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Using the bq77910A with bq34z110 to balance lead acid batteries.

Reba Liggett
Prodigy 30 points
Other Parts Discussed in Thread: BQ77910A, BQ34Z110, BQ77908A, TLE2426

I'm looking into developing a small scale lead acid battery management system using the bq34z110 gas gauge with the bq77910A cell balancing and protection chip as recommended in the datasheet for the 110. The issue I'm running into is using the 77910A for lead acid battery multicell management.

I'm using four 6 V SLA batteries in series, producing 24 V total, which won't overload the 77910A. The problem which I'm facing is the issue of programming the CBV, which I'm assuming is the threshold voltage that triggers the cell balancing function to begin. In the datasheet, it shows that the threshold voltage is only programmable up to 3.9 V. This works for the lithium ion/polymer batteries with which I'm also working but would deep discharge the 6 V SLA batteries. Is there a way to program or stack the chip to get a CBV of ~5.7 V - ~6 V? 

I would think this would be possible since this chip and the 77908 are recommended for use with the 110, despite their lithium ion programming.

Thank you,

Reba Liggett

  • 0
    TI__Guru 54715 points

    Figure 2 of the bq34Z110 datasheet may not be the best representation of a system since protection/balancing functions may be uncommon in lead acid systems and the bq77910A and bq77908A indicated are designed for lithium based chemistries.

    If the protection function is desired with the lead acid system, it may be possible to use the 3cell/6V battery across 2 cells of the '908A with some circuit to simulate the middle cells for the '908A.  This would require some current, several parts and you would need to check to see if the limits could be set to match what you need for your system.  The balancing would need some careful analysis since the '908A would pick one of the 2 simulated cells and the simulated input would need to carry the balance current.  The protector is really targeted for Li cells and using it here could be very complex.

  • 0 in reply to WM5295
    Prodigy 30 points

    Thanks for the answer! I have another question though. What kind of circuit would be necessary? I know with the cell simulator sent with the '910A is a resistor chain, so I was planning on something like that. Will that be enough? I'm not planning on using the protection side, just cell balancing. Is that possible? 

  • 0 in reply to Reba Liggett
    TI__Guru 54715 points

    I have not tried to do this, so it may not work, but the idea would be to provide a virtual cell input for the '910 so one 6V battery simulates 2 cells for the '910.  A resistor divider alone may be a poor choice since the resistance to the '910 must be < 100K to prevent an open cell fault, and low resistance values might be an unacceptable drain on the battery.  The resistors will also have to take the balance current. There may be an IC to provide this virtual ground (TLE2426 for example), but you may not want it to be symmetrical and ICs will have a cost and performance limit.

    It may be possible to use 2 large value resistors to establish a voltage and a couple emitter followers to provide high current from the battery with some small voltage change.  If this is biased off-center, one should be picked for balance instead of the other, and the one transistor would carry the balance current (and resulting power) while the other would carry only the open cell test current.