Hidden Costs of Inaccurate State of Charge

When your phone’s battery hits 20%, it’s hard not to feel a mix of anxiety and dread. We’ve all been there—watching helplessly as the charge plunges from 20% to nothing in an instant. And then there’s the unexpected relief when that last 1% somehow seems to last forever.

This inconsistency happens because the battery percentage, or State of Charge (SoC), isn't measured directly. Instead, it’s an estimate based on complex calculations performed by a small computer. Every energy storage system is made up of numerous modules, each monitored by a Battery Management System (BMS). The BMS is vital for safe operations, ensuring the battery remains within acceptable voltage, temperature, and current ranges.

The challenge is that most BMSs rely on basic calculations and lack the advanced processing power needed to provide precise estimates. This limits their ability to accurately determine the true charge level, which directly impacts battery efficiency.

How Is SoC Calculated?

Accurate SoC estimation relies on two key elements: the battery’s specific chemistry and an estimate of its open circuit voltage (OCV).

Battery Chemistry

Every battery chemistry has its own unique OCV curve, something like a fingerprint. Even within lithium-ion batteries, the curves differ. For instance, the OCV profile of a Nickel Manganese Cobalt (NMC) battery is distinct from that of a Lithium Iron Phosphate (LFP) battery. These unique curves make battery chemistry an essential factor in calculating the State of Charge.

Open Circuit Voltage (OCV)

OCV refers to the voltage of a battery when no external current is flowing, meaning the internal chemical reactions have relaxed, and there’s no voltage drop due to internal resistance. However, accurately measuring OCV requires the battery to be at rest—something that’s rare in systems operating continuously.

This creates a problem. For some batteries, like LFPs, the OCV curve is relatively flat. This means there’s little voltage difference between a fully charged and nearly empty battery, especially between 90% and 20% charge. Because systems don’t often pause to allow accurate OCV measurements, the voltage is usually estimated instead. Even small errors in these estimates can result in significant inaccuracies, with State of Charge errors ranging from 5% to 15%.

Real-World Impact

For large-scale energy storage systems, inaccurate SoC calculations have major implications. To avoid over-discharging, operators often maintain a buffer, keeping an extra 10% charge as a precaution. However, this approach limits how much of the battery’s capacity can be used.

This margin of error translates into financial loss. Many large-scale storage operators face up to 15% inaccuracies in their systems, and in some cases, these errors can climb as high as 45%. The consequence? Wasted capacity, reduced efficiency, and significant missed revenue.

Read the full article here in North American Clean Energy.