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BQ24650 Dynamic charge current control

Jitender Misra
Prodigy 150 points
Other Parts Discussed in Thread: BQ24650, BQ24133

We plan to use BQ24650 as a charger for charging 12V Lead acid batteries (@ maximum current of 10A) in our system as well as to provide a system rail voltage in our system. The input voltage to  BQ24650 will come from a 17V solar panel (Voc= 22v , Vmp=17.5V) . Accordingly the Sense resistor , inductor etc will be sized according to this maximum 10A charging current requirement

We are using BQ24650 because we are interested in using MPPT capability of BQ24650. So as long as solar panel gives enough power output the batteries continue charging at high current ( ~10A) . Of course if the solar panel voltage drops due to illumination conditions accordingly the charge current into the batteries will be reduced. This is fine with us.

However we have one more requirement of overriding the maximum charging current dynamically. Basically if the ambient temperature inside our equipment becomes high we would like to reduce dynamically the charging current to lower values , say 2A. We want to do this so that the heat dissipation inside our equipment can be reduced. (Because obviously heat dissipation inside equipment will be more when the BQ24650 is charging batteries at 10A versus when BQ24650 is charging batteries at 2A.)

So i have following three questions regarding this 

1. From the datasheet of BQ24650,i did not find any way of dynamically changing the charging current, however i was just thinking that is it possible to have some external circuit which can force the voltage on MPPSET to lower than 1.2V ( but greater than 175mv) to reduce the charging current? . Can any such external circuit be suggested by TI ? 

2. This question is on a seperate note. What happens when i provide the Vin input to BQ24650 from a DC source of following vales 

a) say 13-13.5V V - So of course i , know that from this low voltage adequate voltage for charging lead acid battery can not be generated , hence batteries will not get charged - but question is that whether BQ24650 will try to charge the batteries still or not ? 

b) say 24V - this will force the MPPSET pin voltage to be much greater than 1.2V - so does this condition make BQ24650 assume that there is some fault condition and stop charging or will it continue the charging at full current assuming that more than enough power is available for charging

  • 0
    TI__Genius 10240 points
    1. You can refer the bq24133 Figure 22 "Selectable Input Current Limit" circuit. www.ti.com.cn/.../bq24133.pdf
    2. a) No, The device is in SLEEP mode.
    2. b) continue the charging at full current
    An EVM would be very helpful to get better understanding of the performance.
  • 0 in reply to ezhao
    Prodigy 20 points
    BQ24133 does not have MPPSET pin but it has a ACSET pin and the suggested circuit basically changes the adapter current setting for the BQ24133.

    So are you suggesting in BQ24650 case as well to change the voltage at the MPPSET pin by placing an external parallel resistor that gets selectively enabled / disabled to reduce the charging current ?

    Here i would appreciate if you have a figure or a curve between voltage at MPPSET pin and the corresponding charge current. Of course when the MPPSET is at 1.2V we will get the full charging current of 10A ( as set by the external Rsense resistor)

    However whether there would be a linear relationship between voltage being lower than 1.2 and current correspondingly lower than 10 A- implying something like below
    1.2 V - 10 A
    1V - 8A
    0.8V - 6A
    0.6V - 4A
    and so on

    hence i would appreciate if a figure depicting relation ship between voltage at MPPSET pin and the corresponding charge current can be mad available.
  • 0 in reply to ezhao
    Prodigy 20 points

    From BQ24133 datasheet i can see that changing the voltage at ACSET basically changes the current setting for Adapter current. 

    However in our case we have the MPPSET pin for BQ24650. Are you suggesting that in a similar fashion the voltage at MPPSET pin can be changed to reduce the charging current. ? 

    In that case i would appreciate if a figure / curve or a formula can be made available which shows the charging current relationship with voltage at MPPSET pin 

    Also is this relation ship linear  something like below 

    1.2V - 10A 

    1 V - 8A 

    0.8 V - 6A

    0.6V - 4 A 

    etc.  or is it non linear in which case i would appreciate a curve. 

    Basically if above works then we will include a I2C driven digital pot that would reduce the voltage at MPPSET pin dynamically based on temperature condition within our equipment

  • 0 in reply to ezhao
    Prodigy 130 points

    Hello, I am also trying to use the BQ24650 MPPSET input to scale battery charge current based on load current.  I have an EVM and have not the the behavior I would expect.  Using a trim pot between VREF and ground, with the wiper feeding MPPSET, I can attain about a 1% reduction in charge current before it simply shuts off battery charge current entirely.  

    I am going to set up and repeat the test with a buffer amp feeding MPPSET, but expect to find the same results.  Could you please provide more information on using this pin to scale charge current? Specifically: What is the scalable range of the output, and the corresponding MPPSET voltages?

    If this controller is not capable of reducing charge current 50%, could you suggest an alternate that can, is switchmode, and has 10A capacity?

    Thanks