Design a battery fuel gauge for 2S battery packs

From the perspective of cell numbers in series, the battery fuel-gauge world has always been binary. You could either opt for a single-cell gauge or a 2-4 series multi-cell gauge in your designs. The binary world works great if all of your designs fall in either the 1S or 4S bucket – but if it falls in-between like in laptops, wearables and drones, tough luck!

It’s because of the key differences between multicell and single-cell gauges that these two categories exist in the first place. There is of course the pack voltage difference between 1S and 4S, with 1S <5V and 4S in the ~18V range, depending on the type of Lithium-ion cell. With higher voltages, safety considerations for the battery pack increase. For example, in a single-cell pack, you may find an overvoltage protector in addition to the gauge in the pack. But a four series pack typically has multiple levels of safety: independent, redundant ICs for voltage protection and a mechanism to blow a chemical fuse in case a critical safety event occurs with the potential to cause harm to the users of the pack.

The other key differentiating feature between single- and multicell gauges is cell balancing. The series cells in a multicell pack are balanced to equalize the cell voltage and state of charge when the pack reaches a fully charged state. The diagnostic data collected by a multicell device is also richer, with more parameters and levels of those parameters being monitored and recorded.

While you could configure a single-cell device such as a bq2750x or a versatile device such as bq34z100 to work in a 2S configuration, these devices do not provide the comprehensive safety or cell-balancing features of a true multicell device.

High accuracy 2S gauges, like the new bq28z610 and bq78z100, provide both the necessary comprehensive safety features as well as cell balancing to get the maximum runtime and cycle life out of a 2S pack. In addition, the bq28z610 has the unique feature of signaling charge control information to the bq24773 chargers. With the charge control feature, the host can be relieved of playing intermediary in the constant communication between the charger and gauge and focus its resources on other higher priority tasks.  

With the release of the 2S family of gauges, you are now freed from the previous binary selection process and you can design in an optimal gauge solution that provides the trifecta of accurate gauging, safety and cell balancing.

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