WHY IS FUEL GAUGING DIFFICULT IN LiFePO4 (LFP) BATTERIES?

John Goodenough is famously recognized for his contributions to the lithium-ion battery, for which he was awarded, along with Stanley Whittingham and Akira Yoshino, the Noble Prize in Chemistry in 2019. Goodenough is named co-inventor on two US patents, US5,910,382 dated June 8, 1999, and US6,514,640B1 dated February 4, 2003, that describe the foundation of cathode materials of which Lithium Iron Phosphate (LiFePO4) (abbreviated as LFP) is now in commercial deployment. After a series of lawsuits involving A123 Systems, Valence Technology, Hydro Quebec and University of Texas, the latter being the owner of the patents, the courts ruled in 2011 to narrow the scope of these two patents, thereby opening the material to be manufactured by many. Since then, lithium-ion batteries with LFP cathode have been widely available, with China leading the way in their manufacturing.

What makes LFP batteries attractive?

i) In principle, they tend to be less costly to manufacture compared to NMC or NCA cathodes owing to their use of iron and phosphorus, two commonly available ores;

ii) LFP-based batteries operate at a lower terminal voltage (maximum of 3.5 V vs. up to 4.35 V for other cathode types) thus making them less likely to catch fire;

iii) LFP-based batteries tend to have long cycle life, often exceeding 5,000 cycles.

Yet, as we all know well, there is no free lunch. In return for these benefits, LFP-based batteries suffer from lower energy density, approximately half to two-thirds that of NMC- or NCA-based batteries. Energy density is a fundamental property that impacts driving range in electric vehicles.

Measuring state of charge (SOC), also known as a fuel gauge, in LFP-batteries is a significant system-level challenge. Traditionally, the SOC of a battery (i.e., the amount of charge held in the battery, noted as a percentage range between 0 and 100%) is inferred by measuring the terminal voltage of the battery. This earlier blog entry explains in more detail the basic principle of accurately measuring SOC. In summary, the principle relies on a measurable change in terminal voltage as charge is added to or removed from the battery.

In LFP-based batteries, there is a wide portion of the SOC range where the terminal voltage does not change as the battery charges or discharges. And it happens to be in the most useful range of a battery’s operation, between approximately 30% and 95% of the SOC range. In other words, electronics have a very difficult time discerning the SOC of the battery using this voltage method.

So, systems must add coulomb counting to offset this shortcoming. As the name suggests, these are sophisticated and expensive analog electronics that “count” the amount of charge that enters and exits the battery. They work well for relatively short durations of time, but if not calibrated continuously, they tend to drift over long periods of time, hence giving erroneous readings.

This is a serious challenge to the widespread use of LFP batteries. The error in SOC measurement propagates throughout the system and manifests itself as a limitation in measuring state of health (SOH) of the battery. In other words, if one is unable to accurately measure the level of charge in the battery, it becomes increasingly difficult to assess the level of degradation of the battery, and consequently, its “health.”

Good engineers are trained to address uncertainties (i.e., errors) by adding safety margins. So, LFP battery designs incorporate fat margins that further reduce the accessible energy, exacerbating its low energy density limitation.

To be clear, this is not a scientific limit, but rather an engineering limit. In other words, innovation in measurement techniques can overcome this problem and make fuel gauging in LFP batteries accurate and reliable.