As a city transport tool, the electric bicycle has a strong market for its lightness and agility. But what if the bike suddenly runs out of power on the road? Some businesses saw an opportunity here and launched mobile fast-charging stations offering ten minutes of charge for one yuan, which are quite popular with the public.

Yet online users question whether fast charging, with its high voltage and large current, damages the battery and leaves it short of capacity later. Is the claim that fast charging harms electric vehicle batteries true?

As everyone knows, charging a lithium battery has three stages: constant-current pre-charge, large-current constant-current charging, and constant-voltage charging. First, a word about this lithium battery charging process.

Because of the special nature of lithium batteries, either over-voltage or under-voltage can ruin a cell, so today’s lithium battery charge-discharge protection circuits work by measuring the cell voltage and judging whether the battery is in a normal state (neither over-voltage nor under-voltage). Charging a lithium battery is divided into three stages: constant-current pre-charge, large-current constant-current charging, and constant-voltage charging.

When the voltage is below 3.0V, the charger pre-charges the lithium battery with a 100mA current to slowly recover an over-discharged cell; this is a protective measure. Every qualified charger has this stage, and it is here that the question becomes relevant.

Once the lithium battery voltage rises above 3.0V, the second stage begins: the large-current constant-current charging stage. The charging current can now be raised appropriately; depending on the battery it ranges from 0.1C to several C, where C refers to the battery capacity. For a 2600mAh lithium battery, 0.1C means a current of 260mA.

In this stage, the standard charge recommended nationally uses a 0.1C current, which is called the standard charge. But this standard was set many years ago, over a decade back, when lithium battery technology was far less stable than today and large-current charging was not allowed; hence such a standard existed. The only benefit of the standard charge is a stable charging process, with an extremely small chance of explosions; the drawback is that it is slow.

Fast charging means charging in this stage with a current greater than 0.1C. For a 2600mAh lithium battery the standard-charge current is 260mA, and any charging current above 260mA counts as fast charging. Judging by today’s lithium battery level and charge-discharge management chip level, charging at even 1C is no problem, so fast charging is not as dangerous as imagined. Ordinary fast charging uses currents of 0.2 to 0.8C, so it remains safe. Thanks to improvements in recent years, most chargers now are of the fast-charging type.

The final lithium charging stage is constant-voltage charging: when the battery voltage is detected to equal 4.2V, the charger enters constant-voltage mode at a steady 4.2V while the charging current grows smaller and smaller (as the battery fills up, the current naturally falls). When the charging current drops below 100mA, the battery is judged full and the charging circuit is cut off.

Electric bike batteries generally come in two kinds, lead-acid batteries and lithium batteries. Both are assembled from a number of cells (a lead-acid pack made of six or eight units in series and parallel; a lithium pack made of many lithium cells in series and parallel).

At present, the great majority of electric bike battery packs have no cell-balance management, which makes the pack lifetime far shorter than that of a single cell. This also explains why electric bike batteries are not durable and are often scrapped after about a year.

In short, the conclusion is: fast charging means charging with a current above 0.1C. For a single lithium cell this barely affects life or stability, but for an electric vehicle battery pack, fast charging is an act of trading time for battery life.

One last point: charging your vehicle with its standard-issue charger is fine. But beware of charging stations that advertise a full charge in 10 minutes; using such stations will absolutely take a heavy toll on battery life.

In addition, for a battery pack, cell balance matters even more. The reason fast charging shortens pack life is precisely that the pack’s balance problem remains unsolved.