eBike stopped working


New member
Local time
8:02 AM
May 28, 2022
I tried to start my bike today but get absolutely nothing

The bike has a throttle and digital display.
The display won't even switch on.

The battery is 36V 9aH
I've tried a multimeter on the battery and its show 4.17V when the battery is either switched on or off. (see photo)
I presume this means the battery is definitely at fault?
Any idea what could be wrong? Was working fine a few days before.
Charge the battery and test again, note time to charge.

I have already put it back on the charger earlier, its full now.
Took about an hour to fully charge which is about right as it was nearly full before. Charger light then turned from red to green.
Nelson is pretty good at guiding people. Nelson could it be the controller? These ebikes and their weirdness is nuts.
It can't be the controller, I'm reading the voltage directly from the battery.
The battery on/off switch position makes no difference to the voltage and it always reads approx 4.17V
I tested the voltage between each pin of the BMS connector (wire harness)
Voltages ranged from 4.07 to 4.22
I assume thats ok and proves the cells are fine?

Actually, your battery is pretty badly out of balance and any reading over 4.20 is bad, in several ways.

Take readings from the negative to each pin, then negative to next pin and subtract the previous reading.

I am assuming you went 2 to 3, then 3 to 4, etc.

I am honestly not sure why but the method is always Negative to each pin, then on to next and subtract the previous reading.

A "few days before", when it "worked fine". Tell us EVERYTHING about that last ride, any charge after, anything happen to the bike, wet, dry, cold, hot, earthquake, forest fire.

Just so you know, wedging it in a tree 20 feet above ground, pulling a total load over 500 lbs for an extended time, watching it actually catch on fire, seeing huge showers of sparks after shorting two wires, ALL OF THESE, and many, many more, would be things you really need to mention and not leave out.

SOMETHING happened to your bike, and now it does not work.
It was fine on the last ride.
The bike is 5 years old tho, and it did fall over today but not sure that should break the battery?

Pricing a new battery is £160, and given the age of the previous battery do you reckon it's prob time to invest in a new one?
i am guessing there was no problem until it fell over, right? It was only tested AFTER it fell over, right?

NO, it should not break it but it won't do it any good, either, and it DOES NOT WORK right after the fall.

Anybodies opinion on what should, or should not, cause a problem is greatly secondary to sure and certain knowledge of what DID cause the problem.

Check the bike THOROUGHLY for a loose or broken connector. Front to back, strong light, second person to verify, every single pin, take your time, when you get done, if found nothing, pause, take a break, then start at the back and move to the front, slowly, every wire, every conector, every pin.

At 5 years you are about due for a new battery but this one SHOULD have some life and there is likely some problem that the new battery may not fix.

Also possible old battery just died, but the readings are funky.

How hard was the impact, it fell onto what kind of surface, grass or concrete, any bushes or objects in the way, any pokey-type rods or sticks or spears in the way?
ah s**t Nelson, I think you are onto something here...freakin impact is a b**ch....
I'd listen to Nelson right now
Well there was several days between last use and it falling over today so impossible to know if that's what caused it. But it's a distinct possibility.

I checked connections etc and they seem fine, but surely if a voltage reading taken directly from the main battery discharge connection is only showing 4V then the battery is the likely issue? Should it not read 36volts in normal circumstances?
It should read 42V. 4.17 is suspiciously similar to what one, individual series string, out of 10 total, would read. 4.20 would be full, 4.17 a decent time after a full charge, for a 5 year old battery, would be within expectations.

If the impact was severe enough to shake loose a couple spot welds, possible.

That is why I asked if it fell onto (relatively soft) grass, or (not soft at all) concrete. Not getting an answer to that simple question is just ONE reason speculation is more difficult and less precise.

I would bet money you did NOT check the connections as I specified. Most people don't. I usually have to repeatedly insist on this, and very often after a fall, or a crash, this is exactly what they find. Just LOOKING is not good enough.

Double check the output pins. I see five, likely two for charge, two for output, and one for temperature or just unused. Put a new battery in the DVM.

Worked OK, fell over, 4V reading on battery. Do not know how hard the impact was, how much impact the battery case took, do not know, do not know.

Charger went green. That should only happen at 42 V. Check the output ports, AND the input ports. Jostle the battery and listen for loose parts rattling around.
Firstly, many thanks for your responses.
Incredibly helpful

It fell onto concrete, was a heavy enough fall to be honest.

The output pins only get a voltage reading on 2 of them (the ones that show the + and - in the photo). Is that normal?

I did check the connections, I've undone and reinserted every plug.

Is it not also strange that the voltage reading (4V) is the same irrespective if the battery is switched on or not?. Is it possible the switch is wrecked and it's not actually trying to output the full voltage?.
A hard fall onto concrete brings many things into play.

Likely something is broken inside the battery.

On the switch itself, feel it's action for mushiness, or a difference from pre-concrete awareness. May be necessary to open battery and hot-wire switch.

Did you do the shake test?

You can also test the charge port, but this requires some care as pin settings vary.

Also, on the battery itself, I would check the negative against every other port.

Did you individually check every single pin, and every single receptacle, in every plug? They are not necessarily as rigidly held in place as you might think. What it LOOKS LIKE does not matter, that it will not recede into the plug and thereby fail to make GOOD, SOLID, and CONSISTENT contact is what you are looking for. Likely not the problem in this case, BUT IT COULD BE.

It's an old battery, so no great loss if something goes horribly wrong. But, also still usable, so good reason to make an effort on it. May learn something which could be useful if your new, expensive replacement battery also gains concrete awareness.

Also, check your kickstand and parking habits. Ebikes are heavy, kickstands often not designed for weight, and they fall hard, and have expensive parts which do not deal well with concrete awareness.
The rattle test yielded no results

This is a fascinating topic.
I think theres actually a big business for repairing these batteries
I see a few people doing it in North London but no one near me in South London.
They are actually remarkable simplistic from an electrical point of view when opened, so a lot I suspect could be easily repaired if you know how to troubleshoot inside (which I don't)

I appreciate what you're saying about the connectors, and I wouldnt be surprised if one of those has taken a hit as well.
However even when the battery is totally isolated from the bike its not reporting anywhere near the output voltage it should be.
Given the age of the battery, and the fact I can get a replacement for £160 I think thats a prudent step forwards, even if its not the sole issue its something that will need replaced sooner rather than later.
I'm going to keep this old one and keep studying how to troubleshoot the issue with it.
Is there any way I can check the voltage of the full battery in the series circuit when inside the battery?
That would help show if your theory 'If the impact was severe enough to shake loose a couple spot welds' is correct.
Is it just a case of taking the voltage from the top cell to the bottom cell in the series which should read ~42v.

If thats the case then it proves the issue is in the BMS, switch, fuse, lose wiring when trying to subsequently discharge that voltage etc
Inside the battery, the main POS and neg leads are soldered to the BMS.

You can probe those leads directly with a DVM.

Also, you can check the individual balance leads, Neg to #1, neg to #2, etc. Each total must be subtracted from the next reading to give the number for that series string.

#1 gives 4.2, #2 gives 8.3, series two reading is 8.3 - 4.2 = 4.1. Should have two digits after decimal but too tedious to type.

Problem with repairing batteries is that by the time they need repair, they likely also need new cells, and the cost of new cells, plus the labor to rebuild, when you are not paying the labor pennies on the hour, the costs just do not make sense.

Some proprietary batteries actually brick the BMS if you try to replace cells, and the bike will not work without the company BMS.

It is well worth looking for a blown or damaged fuse, or loose wire, etc.
took a video of recording the neg to pin 1,2,3,4,5,6,7,8,9,10

Total from negative to pin 10 is exactly 42V
Now just need to figure out why that 42V isn't being discharged through the port