Other Parts Discussed in Post: BQ20Z45-R1, BQ34Z100

Designers of portable electronics have many flavors of lithium batteries to consider for a design. It’s often difficult to understand how and when certain batteries discharge, especially when dealing with battery-backup applications where short charge periods occur every couple days to replenish self-discharge, and a fully discharge rarely occurs. Lithium-iron-phosphate (LiFePO4) batteries are being used by more and more designers because of their reliability and steady depth of discharge.

I asked Keith Keller, TI Analog Field Applications engineer, to briefly elaborate on why he wrote a recent TI Analog Applications Journal article, “Improved LiFePO4 cell balancing in battery-backup systems with an Impedance Track™ fuel gauge.”  

 Q: As a field application engineer, do you see a lot of customers working with LiFePO4 batteries? Where do they use them? Why?

LiFePO4 batteries are used in certain niche applications, ranging from electric bikes (ebikes) to UPS to power tools to solar-powered garden lights. The cost per cell is higher than a standard 4.2V cell for ~ 40% less energy capacity, making it difficult to use in cost-sensitive applications. Customers typically use LiFePO4 batteries for high-rate discharge current capability (10s of amps per 18650 cell), longer cycle life than standard Li-Ion and better thermal performance. This application note discusses server backup application where this cell chemistry makes sense.



Fig. 1: Map of voltage densities for lithium-based battery cells


Q: What’s the biggest problem today with measuring remaining battery capacity in these batteries?

As you can see from the voltage densities curve (figure 1) LiFePO4 cells have a very flat voltage versus their state-of-charge. What this curve does not show is quite a steep voltage climb at the end of charge. This leads to difficulty in both gauging in the flat portion of the voltage curve as well as balancing of cells with only a charge and rest period in these server backup systems.

 Q: While working on this article, what did you learn that you didn’t realize before?

Millivolts count when gauging and balancing of these cells.  We made a change to the bq20z45-R1 algorithm to meet this need.  Newer battery fuel gauging solutions soon to released by TI, already incorporate these changes to gauge and balance LiFePO4 in a much better way.

 Additional resources:

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