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LiFePO4 and BQ27*** series (fuel gauge or Resistance measurement)

Ran Aloni
Expert 2386 points
Other Parts Discussed in Thread: BQ27000, BQ27500, BQ27010, BQ27210, BQ27541-V200, BQ27200, BQ27510, BQ27541

Hi

 

I would like to take a measurment of cycle count that a single LiFePO4, and maybe other useful information (but mainly cycle count),

I can see the series is suitable for Li - Ion, which has a higher EDV than LiFePO4,

I already have an EV2300 to connect the the HDQ/I2C port to the computer SW,

will it be possible to connect BQ27000 to it?

will it be possible to connect BQ27500 to it?

does anyone has a better suggestion to a different constelation?

 

 

 

  • 0
    TI__Genius 14985 points

    Yes, you could use a bqJunior (bq27000, bq27010, bq27200, or bq27210) or an Impedance Track gauge (like bq27541-V200) to work with LiFePO4 single-cell batteries. 

    bq27500, bq27510, bq27505, bq27520 are designed to sit on the main system board and not inside the battery pack, so they cannot really count cycles very well since they expect different batteries to be inserted periodically.  You really should use a pack-side gauge like a bqJunior or a bq27541-V200.

    Note that you will need to configure them properly no matter which gauge you use.

     

    Best regards,

    David

  • 0 in reply to dMax
    Expert 2386 points

    first of all, thanks for the fast response.

     

    I read a little about BQ27541 V200,

    sounds more than suitable for us in a fair price but:

     

    1)will it integrate with EV2300 and its software (I saw a lot of new functions than in BQjunior), if not what will?

    2)I beleive it does'nt specify (in absolute spec) what is the maximum current he can endure? do you know?

     

    thanks

  • 0 in reply to Ran Aloni
    TI__Genius 14985 points

    Yes, you can use the EV2300 with bq27541-V200.  In fact, the bq27541EVM will also ship with an EV2300.  You will simply need to download the latest Evaluation SW from the product folder on TI.com:

    The current limit is determined by your sense resistor value.  There is not actual current flowing through the gauge IC.

  • 0 in reply to dMax
    Expert 2386 points

    I oredered a development kit for BQ27541

    still I'm worried.....

     

    the Rsense values varies between 5mOHM to 20 mOHM.

    naturally with LiFePO4 we are talking about large currents, it can peak to 50A, but usually about 15A (in high usage),

     

    the power on the Rsense will be 50A*50A*5mOHMS= 12.5W

    the Rsense I know could carry only 2W!

     

    thanks

     

     

     

     

  • 0 in reply to Ran Aloni
    Expert 2135 points

    You can easily find sense resistors rated for up to 5W and they can be paralleled for higher power handling. i wouldn't use them at above about 50% of their power rating though to prevent temperature-induced drifting of the resistance.

    Most can easily handle up to 5x their rated power levels for high current peaks. Just have lots of copper on the PCB where the resistors are soldered on and the heat is quickly pulled away in case you have multiple high current peaks. Then you can design for your 15A levels (just over 1W with 5mOhms), perhaps using a single 3W or 5W resistor, depending on how long your 50A surges last?.

  • 0 in reply to John M
    Expert 2386 points

    thanks, the 50A peaks are very short, still need to be considered because they could be frequent,

    I guess the search for the right Rsense begins...

    unless you can think of a more durable power managment IC,

    thanks

     

     

  • 0 in reply to Ran Aloni
    Expert 2135 points

    I guess no matter which power mgmt IC you use, you'll still have to worry about the sense resistor. :-))  Unless you use a hall-effect sensor, or other similar non-contact sensing method, that is. I've had great luck with DigiKey with searches for my 1, 5, and 10mOhm, 5W, 1% resistors.

  • 0 in reply to John M
    Expert 2386 points

    yes,

    I've found a suitable Rsense

     

    thank you very much

     

    you've been very helpful !!