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Gag gauge ICs for two battery packs in series.

Marco Bedani
Prodigy 30 points
Other Parts Discussed in Thread: BQ2060, BQ2010

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Hi everybody,

I have a question about a particular connection of a gas gauge IC.

I'm working on a project for a system with two NiMH 10-cell battery packs. The two battery packs are separately charged by two battery chargers and monitored by two gas gauge IC (like BQ2010 or BQ2060).

When AC external adapter is not present, the two battery packs are connected in series (i.e: creating a stack with the positive pole of the bottom battery connected to the negative pole of the top battery).

In this situation the gas gauge IC of the top battery will have GND pin not connected to the “real” GND of the circuit, but tied to the positive pole of the bottom battery (so that all the measurements performed by the BQ2010/BQ2060 are referred to that “virtual GND”).

Can anyone tell me if there are any specific issues or problems related to this operating mode of the gas gauge IC?

  • 0
    TI__Genius 14985 points

    Do you plan to communicate with both gauges?  It seems like the main problem to work around will be the shifted voltage level of the communication bus of the top gauge.

    Are these 10 cells in series or parallel?

  • 0 in reply to dMax
    Prodigy 30 points

    The 10 cells of each battery pack are in series (each battery pack providing a +12 V supply).

    My idea is to communicate with both gauges multiplexing I2C bus, and optoisolating the channel referring to the top gauge in order to shift the voltage level to appropriate voltage levels for the CPU.

  • 0 in reply to Marco Bedani
    TI__Genius 14985 points

    That sounds like it should work for communicating with the gauges if the entire circuit is self-contained within the battery pack (like sense resistors, etc.).

    How about charging?  Will they be charged while stacked in series, or charged at the same relative voltage levels when AC is present, and then switched to a series configuration when being discharged?

  • 0 in reply to dMax
    Prodigy 30 points

    The batteries will be charged at the same relative voltage level (+12 V) and then switched to a series configuration when being discharged (AC not present).

    I also came up with an alternative idea.

    Instead of referring the top gas gauge to the positive pole of the bottom battery (i.e. pin VSS connected to +12 V with respect to GND), maybe I could shift all analog voltage levels related to the top battery (current flowing into the battery, battery voltage, ...) with some simple amplification circuitry, re-scaling them to appropriate levels for the gas gauge IC (i.e. pins SB, SR1, SR2, SRC in a BQ2060).

    With this expedient, both gas gauge ICs would be referred to GND and there would be no need to optoisolate I2C bus to communicate.