Reading Assist Level and Battery Charge on the Screen

The battery bars dropped two steps on a hill, then crept back up on the flat. Or you changed the assist and the whole picture changed. Neither is odd on its face. Here's what's behind both readouts.

Assist level: what the number or bars mean

On a pedal-assist bike, the motor works with your legs. Wikipedia puts it this way: "The motor augments the efforts of the rider when they are pedaling"¹. It also says the motor is "regulated by pedaling"¹.

The assist display tells you which setting you've picked. It may show bars, a number or a word. Higher usually means more help, but the steps differ between makers. A middle setting on one bike isn't the same push as a middle setting on another. Your manual lists what each step does on your model.

If you pick a setting and the motor seems quiet, the setting may be at its lowest step or off. Look at the screen first, then check your manual.

Battery gauge: a fuel gauge, with limits

Wikipedia says the state of charge "indicates the remaining energy in the battery pack" and calls it "the equivalent of a fuel gauge"². State of charge is the trade term for how full a battery is. Your screen shows it as bars, a percentage, or sometimes both.

A fuel gauge in a car has a float in a tank. A battery has nothing like that. The system works the figure out from other readings, which is why it's an estimate.

Why the figure moves

Wikipedia describes two common ways to estimate it.

• From voltage. One method reads the battery voltage and compares it with a known curve. But "the voltage is more significantly affected by the battery current"², and by temperature as well. So when the motor pulls hard on a hill, the voltage dips and the gauge can read lower than it will a minute later.
• From current over time. Another method "calculates the SoC by measuring the battery current and integrating it in time"². That's counting energy in and out.

I can't tell you which method your bike uses. Your maker's manual may say. What the sources do show is that voltage-based readings can swing with load, and that explains bars that fall on a climb and recover on the flat.

When the screen says the battery is low

Some makers flag it with a code. One maker says "When the battery is nearly empty and the system detects low capacity for more than 10 seconds, it will trigger error code 66"³. It adds a warning: "If you continue using the bike in this state, it may stop working properly, such as losing riding assistance"³.

So a low-battery message is a prompt to act. Work through it in order.

1. Look at the assist setting. Your manual may say whether a lower setting changes how long the charge lasts.
2. Plan to recharge, and follow your manual's charging steps.
3. If the figure drops fast on a full charge, or jumps about with no load, contact the maker or a dealer.

Charging, storage and fire safety come from your battery maker and your local fire service. This page doesn't give those steps. Don't open or modify a battery pack.

Percentage or bars

If your screen can show both, pick whichever you read faster. A percentage looks exact, but it's still an estimate built from the same readings. Bars are coarser, and one bar may cover a wide spread of charge. For distance left, see the range section. For symbols near the gauge, see the icons page.