an original battery pack that uses BQ27210 for management. Original cells had died and, being a guy with a DIY spirit, I decided to try to replace them with new cells. The battery is now working but showing a symptom described below.

The battery appears to recharge fine. Once fully recharged, I'm getting a 'battery remaining' reading in Windows XP that seems correct judging by the manufacturer specs for a new battery. However, once the remaining battery capacity has steadily gone down from 100% to approximately 44%, the reading then immediately jumps to 7%. From there, it'll again decrease steadily to 0%. I can tell the big jump Windows reports is not based on the real battery performance, because the laptop will remain powered on for a long period of time at 0% reading (probably what power a 44% full battery is still able deliver)

I suppose the three possible reasons for this are

1. Some register value(s) associated with performance of the original cells being remembered by the management chip BQ27210. This results in a wrong capacity reading delivered to the laptop system.

2. Some other problem in the battery management circuit or laptop hardware.

3. Windows XP misinterpreting the data sent by the battery.

Unfortunately I don't have access to a new factory-made battery module that I could exclude/confirm the third option with. At some point, I might try booting from a Linux live CD to get a second OS opinion on the reading.

If anyone with experience in BQ27210 or a similar battery symptom has ideas, I'd love to hear them.

  • 0
    TI__Genius 10235 points

    The reason for the error is likely because the replacement cells did not match the cells in the original battery pack. The bq27210 uses the CEDV algorithm which requires characterization specifically for the battery pack as built in order for gauging to be accurate. It is not going to be able to maintain this accuracy if the pack is changed.

  • 0 in reply to Chase7071949597
    Prodigy 30 points

    Thank you for the input! The datasheet mentioned the 'Capacity Inaccurate' status register flag that is set either by a full reset or every 32 valid learning cycle events. Is this what initializes and eventually causes rebuilding of the characterization curve? If so, should I expect the capacity reading to adjust to new cells when the next full 32 cycle-count mark is reached?

  • 0 in reply to TKay
    TI__Genius 10235 points

    The cycle you described is just for relearning the LMD. I'm actually referring to the compensanted EDV coefficients that are stored in the gauge calculated specifically for the battery pack in development. These values are not likely to match that of the cells you swapped in and will cause accuracy errors.

  • 0 in reply to Chase7071949597
    Prodigy 30 points

    Right. So, in order to get a correct reading, I'd have to recalibrate the chip using sophisticated measuring equipment to match the actual cells.

    Oh well, I'll just keep using the pack as it is. Better knowledge next time.

    Thank you a lot for taking time to explain this all to me, appreciate it.