• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 64 subscribers
  • Views 1238 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Bq27421-G1A Learning cycle

Wayne Uroda
Prodigy 50 points

Hi,

I have a specific question regarding the learning cycle of the Bq27421-G1A when trying to do the initial charge from relaxed.

I have set the following configuration:

Design capacity = 1800

Design Energy = 6660

Taper rate = 180 x0.1Hr Rate (100mA for this capacity)

Opconfig = 0x05F8 (in order to send the software BAT_INSERT command).

So the gauge is setup with these parameters, the BAT_INSERT command is sent and the gauge has simulated its guess at the SOC. Then I discharged the battery to 3.24v and let it sit there for 10 minutes. At this time the important registers are:

relaxing:
Control status: 009E (INITCOMP | SLEEP | LDMD | RUP_DIS | VOK)
Flags: 018E (CHG | OCVTAKEN | BAT_DET | SOC1 | SOCF)
Volts: 3274
Current: 0
SOC: 0

The battery was then charged at max 500mA

0 min: (the sleep flag is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 018E (CHG | OCVTAKEN | BAT_DET | SOC1 | SOCF)
Volts: 3390
Current: 507
SOC: 0

7.6 min: (SOCF is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 018C (CHG | OCVTAKEN | BAT_DET | SOC1) 
Volts: 3714
Current: 504
SOC: 5

26.8 min (SOC1 is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0188 (CHG | OCVTAKEN | BAT_DET) 
Volts: 3826
Current: 500
SOC: 15

194 min (CHG is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0088 (OCVTAKEN | BAT_DET)
Volts: 4182
Current: 362 (current has been reducing since 183 minutes)
SOC: 99

241 min (CCA is set)
Control status: 088E (CCA | INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0088 (OCVTAKEN | BAT_DET) 
Volts: 4195
Current: 97
SOC: 100

241 min (CCA is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0088 (OCVTAKEN | BAT_DET) 
Volts: 4196
Current: 96
SOC: 100

243 min (FC is set)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0288 (FC | OCVTAKEN | BAT_DET) 
Volts: 4195
Current: 93
SOC: 100

253 min (OCVTAKEN is cleared)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0208 (FC | BAT_DET) 
Volts: 4197
Current: 70
SOC: 100

258.4 min (OCVTAKEN is set)
Control status: 008E (INITCOMP | LDMD | RUP_DIS | VOK)
Flags: 0288 (FC | OCVTAKEN | BAT_DET) 
Volts: 4197
Current: 62
SOC: 100

258.8 min (VOK and RUP_DIS cleared)
Control status: 0088 (INITCOMP | LDMD)
Flags: 0288 (FC | OCVTAKEN | BAT_DET) 
Volts: 4197
Current: 60
SOC: 100

The gauge then stays in this state and the charge current keeps tapering down. The gauge does another CCA at 331 minutes.

The current tapers down to 10mA a few seconds after the CCA and the gauge sets the sleep bit.

The gauge then stays in sleep mode for many hours, the charge current tapers off to zero.
During this time the registers are:

Control status: 0098 (INITCOMP | SLEEP | LDMD)
Flags: 0288 (FC | OCVTAKEN | BAT_DET) 
Volts: 4199
Current: 0
SOC: 100

The gauge never sets QMAX_UP or RES_UP and it also never updates "Qmax cell 0" or the Ra tables.

That was a lengthy intro. Here is my question: Does the gauge need a "step" to zero on the charging current in order to consider this a charge termination? Obviously my charger just operates continuously and the current just tapers slowly to zero. 

If it does need a step, under what conditions should this happen? I've read SLUA597 and the quick start guide SLUUAH7, and from these it seems that as soon as FC is set it is OK then to disable the charger. Is this correct or should there be a minimum time after FC sets before removing power to the charger?

I am going to make the necessary hardware modifications so that I can test this out, but I was hoping for a definitive answer on the subject in case I am still stuck after trying this.

Thanks

- Wayne

  • 0
    TI__Expert 4115 points

    Hello Wayne,

    With the '421, you will first need a Qmax update before you can have a resistance update.  Qmax is defined by PassedCharge / (DOD2 - DOD1).  The amount of passed charge that needs to occur for the full learning cycle is 90% of Design Capacity.  The DOD, which stands for Depth of Discharge, is update while the gauge is in relaxation mode.  DOD is the inverse of State of Charge (SOC).  The gauge gets a DOD point while the battery is relaxing.  It will read the voltage and when the voltage is stabilized (dV/dt < 1 uV/s) the gauge will take that voltage reading (OCV measurement) and convert it to a DOD point. 

    So in order to get a Qmax update you first need to get OCV measurement (DOD update that happens in relaxation) have some passed charge (either charge or discharge) and then get another OCV measurement.  At that point you should have a Qmax update.

    After you have a Qmax update you can then have resistance updates (as long as the [RUP_DIS] bit in the Control() register is not set) after you have been in discharge mode for at least 500 seconds.

    I have attached a document that outlines the learning cycle process a bit more.  I think it should help provide some good guidelines for you.

    0042.LearningCycleOverview_bq27421.pdf

    Also, a quick note on the different modes the gauge has:  charge, discharge and relaxation.  The gauge enters each mode on current thresholds.  For the '421 the discharge current threshold is Design Capacity / 16.7, charge current threshold is Design Capacity / 13.3 and quit current threshold (i.e. relaxation mode) is Design Capacity / 25.0.

    Please see SLUA450 for more details on Impedance Track.

    I hope this helps.

  • 0 in reply to Jared Casey
    Prodigy 50 points

    Hi Jared, thank you for that extra information.

    In your experience, what standby current equates to 1uV/s? (ie, what is the maximum standby current of the appliance where the gauge will still take an OCV measurement?)

    I assume that during the relax when fully charged, the charger must be disabled?

    Thanks

    - Wayne

  • 0 in reply to Wayne Uroda
    TI__Genius 14985 points

    Typically if the current is below C/10, or better yet, C/20, the battery voltage will get to relax eventually.  The time it takes to relax depends on the temperature, state of charge, chemistry, recent loading activity (duration and magnitude) and can range from 10 minutes to hours.