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Pack-side vs System-side Fuel Gauge

Adam Johnson
Expert 1310 points
Other Parts Discussed in Thread: BQ27510, BQEVSW, BQSTUDIO

I'm evaluating several TI fuel gauges for a mobile product, particularly the BQ27542 (a pack-side gauge) and BQ27510 (a system-side gauge).  We will use the gauge in a system-side configuration.  Our battery is user-replaceable, and has no protection circuitry, so we also have a battery protection IC on our PCB.

My question is, what would happen if we used a pack-side gauge in a system-side configuration?  What's the difference between the two?

I saw this post on the forum that asks a similar question, but the answer for that party was to use a different gauge, so the question wasn't directly answered.

Thank you,

Adam J.

  • 0
    TI__Guru* 76230 points

    Hi Adam,
    If your cells are user replaceable then you will definitely need a system side gauge. Your options are either the bq27510 or the bq27520. The reason being the gauge will have to have the ability to recognize when a different battery is inserted and determine which of the resistance profiles to use, based on if it is an aged battery or if it is a brand new battery. Only the system side gauges can do this. pls see section 5.7.1 Battery Profile Storage and Selection of the TRM for the bq27520 to understand how this is done.

    thanks
    Onyx

  • 0 in reply to Onyx Ahiakwo
    Expert 1310 points
    Ok, I understand that if we use a pack-side gauge in a system-side application with a user-replaceable lithium battery, there will be poor accuracy when the battery is changed, because the wrong resistance profile may be used. How bad can we expect this accuracy to be? Are there other consequences to using a pack-side gauge in this way?

    Thanks,

    Adam J.
  • 0 in reply to Adam Johnson
    TI__Guru* 76230 points

    Hi Adam,

    The accuracy could be off significantly. it is difficult to quantify the value because it depends on if the swapped battery OCV profile matches the OCV table in the gauge, as well as how aged the battery is. The error could be as much as 50%.  What is the limitation why you don't want to use a system side gauge?

    thanks

    Onyx

  • 0 in reply to Onyx Ahiakwo
    Expert 1310 points
    Is that estimation (up to 50%) true even among batteries of the same chemistry?
  • 0 in reply to Adam Johnson
    Expert 1310 points
    I ask about using a pack-side gauge because we're far enough in our design process that we have prototype units with a pack-side gauge on them, which we will demonstrate at a trade show late this month. We're working to implement a system-side gauge, but until we finish that, we need to know the consequences of our poor choice.
  • 0 in reply to Adam Johnson
    TI__Guru* 76230 points
    If it is for the same battery type, and a new battery, then the accuracy shouldn't be far off. You will get comparable results each time you swap the battery as long as you are not in the flat region of the OCV profile of the battery. The chem updater for bqstudio or bqEVSW has an excel sheet that tells what voltages corresponds to the flat region for the different chem ids. As long as the OCV profile of your battery is not flat like that of lithium iron phosphate cells (the flat region of LFP cells are very wide, almost the entire voltage profile of the cell) you should be able to archive reasonable accuracy each time as long as the battery is not under load, and if i were to put a number to it, i would say less than 10% error.

    thanks
    Onyx