SoH stands for State of Health, a percentage that tells you how much capacity a battery still has compared to when it was brand new. A fresh battery starts at 100% SoH, and that number gradually drops over time as the battery ages and loses its ability to store energy. It’s one of the most important metrics for understanding how much life is left in any rechargeable battery, from your phone to an electric vehicle.
How SoH Is Calculated
State of Health is the ratio of a battery’s current maximum capacity to its original rated capacity, expressed as a percentage. If a battery was rated to hold 60 kWh when new but can now only hold 48 kWh at full charge, its SoH is 80%.
Two physical properties drive this number. The first is capacity fade: over time, a battery simply holds less energy per charge. The second is internal resistance, which increases as a battery ages. Higher internal resistance means the battery delivers power less efficiently, generates more heat, and can’t charge or discharge as quickly. Both factors together paint a picture of how “healthy” the battery is relative to its original performance.
What Causes SoH to Drop
Every rechargeable battery degrades with use, but specific factors speed up that process. The most straightforward one is cycling. Each time you charge and discharge a battery, small chemical and physical changes occur inside the cells. Over hundreds or thousands of cycles, these changes accumulate.
At the microscopic level, a thin layer forms between the electrode and the liquid electrolyte inside the cell. This layer grows thicker over time, consuming lithium that would otherwise store energy. That growth is the single biggest reason lithium-ion batteries lose capacity as they age. It happens even when the battery is sitting on a shelf, though it happens faster under stress.
Heat is a major accelerator. Elevated temperatures speed up the chemical reactions that cause degradation, which is why batteries in hot climates or in devices that run warm tend to lose SoH faster. Charging habits matter too. Routinely charging to 100% or draining to 0% puts extra physical stress on the electrodes and electrolyte, pushing capacity fade along more quickly than staying within a moderate charge range.
How Devices Estimate SoH
You can’t crack open a battery and directly observe its health, so devices rely on a Battery Management System (BMS) to estimate it. The BMS continuously monitors cell voltage, charge and discharge currents, internal resistance, and temperature. It uses these readings to calculate how much total charge the battery can accept compared to its rated capacity.
One common method is ampere-hour integration during charging: the system measures exactly how much electrical charge flows into the battery from empty to full, then compares that to the original rating. More advanced systems use machine learning models trained on patterns of voltage, current, and temperature data to predict SoH without needing a full charge cycle. These algorithms are especially useful in electric vehicles, where the battery is rarely charged from 0% to 100% in a controlled way.
For sealed batteries like those in backup power systems, accurately measuring SoH historically required a full load discharge test, which is time-consuming, expensive, and carries safety risks like short circuits. Modern BMS technology has largely replaced that approach with continuous monitoring.
The 80% Threshold in Electric Vehicles
In the EV industry, a battery is generally considered to have reached its end of life for automotive use when SoH falls to 70-80%. At that point, the battery has lost 20-30% of its original capacity. For a car originally rated at 300 miles of range, that means roughly 210-240 miles on a full charge. The car still works, but range, acceleration, and charging speed may no longer meet the demands of daily driving.
That 70-80% threshold is deeply embedded in industry standards, warranty terms, and research literature. Most EV manufacturers guarantee their batteries will stay above 70% SoH for a set number of years or miles. However, recent research has started questioning whether a single fixed cutoff makes sense for every driver. A study published in Heliyon found that only about a quarter of simulated real-world driving scenarios actually hit a functional end of life right at the 70-80% mark. Factors like individual driving habits, climate, and how “underperformance” is defined all shift where the practical limit falls.
What Happens After End of Life
A battery at 70-80% SoH isn’t dead. It just no longer meets the high demands of powering a vehicle. These retired EV batteries often get a second life in less demanding roles, particularly stationary energy storage. Solar farms, backup power systems, and grid-level storage don’t need the same power density or fast discharge rates that driving requires, so a battery with reduced SoH can still function well for years in those applications.
Before repurposing, each battery pack is assessed at the cell or module level for its remaining SoH and estimated remaining useful life. Cells that still have adequate capacity get reassembled into storage systems. This process extends the total useful life of the battery and delays recycling, which reduces both the environmental impact and the demand for new raw materials. As EV sales continue to grow, the volume of batteries reaching automotive end of life is increasing rapidly, making repurposing both an environmental priority and a growing industry.
SoH vs. State of Charge
SoH is often confused with State of Charge (SoC), but they measure different things. SoC is the percentage you see on your phone or EV dashboard right now: how full the battery is at this moment, like a fuel gauge. SoH is the long-term measure of the battery’s total capacity relative to when it was new. Think of SoC as how much water is currently in a bucket, and SoH as how big the bucket has become compared to its original size. A battery can show 100% SoC (fully charged) while having a SoH of 85% (it holds 15% less total energy than it did when new).