原文地址：https://segmentfault.com/a/1190000014669547

右邊多出來的是 ：after 的border 

HTML程式碼：

<div class="battery"></div>

CSS程式碼：

html, body {
    margin: 0;
    padding: 0;
    height: 100%;
    display:flex;
    justify-content: center;
    align-items: center;
    /*background: linear-gradient(to bottom, teal, aqua); 
 
*/
}
.battery{
    width: 6em;
    height: 3em;
    color: midnightblue;
    border: 0.5em solid currentColor;
    border-radius: 0.2em;
    font-size: 2em;
    position: relative;
    /* 充電顏色 */
    background-image: linear-gradient(to right, green, green);
    background-repeat: no-repeat;
    background-size
 
: 30% 80%;
    /* 給內部充電影象定位  */
    background-position: top 0.3em left 0.3em;
    animation: charge 5s steps(8) infinite;
}
.battery::after{
    position: absolute;
    content: '';
    width: 0.5em;
    height: 2em;
    background-color: currentColor;
    top: 0.5em;
    right: -1em;
    border-radius
 
: 0 0.2em 0.2em 0;
}
@keyframes charge {
    from {
        background-size: 10% 80%;
    }
    
    to {
        background-size: 95% 80%;
    }
}