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Battery Management - Gas Gauge Forum
Can't reach 100% with BQ27200
I tested the charger BQ24070 with the fuel gauge BQ27200 and I can't reach 100%. I installed the software for the eval board and execute a DONE command after eache charging cycle.
1) Discharge the battery to 3v.
2) Charge the battery at the max with the charger.
3) execute the command DONE.
4) restart at step 1).
The maximum reached with the fuel gauge is 95%.
Are you saying that even after sending the DONE command that the charge is not reaching 100% or is it that you are having to send the DONE command because SOC only reaches 95% on its own and requires the DONE to push to 100%?
After a DONE, the % is at 100%. But the next charge will not reach 100%. The max value reached is 95%.
Then the charge termination conditions are not being met.
The bq27200 will adjust to 100% SOC once the charge termination is detected. The conditions that must be met to detect charge termination is for the voltage to be above the qualification voltage that is set with the QV1 and QV0 bits in EEPROM and the charge current must taper to below the current programmed in TAPER register of EEPROM. The taper current detection is going to have a dependency on the sense resistor value being used in design.
The 5th, 6th and 7th charge has stopped at 95%...
Did I have to change Taper value in EE or something like that?
After the learning sequence did I have to restart the learning sequence if the IC lost his power?
If you want the SOC to be forced to 100% upon charge termination then you must set TAPER register so that it is based on a current threshold that is a bit higher (considering charger and gauge tolerances) than the actual auomatic charge termination of the charger,
If the IC loses power then all learned data is lost and capacity is initialized to 0%. The bq27200 is a pack side gauge which means that it should reside in the battery pack. The ony time it is expected to lose power is if the battery deeply discharges hence it will be at 0% capacity.
Finally, did the DONE command is necessary or the IC will learn by himself the batt caracteristics?
The DONE command is sent at end of charge (full charge) only if the gauge can not detect charge termination on its own.I still want to understand why you needs to send the DONE command when the gauge should be detecting charge termination on its own. I may only understand what is going on if the you describe in detail what you do to make learning cycle. Details must include EEPROM contents, discharge current rate, charge termination conditions and order of events.
The battery characteristics are only learned if the battery is charged completely and it is then discharged to at least below the EDV1 voltage threshold that is set in EEPROM.
Here is the sequence I did
1) I installed the battery on the fuel gauge. (at this moment, the batt level is unknown).
2) I start the charger. (max reached = 52%)
3) Discharge the battery to 2.93v
4) recharge it. (at this moment the level is always 52% if I don't execute a DONE).
Is it possible that the battery don't reach the EDV1?
What is the EDV1 value?
The EDV1 threshold is one of the EEPROM values that establishes the capacity at 6.25% and when the capacity is learned with the bq27000. If it is still default value then it will be higher than 3V.
Can you indicate the following:
Have you ever had the EEPROM values programmed? If so what are the values?
In step 2 why don't you send the DONE command?
Can you describe your charger behavior? What's the charging voltage? At what current does it stop charging at full capacity?
We keep all the EEPROM values at the default values, so we didn't write in the EEPROM.
We used the DONE command at each charge, I forgot to mention it in the sequence.
The average charge current is a little bit over 1000mA and begin to drop at 30% of charge and the charge stop at 52% (for the first charge, before we do the first DONE command). The taper current is set at 102mA when I read the EEPROM. So I think the charger stop to charge the battery when the average current reach the taper current value.
Do I have to change something in the EEPROM or with the default values we are supposed to reach the 100% state of charge?
Is there a specific sequence to get a valid learning cycle, because the Capcaity Inaccurate flag seems to be always at 1?
Thanks a lot!
The EEPROM must be programmed so that the ILMD is close to the design capacity of the battery, the TAPER is just above the taper current at which the charger stops and the EDV1 at the voltage at which expect that 6% of capacity is remaining. If none of these EEPROM registers are programmed accordingly you will not be able to have the bq27000 learn capacity accurately.
Ok, thank you! I'll do that. Also, do I have to change the value of QV1, QV0? I know that there are 4 Voltage values depending on the value of QV1 and QV0. (3968mV, 4016mV, 4064mV and 4112V). I don't know which one to select.
Select the QV combination that provides the highest voltage possible but that is still less than the minimum voltage for your charge when in constant voltage mode (typically 4.2V). When determining minimum voltage while in constant voltage charge mode consider the tolerances of your charger and the tolerances of the bq27000 voltage measurement.
I'm finally able to reach the 100% without using the DONE Command. But I noticed that when I reach the taper current I'm not a 99% and go to 100%. Sometimes I jump from 97% or 98% to 100% without reaching the 98% or 99%. I tried to drop the taper current a bit, but with the HEX values I can't get accurate values. I need like 111mA taper current, but the nearest value is 114mA. Is it a problem of QV1/QV0 or a taper current problem due to the non-accurate hex values. Is it normal to jump 2 or 3 % at a time when I reach the taper current?
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