Ebike battery alternative

[Wolvyr]

I'd like to start by saying I'm a newbie here, I've been lurking for a while but it's my first post. So with that said, if I'm posting in the wrong section, please don't hesitate to point me in the right direction. Thanks
Now on to topic.

So I recently got my hands on my first electric bicycle. It's a second hand bike, and is in pretty bad shape. The motor is of unknown wattage, though the previous owner assured me it is 'very fast', whatever that means. The battery pack is made up of 3 lead acid 12v battery packs with 12ah each. They are extremely heavy.

So I got this idea of collecting 7 5000-10000 mah power banks for smartphones and connecting them together with a dock with 7 male usb connectors. These 7 connectors on the dock will splice together to make a single wire that becomes a 3 pin connector compatible with my motor.

By using 7 of these, it produces a total voltage 35v which should work with my 36v motor (unless I'm mistaken).
However, I'm not too sure on how the current will work out for this. Most of these power banks have two female usb sockets, one with 2A and one with 1A.

So my question is, could this work? Is there a major flaw in my plan? Have I been blindingly ignorant to a fundamental law of nature?

Please educate me, pedelec community.
Thanks
Edit: a few misspellings

 

[Alan Quay]

Not going to work I'm afraid. Even if you series up a bunch of 2a USB ports to get 35v, you will still only have 2a to play with.

You will need around 15a for a half decent battery.

 

[Wolvyr]

I see. Thank you for your reply
Edit:
Unrelated, but does anyone have any suggestions as to how I find out the wattage and current drawn from my hub motor, since the previous owner could not give me this information?

 

[Alan Quay]

I see. Thank you for your reply
Edit:
Unrelated, but does anyone have any suggestions as to how I find out the wattage and current drawn from my hub motor, since the previous owner could not give me this information?

Assuming a business motor (3 wires indicates it is):
It's the controller that will determine how much current it gets, the battery will determine how many volts.

The motor will have a rating that determines how much power (V x A) it can handle.

Post some pictures and we can make an educated guess.

 

[Wolvyr]

I believe the controller is located below the battery compartment. The hub motor isn't shockingly large at all. I would guess 250-300w but I'm less than a novice in this area.
The bike has stickers saying 'Sakura' on it, but I haven't found anything like this bicycle on the Internet under this name. Is there any other information I should post?

 

[Alan Quay]

The controller will probably have more information on it. If nothing else the no. Of FETs will give us a clue.

 

[Wolvyr]

Okay. I plan to take an inside look on the weekend. I will post anything I find.
If I purchase commercial a 36v 10ah battery, will it be safe to run on my bike without any modifications, or should I not do this?

 

[D8ve]

You should get better range and performance. Just make sure it has a battery protection system to stop you draining the battery to much.

 

[Wolvyr]

My bike currently has a 3 pin connector on it. Will it be safe to chop off the connector from the commercial battery, splice on a compatible 3 pin connector and use that?

 

[Alan Quay]

My bike currently has a 3 pin connector on it. Will it be safe to chop off the connector from the commercial battery, splice on a compatible 3 pin connector and use that?

A battery has two connections. You need to understand what the extra connection is and how its connected.

Any 36v battery will do the job, as long as it can supply enough current.

There are normally 3 large wires from the controller to the motor, and sometimes a number of smaller sensor wires.

 

[Wolvyr]

After looking inside, the controller had a faint marking on it saying 36v 16A. So I can conclude the theoretical power output is 576w.

I put the bike together and took it for a test run, and everything was fine for the first 1-2miles on full throttle. Though when I put the motor under more strain, going up a semi steep hill, the motor began cutting out on me. The power would judder on it and would only maintain any consistent spin when keeping the bike at 5-6mph, when it had been reaching 16-17 just 5 minutes before. When I lift the back wheel off the ground and pull the throttle all the way, the motor spins fine, no juddering.
Any ideas on why this happened?
I took the bike home and put it on charge, but the charger showed the 'full charge' green light when I did so.
Are the batteries dead? Is the controller busted? Did I kill the motor?
I am very puzzled.
Thanks

 

[flecc]

After looking inside, the controller had a faint marking on it saying 36v 16A. So I can conclude the theoretical power output is 576w.

I put the bike together and took it for a test run, and everything was fine for the first 1-2miles on full throttle. Though when I put the motor under more strain, going up a semi steep hill, the motor began cutting out on me. The power would judder on it and would only maintain any consistent spin when keeping the bike at 5-6mph, when it had been reaching 16-17 just 5 minutes before. When I lift the back wheel off the ground and pull the throttle all the way, the motor spins fine, no juddering.
Any ideas on why this happened?
I took the bike home and put it on charge, but the charger showed the 'full charge' green light when I did so.
Are the batteries dead? Is the controller busted? Did I kill the motor?
I am very puzzled.
Thanks

The 576 watts is consumption not power, and no motor is 100% efficient so the net maximum power is probably nearer 400 watts and that only available within part of the revs. range.

Sakura's two wire brush motors were always rated as 200 watt nominal to comply with the law at the time of supply, so it's very likely that yours is also rated at 200 watts. 250 watts only became legal on 6th April 2015 and Sakura bikes haven't been sold here since then, having disappeared from the market long before.

The cutout symptom you report is very much like that of dud batteries, unable to continue supply under high load when consumption is the maximum. Uphill it will only climb at well below top speed, typically around 6 to 8 mph, depending on hill steepness, unless you put in considerable help though the pedals.

Once back on the flat it should run at up to about 15 mph ok.
.

 

[Wolvyr]

Once back on the flat it should run at up to about 15 mph ok.
.

I wish that were the case. Even now in my garage, the motor judders and will now put out power like it did before I put it up that Hill.
Could the juddering be a symptom of a broken fuse? I used a 20A mini blade fuse in the battery, if that makes any difference.

 

[Alan Quay]

No, fuses either work or fail completely.

As Flecc says, most likely a battery problem.

 

[flecc]

I wish that were the case. Even now in my garage, the motor judders and will now put out power like it did before I put it up that Hill.
Could the juddering be a symptom of a broken fuse? I used a 20A mini blade fuse in the battery, if that makes any difference.

No, definitely not a broken fuse. Carefully check the three wire connections at all points from motor to controller, since a faulty connection in any of those can cause exactly that juddering.
.

 

[Wolvyr]

No, definitely not a broken fuse. Carefully check the three wire connections at all points from motor to controller, since a faulty connection in any of those can cause exactly that juddering.
.

I'm fairly certain the juddering isn't due to the circuit being disconnected then reconnected by a faulty connection. This is because when the motor isn't burdened by a heavy load, the motor spins fine. The juddering must be due to the motor being too weak to carry the load.
So the question is, why was the motor strong enough at first, but then became weaker after some distance?
I guess it's possible to be faulty batteries but a volt meter shows that they all hold voltage quite well. Could it still be a faulty connection?

 

[flecc]

A faulty end of life battery wil not cause juddering, only the cutting out under high load.

Juddering is usually due to erratic phase firing which can happen with a faulty connection or possibly an internal fault in one of the motor phase windings.

Can you check for continuity around the three motor wires. First disconnect them from the controller and check that there is continuity with negligible resistance between any pair of wires. If you find a pair open circuit you've found the problem.
.

 

[Alan Quay]

SLA's don't have a BMS, so a low voltage condition will be detected by the controllers LVC detection. At this point the controller will switch off, the current will drop, the voltage will recover and the controller will re-start.

It could be other things, as Flecc has described, but I've seen this behavior with SLA's

 

[Wolvyr]

A faulty end of life battery wil not cause juddering, only the cutting out under high load.

Juddering is usually due to erratic phase firing which can happen with a faulty connection or possibly an internal fault in one of the motor phase windings.

Can you check for continuity around the three motor wires. First disconnect them from the controller and check that there is continuity with negligible resistance between any pair of wires. If you find a pair open circuit you've found the problem.
.

I struggle to see that a faulty connection could be the cause here due to the fact the wheel spins without judders when the heavy load is removed by lifting the wheel. I do no believe there is erratic phase firing because of this fact, as the wheel still spins at a constant speed when under less stress.
However, I will still investigate this possibility.
When performing repairs on the bike, I noticed the wires to the motor were exposed and had been causing many many broken fuses for its previous owner. I attempted to open the motor casing and replace the wires altogether, though failed to take apart the casing. I simply could not pry the face off after removing the screws. Any suggestions?
So Instead, I ran heatshrink along each wire separately as far down as I could and then heat shrinked the two wires into one. I suppose it is more than possible there is still exposed wiring further down into the motor that is causing a problem. Even so, wouldn't that just cause the fuse to break, breaking the whole circuit and stopping all power to the motor altogether?
Edit: bad autocorrects
Another edit:
I have been foolish. Before I said that the juddering did not occur when heavy loads were removed. I left the batteries to charge a little despite the charger showing the green light, in case they needed more power. This had no effect. However, when testing whether it did, I found that if I lift the back wheel and apply the throttle straight to max, the juddering >does< still happen, despite the load being removed. However, if I give it a fast, but still gradual and paced twist to max, the juddering does not occur when the wheel is lifted. I am sorry for having any doubt, Mr flecc.

 

[Wolvyr]

SLA's don't have a BMS, so a low voltage condition will be detected by the controllers LVC detection. At this point the controller will switch off, the current will drop, the voltage will recover and the controller will re-start.

It could be other things, as Flecc has described, but I've seen this behavior with SLA's

I apologise for asking, but could you maybe explain what an SLA and LVC is?
Edit: I see SLA is sealed lead acid, still researching for LVC
And LVC would be low voltage cutoff. So how exactly would one repair the problem with the LVC?