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Part Number: BQ34110
After running multiple logging discharge cycles, I have noticed that FCC never updates itself by more than 256 mAh. In the BQ34110 TRM, it says "FCC cannot be reduced by more than FCC Learn Down or increased by more than FCC Learn Up during any single update cycle". "FCC Learn Down" and "FCC Learn Up" are bolded and italicized, which I assumed meant that they are data flash parameters that can be altered. However, there is no such parameter in BQStudio, and there is no other mention of these parameters elsewhere in the TRM.
Is it possible to increase the FCC Learn Down/Up limit to greater than 256 mAh? Or, if the parameter has only enough space for a 1-byte number, is there a way to turn off the limit entirely? I am asking this because it is clear that our batteries have a full charge capacity that is significantly lower than the Learned Full Charge Capacity reported by the chip (after fully charging, the RC value is approximately 5 Ah, while the learned FCC is approximately 7.8 Ah). I know that Learned Full Charge Capacity is a data flash parameter that can be changed, but I would rather let the chip measure it automatically, just in case our charger isn't working properly somehow. At the same time, I don't want to have to do 6 or 7 charging and discharging cycles because the Learned FCC can only update 256 mAh at a time.
Thanks again for any advice you can give.
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In reply to Bryan Kahler:
In reply to ColinY:
After more testing, I'd just like to add that there seems to be a difference between the chip's remaining capacity counter vs. the actual nominal design capacity of the battery. Even after using a fresh battery and running a discharge and charge cycle, the remaining capacity never charges back up over 5.5 Ah when the battery is rated 8 Ah, even when the voltage hits around 13.5 V (we are using 12V, 8Ah batteries). If we discharge them again from this 5.5 Ah point, Time to Empty (TTE) and State of Charge (SoC) estimations decrease linearly and continuously, and TTE seems to be very accurate. However if we disconnect and reconnect the chip to the battery in order to trick it into thinking it is at 8 Ah, 100% SoC, the TTE and SoC will decrease linearly to about 30% SoC until it gets close to EDV2, then drops abruptly to 7% SoC and 5 minutes TTE. This makes me think that the 5.5 Ah, ~60% SoC value is more trustworthy. But since the SoC is always at 60% max, FCC will never be able to update since SoC needs to be near full for it to qualify as a valid discharge. And even if we can trick the chip into thinking we're at FCC of 8 Ah, running an discharge cycle will only update the FCC by 256 mAh. Should we be just setting our design capacity to around 5.5 Ah?
It might be easier to understand with the logs. In this log we did a full discharge from full charge capacity, 100% SoC. Then we charged it back up, but it only reached 65% SoC. I realize that the current doesn't taper off, but the voltage is already high enough that RC should be much closer to 8 Ah.
This log is a continuation from the previous log, when it started discharging around the same voltage as the first log, but only around 65% SoC. The RC, SoC and TTE of this log is much more continuous and doesn't jump abruptly.TestDCLog-4-9-19-1338.log
Just another update for whenever somebody gets the time to look into this problem: we charged the battery fully all the way to 100% capacity (close to 8 Ah) by using a power supply (instead of tricking the chip into thinking it's full SoC). The battery voltage peaked at about 13.5 OCV. We then started discharging from that point at full load (12A) and we had the same problem of the BQ34110 overestimating the time-to-empty and SoC values again (see attached log). At this point we are also wondering if our battery profile might be the problem. The CEDV calculator returned FCC OCV of 11.9V when it seems that our battery isn't full charge until it's closer to 13.5 V. I've also attached the battery profile and GPC logs to this post. FinalParams6s.zip
Thanks for the reply, I've attached the gg.csv file here: BQ34110GGCSV.xlsx We've seen underestimation at both lower and higher rates at room temp, so we're redoing both logs. I'll let you know if there's any improvement.
Finally got done with re-characterizing the battery. We actually redid all 6 logs- we thought we hadn't charged up the batteries the full way during the first set, so that might be the problem. With the new characterization, we are seeing a reduced error, but the TTE is still dropping from 11 minutes to 3 minutes. This is our characterization logs:
This is the parameter zip file we received from TI:
This is an example discharge with the new parameters:
I tried messing around with the FitMaxSOC% and FitMinSOC% in the config. I haven't actually seen any posts or info about how to select these numbers. From what I noticed, the SOC Error% that are returned from TI vary about 1-2% when adjusting these values. It seems like with a lower percentage error, the RC/TTE values don't drop as heavily. For example when the SOCerror% for roomtemp_lowrate was 1.77%, the TTE dropped from 16mins to 3 mins once it hit EDV2. When the SOCerror% was 0.73%, the TTE dropped from 11 mins to 3 mins. I'm not sure if this is correlation or coincidence.
The problem with this is that it's impossible to get all 6 condition (temp + rate) error % under 1%. If I change FitMax/Min so that one condition error goes down, another condition error will go up. It might be possible to change the CEDV parameters dynamically based on discharging environment conditions, but this would be a pain to code.
I'm interested to hear any advice or suggestions you can give. Thanks for your time.
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