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about max error of gas chip for battery pack
Updated:2011.08.17 Source:Laptop Battery | Laptop Batteries | New Replacement Laptop Notebook Battery | Laptop Lithium Battery Clicks:
 

Q:

I have programmed a fresh BQ20Z95  with my *.senc file. I complete the calibration and wire it to the battery pack. When I read the max error it is at 145% it drops to 1% after 2 to 4 cycles. A battery design we buy displays a max error of 5% with zero cycles. What am I doing wrong?

A:

Max Error of 145%:

This error happens when there is a discrepancy between `Cycle count` constant in SBS Configuration data-flash and hidden constant (Number of cycles where last Qmax was updated).

When a GG file with data-flash constants is imported and written to a pack that have already been cycled (i.e. a pack with a non-zero value of the hidden constant). This process changes the `Cycle count` but does not change hidden constant, causing an error when calculating Max Error. Did you create your *.senc file by updating GG file to a cycled pack?

To avoid this problem when updating GG file to a cycled pack, you can:

1. Before writting a new GG file into a target pack that have been cycled, alwaysre-write the default firmware first. This will make sure that hidden constant is zero.

2. Update chemistry data to the target pack.

3. Make sure in the GG file, the The SBS parameter "cycle count" = 0

4. Program the GG file to the target pack

Note that even if this error happens, it will clear itself after first Qmax reading is taken (which requires one relaxation/discharge/relaxation cycle). MaxError will than get reset back to 1%. That's why you saw the max error drops from 145% to 1%. Also note that none of these affects actual accuracy of the gas-gauge, but only the reporting of MaxError is wrong.

Max Error of 5%
This means the battery pack has not finished it learning cycles. Only Qmax is learned and resistance is not.

Run another cycle (charge to full - rest for 2hr, discharge to terminate voltage, rest for 5hr)  and Max Error should drop to 1%.

Please refer to application book slua404 (ch11) for information on "Preparing Optimized default Flash Constants"