100% Charging and Safety Margin (Error Margin)

Hello, this is Dr.EV. In this article, following our previous discussion on “battery charging and management methods,” we’ll explain the misconceptions surrounding 100% charging and safety margins (error margins) in electric vehicle (EV) batteries in the simplest way possible. Although the content may be a bit lengthy, we’ve organized it to focus on the key points.

When developing EVs, manufacturers focus on critical indicators such as battery lifespan, driving range per charge, and charging speed. To achieve these targets, the Battery Management System (BMS) must accurately estimate the battery's State of Charge (SOC), State of Health (SOH), and State of Power (SOP).

These are referred to as estimations because they are not directly measurable. Instead, they are inferred from real-time measurements such as voltage, current, and temperature. Even top-tier manufacturers report errors of around ±3% for SOC and ±5% for SOH and SOP.

During battery and pack manufacturing, minor deviations (tolerances) are inevitable. For example, when producing a cell designed for 10Ah and 4.3V, the actual output may vary slightly — such as 10.1Ah or 9.9Ah. Additionally, when multiple cells are connected to form a pack, slight resistance differences in the connections can result in minor energy variations between cells. Therefore, the total error, including manufacturing tolerance and BMS estimation error, may be around ±5%.

Automotive BMS units are developed to meet the highest safety integrity level (ASIL-D), ensuring that the probability of exceeding such error margins (e.g., 5%) is extremely low — as low as 0.0000002%. However, this level of assurance requires massive sample testing, which depends heavily on the manufacturer's investment in quality and safety.

If we assume a ±5% total error, manufacturers typically secure a 5% safety margin on the battery level.The problem is that a larger margin reduces the usable range per charge, which directly affects the vehicle's advertised range. As a result, some manufacturers may reduce the safety margin and accept a higher failure risk, while others may sacrifice some range to retain a larger margin.

Therefore, reducing BMS estimation error by even 1% can significantly improve range, lifespan, and charging performance. In fact, a higher safety margin may suggest a lack of technical sophistication. In some cases, manufacturers with higher errors may aggressively reduce the margin, potentially increasing the risk of fire or accelerated degradation.

Manufacturers also factor in the frequency of stressful conditions such as 100% charging, high-power output, and extreme temperatures when setting their warranty period and mileage. This means that under typical conditions, the warranty can be met without special battery management.

The common belief that “batteries don’t require management within the warranty period” is not incorrect. However, usage beyond the warranty period is generally not expected by manufacturers. In our previous post analyzing real-world data from 1,500 Tesla vehicles, most battery degradation patterns aligned closely with the warranty period — typically around 8 years or 200,000 km.

If manufacturers were confident in longer battery life, they would reflect this in the warranty itself and use it for marketing as a sign of reliability. While some EVs may demonstrate extremely long battery life, this is often due to positive deviations in error and should not be generalized.

Here we mainly discussed SOC, but SOH is closely tied to the warranty period, while SOP is directly related to charging speed and vehicle performance. Thus, accurate estimation of all three parameters (SOC, SOH, SOP) is a core technical competency and a key to competitiveness in EVs.

In summary, unless your usage falls under very specific and extreme conditions (e.g., parking a fully charged car in hot weather with thermal management turned off), you can safely use your EV within the warranty period even with 100% charging or occasional high output.

However, if you plan to keep the car beyond the warranty, managing the battery to reduce degradation will be beneficial. Just as battery degradation matters when buying a secondhand laptop, no one wants to buy a used EV with a heavily degraded battery.

Lastly, the statement “100% charge isn’t truly 100% because of the safety margin” is inaccurate. If the margin is negatively offset (i.e., the actual capacity is lower), then a displayed 100% could indeed mean a fully charged battery.