Fuel gauging AB battery pack

Peter,

It would be easier to use the dissimilar cells in parallel with one another, so that each series stage has the same capacity. e.g Use a 1000mAh cell in parallel wit ha 2000mAh cell in each series stage. It sounded like you wanted to have a 2000mAh series stage and a 4000mAh stage. The problem with unbalanced stages in that the smaller one will charge and discharge much faster. You will probably need a device with active cell balancing to try  to keep get them to terminate charge and discharge at the same time. If you use the configuration that I mentioned first, then the gauge will see them as two 3000mAh series stages.

Tom

Tom,

Thank you for the quick answer.

I understand that the best scenario is to have the cells paralleled, then each parallel combination would be considered as single cell by the fuel gauge. Unfortunately, this solution brings some disadvantages as it would require wiring between the middle terminals and a risk of unprotected short circuit. From a safety perspective, two separate strings, each with its own protection FETs is the best solution.  

Perhaps, I was not very clear in the beginning - is there a solution from TI, that I can have two strings with own protection and a common fuel gauge, for example bq34z100 reporting the total battery capacity? 

I have used in the past MAX1538, but it is for hot-swappable independent batteries.

Best regards,

Peter

Tom,

I realize my explanation was not very good, please see the sketch with the battery configuration attached.

Regards,

PeterM 

As far as I know, there aren't any gauges from TI that can gauge multiple series strings of cells at once. if you don't want cell type A and B being paralleled, your best option would be to use an individual gauge and protector for each series string of cells.

Disadvantage of this are that your host microcontroller would need to read the capacities of each string individually, and increased circuit complexity since you'd multiple sets of the same gauge circuits for each series string, and you'll also need an I²C multiplexer like the TCA9458A since TI's gas gauges can't have their addresses reconfigured.

Thanks Jason,

What do you think of the idea to use two independent protection circuits and a single bq34z100 for the whole battery, measuring the current over common sense resistor and the voltage at the OR point of the two strings?

Regards,

PeterM

It wouldn't work very well (if at all), unfortunately. Impedance Track is cell and chemistry-dependent (even different Li-Ion batteries from the same manufacturer can have different discharge curves), and if one protection circuit activates but not the other for some reason, the gauge on the disconnected battery will still register current and this could drastically throw off the battery capacity calculations. Additionally, if both protectors disconnect, then the gauge loses power (also not an ideal situation).

If completely separate battery strings (for each battery type) is ultimately the plan you want, you could use, for example, a bq40z50 which will integrate the gauge and protector for one series string of cells. This won't fix the issue of avoiding duplicate components, but at least you won't need a separate gauge and protector.

Is the schematic below the sort of idea you wanted for your pack design?