Why do my Urbanbiker Viena electric bike batteries keep failing?

Question

Around late 2019, I bought an Urbanbiker Viena new off Amazon for my daily work commute (around 15km/day at that time, now it’s a bit lower at about 10km/day).

After about 4000km, my (first) battery started failing, with symptoms similar to How to make a 10 year old electric bike battery work better? :

Even after a full-night charge, the bike’s display would display a full charge, but as soon as (or just a few hundred meters after) I start pedaling/demanding power, the battery display would drop to nearly-empty, then suddenly stop (sometimes displaying a flashing "maintenance" icon).

I contacted Urbanbiker support, and after some remote troubleshooting, I sent the battery and the display (but not the controller) for diagnosis. They couldn’t find any fault, but they sent me a new display nonetheless along with the same battery.

Changing the display did nothing, and the battery eventually died for good. Support then told me I would have to replace it, but at 700€+ new, I opted for a second-hand one that the seller described as lightly-used (they had their e-bike stolen, and hence sold their spare battery).

However, that second battery is now displaying the same behaviour, after only 500km of use by myself.

The batteries are marked as 48V 17.5Ah, and the provided charger is marked as 54.6V 2.0A out (I can provide pictures).

FWIW, lazy as I am, I often use the maximum assistance mode, although the place I live (in France) is pretty much 100% flat. Urbanbiker support told me that’s what causes my batteries to degrade… but somehow I’m not convinced.

Is that normal that those batteries fail that fast? What can I do to repair them and especially prevent this behaviour from happening again?

I’m on the verge of selling this bike as I’m unsatisfied with other issues (assist is not "snappy" enough from a stop, though acceleration is then good; and unassisted pedaling feels overly sluggish), but I’m willing to give it a last try.

EDIT: I've eventually ordered a refurbished Moustache e-bike from https://upway.fr/ as a replacement and will sell the Urbanbiker Viena to recoup the cost.

Answer 1

It is not normal that a well maintained, properly manufactured battery pack built with quality Li-Ion cells using a proper battery management system (BMS) fails that fast. Now, I can't tell if the Viena's one checks these marks without tearing it apart and inspecting the internals, but I can say that I have already seen similar issues on a lightly used 3-4 years old battery pack on an ebike from a well known bike manufacturer.

One potential cause that could explain the problem is that since there are a certain quantity of Li-Ion cells in series to get a 48V battery (usually 13 for a nominal voltage of 48V (13 * 3.7V) and a charging voltage of 54.6V (13 * 4.2V)), the BMS needs to be able to balance them to make sure all cells in series are within a few mV of each others. If one of the p-group (group of cells soldered in parallel, probably 7 cells for your pack assuming each cell has a capacity of 2.5Ah (7 * 2.5Ah = 17.5Ah)) has an abnormally lower voltage compared to other p-groups because of either:

• A damage cell within the p-group
• The BMS not doing balancing at all
• The BMS doing a poor job at balancing
• The use of Li-Ion cells that have a high leakage current
• A broken or damaged BMS voltage sense wire
• Other reasons

Then it could happen that while your are using your bike, this particular p-group triggers the BMS undervoltage condition (usually around 3V) which cuts the output even if every other p-groups are still at a valid operating voltage (e.g. 3.5V).

On a 48V battery pack I troubleshooted recently, the voltage on the battery connector (which is the output of the BMS) was 0V, but after bypassing the BMS and probing the Li-Ion cells directly inside the battery pack, I could read 46V which gives an average p-group voltage of ~3.5V. But, when probing every p-group individually, I could find out that the p-group 4 and 5 were shorted together, making a differential voltage reading of 0V for p-group 5.

All that to say, there are chances that the pack could still be rescued but it would definitely need a check from a qualified electronics technician and I really insist on the term qualified since the manipulation of such batteries can be hazardous.