BQ20Z655-R1: Cell Balancing: Can't get the 655 to execute balancing. Need to learn what knobs we can turn to make this happen.

Doug Jensen

Part Number: BQ20Z655-R1

Cell balancing: We need to know what we need to do to get the 655 to perform cell balancing because it is not doing it with every pack, and it’s absolutely not clear what ‘knobs we can turn’ to make balancing happen on packs that clearly need it to happen, but more importantly have the 655 automatically balance in the field without 'engineering' intervention.

Breaking this down: What are *all* the conditions for balancing to happen and what parameters can we adjust to make balancing occur? Furthermore, please break down each condition into what causes them to occur etc. (e.g. If a QMax update is required to occur, what causes a QMax update to occur and what parameters can we adjust to make a QMax update occur?)

We have 3S1P packs with one cell that is over 250mV different from the other two (which are within a few mV of each other) but we can’t get the CB bit to turn on. There is conflicting (or at least misleading) information in the TRM on when balancing will absolutely, positively happen. A simple example is the text:

“Cell balancing is active after a Qmax update has occurred with the FC bit set and OCV taken. This state can be determined by the Update Status being set to 0x0E.”

- We know that the FC bit does not have to be set for balancing to occur (if the FC bit was set the pack would be fully charged so balancing can’t happen since charging has completed.)

-We know the Update Status does not need to be 0x0E because we have a pack with CB set and Update Status = 0x06.

- The TRM doesn’t tell us the ACTUAL sequence of events to cause a QMax update – it only vaguely states when it might occur: “The bq20z655 updates the no-load full capacity (QMAX) when two open circuit voltage (OCV) readings are taken…” When does this happen? How can we force it to happen with gg parameters?

Bottom line: We have many packs in need of balancing which we expected the 655 to perform automatically but it does not. What do we do to make it happen automatically? Please be specific and describe all dependencies down to things we can control and measure – please do not say “it will happen when X happens” unless you tell us how to make X happen.

Some of the parameters you might want to know:

The Rest CIM time is 20 seconds

The Active CIM time is 20 seconds

Our Min Cell Deviation is set to 1650 (with 150 ohm balancing resistors and the datasheet values of 0.4 for duty cycle and 3.6 for V.)

Config C = 0x0001 (RSOCL=1)

We would like to get in direct contact with someone who can help us out - a few minutes on the phone would save hours in forum effort.

Thanks for the help! - we need it, and we need it quickly.

over 7 years ago

0 Swaminathan Ramanathan75031 over 7 years ago

TI__Expert 6575 points

This is quite an old device so it might take a couple of days to check with our firmware team and get all the information you have requested.
It would be very useful if you are able to post the gg file that you are using for production of these packs, so that we can focus specifically on the way these packs are setup.
Regards,
Swami

0 Doug Jensen over 7 years ago in reply to Swaminathan Ramanathan75031

Prodigy 175 points

Swami - Attached is the current golden gg file we are using. As such it doesn't reflect the imbalanced cells but clues you into our setup. Lemme know what else you need to speed things up.

DougDJensenRev B.1 (SN INU1-01480).zip

0 Swaminathan Ramanathan75031 over 7 years ago in reply to Doug Jensen

TI__Expert 6575 points

Hi Doug,
The statement in the TRM “Cell balancing is active after a Qmax update has occurred with the FC bit set and OCV taken. This state can be determined by the Update Status being set to 0x0E.” is indicating that the gauge needs Qmax to calculate the time required for balancing, since the gauge is trying to balance the capacity difference between cells.
It is however not clear from that statement that even though the gauge has calculated the cell balancing times, the actual balancing will not start till the next charge cycle.
Here are some more details for the calculation of the balancing time:

What is DOD (Depth of discharge):
1. DOD provides the gauge a reference that is independent of temperature and load to correlate the relaxed open circuit voltage of a cell to the cell specification maximum capacity of the cell.
2. A DOD of 0% correlates to the maximum open circuit voltage of the cell specification (Not the application specification), where the cell has the maximum theoretical capacity at 25C and no load.
3. A DOD of 100% correlates to the minimum open circuit voltage of the cell specification (Not the application specification), where the cell has 0 capacity at 25C and no load.

Qmax update conditions:
1. [VOK] is set during the charge or discharge cycle for the Qmax update:
   a. If [VOK] is set while gauge detects either charging or discharging then a Qmax update at the next relaxation period is possible.
   b. If at entry to relaxation VOK is set and then clears when DOD is measured, then there has been a successful Qmax measurement. Note: VOK will stay cleared until a charge or discharge commences, then it will set again.
   c. If [VOK] is not set at the start of charge or discharge:
     i. Temperature is outside the range 10C to 40C (See 3.)
     ii. Charge or discharge started in the disqualified region for the cell (See 4.)

2. [R_DIS] is not set during the charge or discharge cycle for the Qmax update:
   a. During discharge if the calculated resistance update is –ve, [R_DIS] will be set. This may happen under the following conditions:
     i. Bad chemID selection
     ii. The RESET command was sent when the voltages were not relaxed leading to an incorrect DOD measurement
   b. Offset error as described in 6. exceeds 2% of Design capacity

3. DV/DT 4uvolt/sec:
   a. How it is checked:
     i. Gauge will sum the 1 second samples of voltage for 100 seconds and store this as V1
     ii. Gauge will sum another 100 seconds of Voltage (1-second samples) and store as V2
     iii. If |V2-V1| <= 40 then the Cell voltage is considered relaxed and the DOD is measured for Qmax update

4. Must be between 10C and 40C:
a. DOD is accurate within a temperature range, so to minimize DOD error due to temperature it is only considered for Qmax update if the temperature is between 10C and 40C

5. Voltage Disqualification Range (Flat region found in \bq Evaluation software\Plugins\Chemistry\chemdat.txt):
a. The DOD error can be large in the flat region of the cell (small voltage change for a large change in DOD)
b. Only DOD readings outside the flat region are considered for Qmax updates

6. Offset Error (CC Deadband):
   a. The gauge considers the “CC Deadband” as an unavoidable error that is always present. This value is integrated after every DOD measurement, and if this integrated value at the next DOD measurement, qualified for Qmax update, is greater than 1% of the design capacity, it will disqualify the Qmax update.
   b. This is so that any error due to the CC deadband does not impact the accuracy fo the Qmax update.
   c. Example:
     i. If CC Deadband = 10microV
     ii. If Sense Resistor = 10mOhms
     iii. Offset current = CC Deadband/Sense Resistor = 1-mA
     iv. If Design capacity = 1000-mAh
     v. Allowed capacity error for Qmax = 1% = 10mAh
     vi. Allowed time between two DOD measurements for Qmax update= Allowed capacity error for Qmax/Offset Current = 10/1 = 10hrs

7. Capacity Delta greater than 37% (DOD not RSOC):
a. The difference between the starting DOD and the ending DOD must be > 37%
b. Typically looking at he passed charge and comparing it to the Qmax will give a good idea of the change in DOD.

Once the Qmax has been updated, the gauge can set the balancing timers as required to balance the capacity difference between the cells based on the "Min Cell Deviation", and the balancing can start during the next charging cycle.

Regards,
Swami

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BQ20Z655-R1: Cell Balancing: Can't get the 655 to execute balancing. Need to learn what knobs we can turn to make this happen.