The 5 most common causes of a faulty e-bike battery

If we had to name one thing that nearly everyone runs into sooner or later, it is this: a battery fades.

At first an e-bike feels like a kind of superpower. You do your usual rounds without effort, you barely think about charging and the range seems endless. After a few years the first doubt sets in. You ride the same route in the same conditions, and still you find yourself glancing at the battery light sooner than you used to.

What we often hear then: “It isn't dead, it just runs flat quickly.”

And that is exactly what is happening. An e-bike battery contains lithium-ion cells. They wear, just like tyres or brake pads. Not because you are doing anything wrong, but because it is chemistry. Use (charge cycles) wears them, and so does time on its own. Heat speeds the process up, and a battery that often sits fully charged, “waiting” for the next ride, ages faster than people expect.

What you can do yourself:

There is no need to get anxious about it, but a few habits do make a difference:

• try not to leave a battery at 100% for weeks on end,

• avoid storing it in the heat (a car in the sun, the loft in summer),

• and top it up in good time instead of running it right down every time.

What we do at KWS Seuren:

We test whether it really is only loss of capacity, or whether something else is going on (an imbalance or the electronics, for example). If the cells are worn, we look at whether a cell replacement (re-celling), in which we replace all the cells with new ones, is possible and makes sense for that type of battery.

This may well be the most frustrating one. To the customer it feels as if the battery “died” overnight, while the cause usually started months earlier.

The scenario is nearly always the same:

• In autumn the bike gets used less.
• The battery stays on the bike, or lies somewhere in the shed.
• It happened not to be very full any more.
• And then, for months, nothing.

In March the first fine day arrives. You want to ride again, click the battery in, press the button, and nothing happens. Charging does not help. The charger stays green. Or no light comes on at all.

What has usually happened is deep discharge.

A battery always loses a little charge, and the electronics (the BMS) also use a small amount of energy to keep themselves on standby. Wait long enough and the voltage drops below a safe limit. The battery then locks itself to prevent damage or danger.

What you can do yourself:

This really is the difference between “years of use” and “bad luck after the winter”.

• Put a battery away at around 40 to 60% charge.
• Top it up once every 4 to 8 weeks.
• Keep it indoors if you can: dry and not freezing cold.

What we do at KWS Seuren:

We start with a safe diagnosis: is it deep discharge, or something else (charger, connector, circuit board)? Then we decide whether “waking” the battery is responsible, and whether the cells are still healthy. Safety always comes first with us.

Many people think of an e-bike battery as something “hard” and “sealed”. It feels solid and it was designed for outdoor use, so surely rain can do no harm?

Until the complaints start, and they are vague and annoying:

• The bike sometimes cuts out, but not always.
• Charging works one time and not the next.
• You see corrosion or a green deposit on the contacts.
• Or it started suddenly after a wet spell.

Water damage is treacherous because it rarely causes trouble straight away. It creeps in. Contact points oxidise and a circuit board starts to corrode. A leak or condensation develops, and all at once your battery stops working.

Common triggers that we see again and again:

• a pressure washer,
• always leaving the battery on the bike outdoors,
• damage or a small crack in the casing that you have not noticed.

What you can do yourself:

• No pressure washer on the battery or the connections.
• Take the battery off if you leave the bike outside.
• Keep the contact points dry.
• Check the seals and edges now and then.

What we do at KWS Seuren:

We check the casing and the contacts for traces of moisture and corrosion, and do the same for the electronics. In insurance cases especially, it matters that the cause of the damage is recorded objectively.

Sometimes a battery comes in and the customer says: “It won't charge any more, so it must be worn out.”

Then we measure it, and the cells turn out to be in decent shape. The problem is not in the “fuel” but in the “computer”.

An e-bike battery has a BMS: a system that monitors and protects the cells and communicates with the bike. When that system fails, you get complaints such as:

• the battery no longer charges,
• the bike does not recognise the battery,
• the indicator behaves oddly,
• error codes or communication problems,
• or cutting out under load for no clear reason.

Electronics can fail through age, moisture, knocks or peak loads, and sometimes through known weak points in certain battery types.

What you can do yourself:

• Use the right charger.
• Avoid moisture and knocks (those two cause a striking number of electronics problems).
• Be gentle with cables and connectors when you pull them out.

What we do at KWS Seuren:

Diagnosis is everything here. Our strength is finding the real cause and a fix that lasts: we repair or replace the electronics where that is possible, and tell you honestly when it is not worth doing.

This is the kind of complaint that drives people mad, because the behaviour seems to make no sense.

Your battery shows 60%. You set off. You reach a hill, or you accelerate hard, and bang: it cuts out. You switch the bike off for a moment. Wait a minute. And suddenly everything seems normal again.

What is usually going on: a battery is made up of several cell groups. If one group is weaker than the rest, its voltage sags faster under load. The BMS sees the lower limit and switches off to prevent damage. The battery then seems “suddenly empty”, but when you let it rest the voltage recovers a little and it appears to work again.

What you can do yourself:

• Treat this as a serious signal. Riding on while it keeps cutting out often makes it worse.

• Have it checked before you end up stranded.

What we do at KWS Seuren:

We measure where exactly the weakness is: general ageing (cause 1), or one cell group that is out of line with the rest. From that we decide whether a repair or a cell replacement makes sense.

Add up the stories from customers and bike shops and the insurance cases, and you nearly always end up with the same five:

1. ageing and falling capacity,
2. deep discharge after winter storage,
3. water damage and corrosion,
4. faulty electronics or BMS,
5. cell imbalance and cutting out under load.

The good news: winter storage problems and water damage in particular can usually be prevented with simple habits. And when a problem does arise, a good diagnosis is the difference between “throwing it away” and “riding on for years”. That is why KWS Seuren exists: to diagnose a battery safely, with the technical knowledge that takes, and to fix it for the long term where that is possible.