Deeply discharged e-bike battery? Here is how to get it back

Lithium-ion cells work between about 2.5 and 4.2 volts. If the voltage drops too far, the BMS disconnects the battery to protect the cells. If the battery is then left lying for months, self-discharge pulls the voltage down further. Below 2 volts per cell, copper starts to dissolve inside the cell. From that point there is real cell damage, and charging becomes risky.

• The charger does not start, or switches off again straight away.
• Not a single light comes on, even after a few hours on the charger.
• The display stays black and the bike does not switch on.
• The battery has stood unused for a long time. The classic case: put in the shed in October, and in spring it does nothing.

Does your battery still respond, but is there a fault? Then it is probably not deep discharge. Try resetting your e-bike battery first.

Almost always by standing unused. A lithium battery loses a few per cent of its charge every month, and the BMS itself draws current too. Put a half-empty battery away in October and the voltage can be below the limit by spring. Every spring our workbench is full of them. Those batteries did not break. They were left alone for too long.

There are two things you can safely try: leave the original charger connected for a few hours (some BMSs take a while to wake up) and check that the contacts are clean. Nothing beyond that. Keep away from the tricks you find online, where the battery is "jump-started" with a power supply or a second battery. With real deep discharge, copper dendrites may have grown inside the cell. They cause a short circuit as soon as you force it. In the worst case the whole thing catches fire. That is why a deeply discharged battery belongs on the test bench first.

Every battery that comes in to us goes through the intake test first. We measure the voltage of every cell group, check the BMS and determine the remaining capacity. After that you get one of these three answers:

1. Revive. If the cells are undamaged, we charge the battery back up in controlled steps and test it under load. You get it back with a test report.
2. Cell replacement (re-celling). If cells are damaged, we replace the complete cell pack with new brand-name cells, with a 2-year warranty. You can read more about that on e-bike battery cell replacement.
3. Advise against it. If neither is worth the money, we simply say so. And we give you an alternative, for example a refurbished battery from our stock.

If the product page for your battery shows no price, you get a quote first, after the measurement. You decide what happens. If you decline the quote, you pay the €95 inspection fee. After more than 45,000 batteries we usually know quickly where we stand.

• Store the battery over winter at 30 to 60% charge. Not full, not empty.
• Keep it cool and dry, ideally between 10 and 15°C. Not in freezing cold and not next to the radiator.
• Check every two to three months whether it still has enough charge, and top it up if needed.
• Never put it away completely empty. That is by far the most common cause we see.

Storing it well adds years. More on that in our article on the lifespan of an e-bike battery and our tips for your e-bike battery in the winter months.