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Bare Li-po cells in parallel with BQ29700

Alex Cheung
Prodigy 40 points
Other Parts Discussed in Thread: BQ24073

We are building a portable device with the BQ24073 Charger and the BQ27421 Fuel Gauge.

The plan for the low quantity prototypes is to use 17cells of 35mAh in parallel.

  1. Is it better to have one BQ29700 on each of the 35mAh cell? From my understanding the capacity maybe too low to trigger the OC



  2. Is it feasible to have the 17cells in parallel and just use 1 BQ29700 for all 17 cells?
  • 0
    TI__Guru 54715 points

    It depends. 

    1. For a 35 mAh cell, the short circuit current may be rather low.   To trip a SCD the FET resistance may need to be rather high which may cause more loss than you want in your system.  Protecting each cell is also a lot of protectors and FETs in the system so it will add cost and space.

    2. Paralleling cells for increased capacity is very common in industry, the cells act as a single cell with larger capacity.  17 cells is a large number but the resulting capacity is moderate (595 mAh).  Check with your cell supplier on how to best connect the cells.  This would allow a single protector and FET set to provide protection, and should be easier to select a FET to meet the device trip settings with a reasonable loss.  It may also make behavior of the battery with the bq27421 system side gauge more predictable since the battery should always be the same size.

    Some users prefer to have parallel batteries rather than parallel cells in the same battery since a bad cell will disable the entire battery.  When the cells are grouped in modules the bad module will be isolated by the protection allowing the system to remain powered. How to group the number of parallel cells into battery modules is a system consideration requiring thought about the behavior of the system and whether loss of one module from protection would cause an overload of the remaining module(s).  Similar to connecting cells in parallel, how to connect the modules in parallel in the system also must be planned. Also consider the behavior of the gauge if the capacity is suddenly decreased because of protection of a module.