A supplementary compensator for Texas Instruments gauge IC state-of-charge estimation for lithium-ion batteries is proposed and a hardware platform is designed to verify the proposed compensation. A sudden change in TI gauge SOC estimation sometimes happens near the end section of the battery discharge curves. The reason is that there are non-modification regions posed by the gauge due to larger parameter variation caused larger SOC estimation errors. By designing the OCV extraction hardware and find that the difference of slopes has a big drop when approaching to 50% SOC and the value can be identified. The slope of 50% SOC is a fixed value which can be applied to SOC compensation. By analyzing the discharging curves for many vendors' lithium-ion batteries, we find that there are two conditions can be applied to compensate the SOC estimation. One is the specific difference of voltage slopes during discharging and the other is a specific discharging slope when SOC equals 50%. These check points can provide nearly consistent estimation of SOC for batteries with different cycle times and different ambient temperatures. An add-on platform is designed with the embedded voltage-gradient checkpoint and EMF table and the experimental results show that the SOC estimation accuracy is improved. The accuracy of SOC estimation with the help of the proposed compensation is increasing up to 11% under different variant current profiles.