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bq27531-G1 & bq24192 Charge Control

Seth Johnson
Intellectual 380 points
Other Parts Discussed in Thread: BQ27531EVM, BQ27531-G1, BQ24192

For these questions assume I'm using the bq27531EVM (bq27531-G1 & bq24192), using a new 10Ah battery (and Design Capacity set to 10000) at room temperature, and my power source provides 17V @ 10A.

1) How does the SOH affect the way the battery is charged? I have read the section "3.2.4 Multi-Level Charging Algorithm" in the bq27531-G1 TRM but it doesn't go into details about it. Is it linear so that when the SOH reaches say 85% then the charge current is overall reduced by ~15% and the charging voltage is raised by ~15%? How much of an effect does the SOH play on the charging algorithm?

2) I am assuming that since the default Charge_Current values at room temp are 100 that the charge current would be limited to 4.5A since 1C of 10000 mA is higher than the 4.5A limit of the bq24192. Is that correct?

3) Is it also correct to assume that it would be useless to program the Charge_Current values to anything above 45 in this case? That way in theory the charge current is limited to the same 4.5A.

4) Say that I was using Charge_Current values of 45 (normally a 4.5A charge current, right?) but the SOH of the battery was 85%. Would it take ~15% longer to charge the battery because the bq27531 is compensating for the lower SOH?

5) I see the Charge_Current settings are based on the C value of the pack. Is the C value used in the charging algorithms just read from the Design Capacity?

  • 0
    Intellectual 380 points

    Regarding question 5, I see that in page 46 of the TRM it uses the term "Design Capacity" and "C" interchangeably to describe LoadSelect value 4 in Table 6-1. I'm not sure what else C could be based off of so I'll assume that's what it is.

  • 0 in reply to Seth Johnson
    TI__Genius 10225 points

    1) The SOH compensated charge control will take in consideration the health of the battery for reducing the charge current. This is not a linear function. When enabled it takes place when the battery is considered degraded as learned from the Impedance TrackTM gauging algorithm. The voltage is not increased when reducing the charge current as you are speculating. The SOH compensated charge function is mainly for reducing stress on the battery during charge when it has become degraded and not necessarily trying to keep the charge time equal to when the battery is new.

    2&3) Yes, this is correct. Design capacity is the C Rate that is used to determine the charging current. If Design Capacity is 10Ah, you won't get any more charge current by increasing the Charge Current values above 0.45.

    4) Assuming that charge takes place at 85% of original charge rate then it would take longer to charge the same battery. This assumption is based on that the battery has the same usable capacity though. In the case of SOH compensated charge control, when the charge current rate has been reduced, the battery has been determined to have degraded which means that its usable capacity has decreased. If its usable capacity has decreased then that battery doesn’t require the same amount of charge to reach full capacity as it needed when it was new. Charge time is not linearly related to charge current rate. Because of this it is not obvious by how much the charge time may extend if reducing the charge current for each particular battery. In general it is most likely that with reduced charge current that battery will take longer to charge.

    5) Yes.