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BQ40Z50-R2: connecting 2 x 3s1p in parallel for redundancy/monitoring

john taylor
Prodigy 90 points
Part Number: BQ40Z50-R2
Other Parts Discussed in Thread: BQ25713, , BQ2947

Hi, I am currently working on a charger/batterypack for use with a portable measurement solution.

I have drawn heavily from the pmp40441 reference design as I have based the charger off of the BQ25713.

I am modifying this design to include a small lcd display for diagnostic info (cell voltages, charge current, etc...) 

Thinking of using the BQ40Z50-R2 for the battery monitoring side of things and here is where my confusion begins.

I am using custom battery packs consisting of INR1860-E35 (samsung 18650 3500mAh) in a 3s1p configuration, 

using the BQ40Z50-R2 as the monitor/gauge will give me the diagnostic data I want, but not the capacity I need.

We have gone with 18650's as the main cells because this particular system is portable, and downtime is costly,

we needed something that was high capacity, rechargeable, and available on short notice.

I know I could simply make a 3s2p pack using one BQ40Z50-R2 and I would get the capacity I want and be able to read the (3) cell voltages from the pack.

The issue here is I want redundancy and also the ability to monitor every (6) cell individually. I'm confused about how to get this to work with my current setup.

I know I need to use some kind of power ORing to isolate the packs from each other, but how do I go about getting this to work? 

These packs will be installed and calibrated at manufacture, so I don't foresee too much of an imbalance between packs, but I need assurances that if cells in one of the packs is replaced and the new cells are at a different charge than the other pack the system can handle this. Can 2 x 3s1p be placed in parallel utilizing 2 BQ40Z50-R2 as individual pack controllers, can the voltages on both packs be different and still charge/discharge the way the system expects? How do I go about achieving something like this or better-I am open to ideas.

Also 

  

  • 0
    TI__Mastermind 42535 points

    Hi John,

    This cannot really work, the reason being 2 3s1p packs getting 2 different current paths to gauges will end up with different characteristic resistances due to differences in battery resistances. It's very difficult to get 2 matched packs and secondly even if you do, there's no guarantee in the field that one pack wouldn't likely be weaker than the other. This can lead to issues with balancing, charging and dsg termination.

    Your best bet in this case is a 3s2p single gauge solution. This won't let you monitor 6 individual cells but you can monitor a stack of two with all the benefits of added protection, balancing and gauging.

  • 0 in reply to Batt
    Prodigy 90 points

    Hi Batt,

    Thanks for the quick response. Not the answer I wanted, but it's probably the one I needed. I knew getting it to work the way I wanted would be more time consuming than just getting it to work right. I updated the diagram, should I include BQ2947 for extra protection? Also do you have a reference for determining how much cell-balancing I should need with this particular setup?