Stop Charging at About 80% with a Smart Plug Timer

Why this happens

Lithium cells age more slowly when they are not held at full charge for long, and most daily rides do not need a full battery. Many two-wheelers have no built-in charge limit, but charging speed is fairly steady in the middle of the charge, so a timer can cut power at an approximate level. The vehicle's BMS still protects the battery; the timer only switches the charger's mains supply off.

What you need

Step by step

1. 1

   Check the manual first

   Look in the manual and app for a built-in charge limit; if your vehicle has one, use it instead of a timer. Also check whether the manufacturer says anything about using timers or interrupting charging. Some recommend regular full charges for balancing, which you will still do every week or two.

2. 2

   Choose a correctly rated smart plug

   Read the charger's input current on its label and choose a smart plug or timer rated well above it, with an earthed 3-pin socket that fits your charger plug. Buy a product with a recognised safety certification. Do not use a smart plug that gets warm, has a loose socket or is only meant for small loads such as lamps.

3. 3

   Measure your charging rate

   Over a few normal charges, note the starting gauge level, the time and the level after, say, one hour. Work out the percentage gained per hour in the middle of the charge, for example from 30 to 70 percent. Charging often slows near the top, but the middle section is usually fairly steady and is what the timer relies on.

   Tip: Record the starting level and temperature too; charging is often slower when the battery is cold or hot.

4. 4

   Calculate the timer duration

   Subtract your typical starting level from 80 and divide by your percentage per hour. For example, if you usually start at 30 percent and gain 25 percent per hour, you need 50 divided by 25, which is two hours. Add a few minutes margin rather than cutting it fine, and treat 80 percent as an approximate target.

5. 5

   Set up the plug and charger

   Plug the smart plug straight into a sound, earthed wall socket, not into an extension board. Plug the charger into the smart plug and connect the charger to the vehicle in the order your manual recommends. Set a countdown or schedule in the app to switch off after the calculated duration.

6. 6

   Check the result and adjust

   When the plug switches off, note the gauge level. If it reads 75 or 85 percent, adjust your timing. After a few days you will know the right duration for your usual starting levels. Unplug the charger from the vehicle once it has switched off, rather than leaving it connected for hours.

7. 7

   Keep regular full charges

   Every week or two, or as the manual recommends, skip the timer and do a full charge while you are at home, letting the charger finish so the BMS can balance the cells. Do the same before a long trip. Note these full charges in your log.

How to tell it worked

On daily charges the gauge reads close to 80 percent when the smart plug switches off, the smart plug and socket stay cool, and you still do a full balancing charge on a regular schedule.

Questions