Hi, been having trouble with my wisper 705 bike, got a new rear tyre fitted in late 2024 and every time I've went to go out and use it since then, the power just randomly cuts out, sometimes it cuts out after a short distance other times I can go half a mile before it cuts out, I thought I'd ask here to see if anyone else has had this problem before I go spending money to get it fixed, would really appreciate some help with this

There's probably a connector to the motor which you disconnected when you took the wheel off. Then when you reconnected you didn't push it fully home. There are probably pointers on both sides of the connectors, make sure they are fully pushed in right up the line. It can be quite stiff.

Thanks, I'll try that when I get home, it's annoying me that it's just sitting there not getting used haha

Thanks, I'll try that when I get home, it's annoying me that it's just sitting there not getting used haha

If it started after the tyre change, it's almost certainly a motor cable connection issue. If the above doesn't fix it, you need to look the other end of the motor cable where it's connected to the controller. After that, there are a few other possibilities, so in the meantime, you need to tell us how old it is or which version. Photos would help.

I have a Wisper 705 step through which is doing the same. It's been to two bike repairers. One tightened things in the controller but it's still cutting out when I increase the power levell or put the turbo assist on. If I turn it off and on it will work on level 1 assist but any increase and it cuts out. I fully charged it and did 5miles yesterday... Quite flat so not much assist required, but it did cut out when I put the turbo on to cross a road.

When home I put it on charge and the light went green afer only 25minutes so not sure it's the battery. Incidentally... The displays always show that its full when it's cut out

2 hours ago, Elaine T said:

I have a Wisper 705 step through which is doing the same. It's been to two bike repairers. One tightened things in the controller but it's still cutting out when I increase the power levell or put the turbo assist on. If I turn it off and on it will work on level 1 assist but any increase and it cuts out. I fully charged it and did 5miles yesterday... Quite flat so not much assist required, but it did cut out when I put the turbo on to cross a road.

When home I put it on charge and the light went green afer only 25minutes so not sure it's the battery. Incidentally... The displays always show that its full when it's cut out

That sounds like a battery where the cell groups have got badly out of balance.

I had that sort of thing once when I had left a bike uncharged over the winter. It had been at about 37 volts when I stopped riding it in the autumn and I checked overall battery voltage about once every month while it was in store, but unbeknown to me, one group of cells was quietly losing voltage and this was not really obvious from the overall pack voltage because there were ten cell groups in series. When I put the battery on charge around April, ready to take it out, the pack quite quickly stopped charging and the red light went out, so I jumped on the bike and set off. Within about three or four miles the pack shut off.

When charging, the BMS cuts off and stops accepting charge current once any one of the cell groups reaches 4.2 volts. If one or more of the groups was very low, it might only have got to 3.4 volts when the charging stopped. Then you set off on a ride thinking you have a charged battery and a few miles later that low group comes down to the low voltage cut off point and the bms shuts the battery down. That seems to fit your symptoms. It might be that.

Edited Wednesday at 08:373 days by Tony1951

One indicator for the electrical novice of whether the battery is in balance or not, is to measure the overall pack voltage when the charger cuts off. If it is a 36 volt battery, you should see a voltage as near to 42 volts or 41.9 volts as possible. If it is down around 41.1, you have a group of cells, or maybe more than one group, well below the ideal fully charged voltage.

If you measured the individual cell groups you might find a situation like this:

|4.19 | 4.0 | 4.1.9 | 4.18 | 4.19 | 4.1.8 | 4.19 | 3.51 | 4.19 | 4.18 |

A battery in that state would be hopelessly out of balance, but would still show an overall voltage off the charger of 41 volts. Add them all up and divide by the number of groups - 10 in this case.

You can see there that the second and the eighth cell groups are low, especially the eighth one. That would soon make the battery shut off when you started riding it, because that 3.51 volt cell group would rapidly decline to the point where the BMS shut the battery down, even though most of the cells have plenty of charge in them. The BMS does that to protect the cells that are low. If you take them too low they will be ruined or even dangerous afterwards.

You can only establish this as the cause of problems on your battery, after you have done all the previously suggested 'Is everything fully plugged in?' tests, and then looking at the overall battery voltage when the charger cuts off. That might show on your screen, or you might need to measure it at the output port on the battery with a multi meter and the battery switched on.

Looking at individual cell group voltages necessitates opening the battery case and examining very carefully. This is not for everyone. You need to be careful if attempting that.

Edited Wednesday at 09:043 days by Tony1951

Thanks for your reply... I will show it to the bike shop tomorrow when I take my bike there as they said they could check the battery for me. 🤞

Knackered battery syndrome or a battery badly out of balance, simply put when one applies more current ones gets voltage sag or for a balance issue one has a bad cell group with a low voltage reading.

The important factor omitted is the the battery age ????

To check the battery balance the LBS will have to physically open the case and measure the 10 cell groups to see if one is out of balance.

10 hours ago, Elaine T said:

I have a Wisper 705 step through which is doing the same. It's been to two bike repairers. One tightened things in the controller but it's still cutting out when I increase the power levell or put the turbo assist on. If I turn it off and on it will work on level 1 assist but any increase and it cuts out. I fully charged it and did 5miles yesterday... Quite flat so not much assist required, but it did cut out when I put the turbo on to cross a road.

When home I put it on charge and the light went green afer only 25minutes so not sure it's the battery. Incidentally... The displays always show that its full when it's cut out

Unfortunately, many dealers and bike shops don't have a clue how to diagnose and fix ebike problems. There are many different things that could cause the problem you have. To be sure, someone needs do do some electrical measurement. If you're not able to do that, tell us approximately where you live so that we might be able to recommend a repairer. If you can get it to Telford, I'll do the diagnosis for nothing and repair it for whatever any bits cost that are required to fix it.

Other than that, one thing that you can try that doesn't cost anything is to leave it on charge for several hours after the green light has come on the charger. If that improves it a bit, do it again and again until your battery gives the full capacity again. It's best to have the battery outside if you do that. The risk is almost zero, but best to be safe. If there is no improvement, either your battery is worn out or you have a connection issue. perhaps you could tell us how much the battery has been used since it was first purchased.

Thank you for your reply. My bike is about 5yrars old.... Has done 800 miles.

14 hours ago, Nealh said:

Knackered battery syndrome or a battery badly out of balance, simply put when one applies more current ones gets voltage sag or for a balance issue one has a bad cell group with a low voltage reading.

The important factor omitted is the the battery age ????

Bikes 5 years old done 800 miles.

13 hours ago, D8veh said:

Unfortunately, many dealers and bike shops don't have a clue how to diagnose and fix ebike problems. There are many different things that could cause the problem you have. To be sure, someone needs do do some electrical measurement. If you're not able to do that, tell us approximately where you live so that we might be able to recommend a repairer. If you can get it to Telford, I'll do the diagnosis for nothing and repair it for whatever any bits cost that are required to fix it.

Other than that, one thing that you can try that doesn't cost anything is to leave it on charge for several hours after the green light has come on the charger. If that improves it a bit, do it again and again until your battery gives the full capacity again. It's best to have the battery outside if you do that. The risk is almost zero, but best to be safe. If there is no improvement, either your battery is worn out or you have a connection issue. perhaps you could tell us how much the battery has been used since it was first purchased.

I live near Leicester. If you could recommend a shop that really does know what they're doing.. I woukd be grateful.

Sadly very few if any LBS will entertain opening a battery , as they will not have the knowledge let alone the time and cost implications.

At 5 years old, one suspects it needs replacing unless a nearby amateur expert can take a look.

Edited Thursday at 10:152 days by Nealh

6 hours ago, Elaine T said:

Bikes 5 years old done 800 miles.

When you don't use a battery for a very long time, it will go out of balance. With only 800 miles, it's probably not worn out, so could probably be revived by re-balancing. You have to open it up to check the state of balance, and you can rebalance it quickly with the right equipment when it's open like that. The longer way to rebalance that works with some batteries and chargers is to leave it on charge for a long time after you get the green light, like I mentioned above. You have to do that several times for several hours each time to gradually bring it back to a balanced state. Did you try that yet? If not, do it and let us know if there's any improvement.

If that doesn't work, can you show us a picture of the battery removed from the bike so that we can see which version it is. If you're lucky, you can get one quite cheap (£169) from Yose Power. I fixed a Wisper with one, but I'm not sure which model ebike it was.