Upgrading advice, 1000W 48V China kit

[utsi]

Hi there, ebike experts!

I've built a couple of ebikes now using cheap China-kits, one 1000W rear hub and one 500W front hub for winter usage. What I want to ask about is my primary build:

• 
  - 1000W rear hub motor, 48v brushless.
  - 1500W controller with 3-speed switch
  - 48v 20Ah li-ion battery https://www.aliexpress.com/item/Gre...32401868802.html?spm=a2g0s.9042311.0.0.RQyzVl
  - Mountain bike with 135mm rear dropout

These components are working wonderfully together and I am able to get ~ 37mph. But ONLY when the battery is fully charged, and only for the first few minutes. After that the speed drops to 31mph and riding becomes less fun.

Now I'm wondering if the Chinese seller was bragging when he wrote that the battery was suitable for up to a 1500W-2000W motor

What I'm considering:

1. Buying a more powerful 48V li-ion battery, would that give me the juice I need?
2. Upgrading the controller to 60V and get a 60V li-ion battery. I'm already pushing my motor, would this fry it?
3. Upgrading everything to 72V components. I've looked at MXUS 3k-motors, would a 10lbs battery be able to power it or would I need something big and heavy? Also, would a regular mountain bike suffice? I live on the 3rd floor so it needs to be portable-ish.

Note: I live in Norway so overheating has never been a problem.

Looking forward to hearing your advice

 

[amberwolf]

If it already does the speed and everything else you're after, get a better (and bigger) battery that has less voltage sag (has a higher C-rate and/or more capacity), and has more capacity to stay in the speed range you want for longer.

Keep in mind the faster you go the more power it takes to overcome air resistance, so you can also do some minimal things to decrease drag while riding, and you'll use less power to maintain the same speed, so the battery would last longer. And/or you could go faster if the drag had been slowing you down before, though it'll use power faster and you'll have less range.

You may also be able to use a higher voltage battery, one they call a "52v", which has a full chargeed voltage of around 58v (while the "48v" batteries are full at around 54v). Most controllers have capacitors and FETs that can handle that. If you're considering replacing the controller anyway, there's no harm in trying it out--if it dies, you haven't really lost anything.

So the higher voltage battery will have a slightly higher top speed and will stay a little faster than the other one for longer.

Exactly how much faster depends on how much voltage sag the battery has under load, and that depends on the quality of cells in it, and the kind of cells in it, and how they're all connected, and how much capacity teh pack has.


If you have other things that need improving, too, it may be worth changing out mroe than the battery.


As a note, teh MXUS 3k is heavy; a bit more than 20lbs bare motor, unless you need the torque and/or power to overcome hills or something like that, it's not worth the extra weight and other things you'll hae to do to use it. For one, you'll need to drill new smaller spoke holes in it to use non-motorcycle spokes, or you'll have to use spoke head washers. Bicycle spokes will just go right thru the holes it already has. :/ (and if you use MC spokes you'd want to use MC rims so you don't break the rim tensioning the spokes, and then the wheel is even heavier).

I use them on my already-heavy trike because I use the startup torque to get going quickly even with a really heavy (hundreds of pounds) load, but it's a lot of weight for a bicycle.

 

[utsi]

Thank you for your response, amberwolf!

I see some sellers are offering pre-built MXUS 3k wheels but considering the extra weight when also adding a much bigger battery I think you're right, the thing will be a monster and too heavy to carry 3 floors every day.

About your "52v" option, wouldn't it be better performance wise to go for a 60v controller/battery combo since the battery is the expensive part anyway? After searching the forum I've seen threads about people overvolting 48v motors to 72v, so 60v is maybe a breeze?

If the answer is "yes", do you have any recommendations on what kind of battery to get?

 

[dogman dan]

yes, the battery seller was lying, sort of. It does what he says, but it does not, and never will like it.

So some of your problem likely is sag under load, causing a lower voltage than you could have, mid ride.

However, the only real route to speed will be a higher voltage,( 72v 20s, or at least 18s that charges to 16.8v more than 14s. ) and much higher c rate battery. Those who want big speed, and low sag on a budget tend to go RC lipo.

Charge and store that shit outside, and IMO, same for what you have now.

Upgrade the motor too later, if you need it. But your existing motor will do 10 miles at 35-40 mph without melting down. Just about the distance you will get from 72v 10 ah which is 720 watt hours. Your battery now is 960.

Get a controller that can run 48v or 72v of course, so you can still go for long long rides ( at 30 mph) by carrying both batteries.

 

[utsi]

Thanks Dan, your answers are always helpful!

Safety > price, I have a kid in my flat so I need to be able to store/charge my batteries inside. Do you think my li-ions should stay outside?

Variable controller, that's a good idea! Maybe I should look for one that is rated for 3kW.

The Chinese sellers typically do not write anything about "s" or "c", can you give me a hint on what to look for when browsing batteries on aliexpress or eBay? "Maximum continous discharge" is what I've been looking at so far.

 

[utsi]

How about this controller:

https://www.ebay.com/itm/36v-48v-60v-72v-40A-BT-PROGRAMMABLE-E-BIKE-BRUSHLESS-ELECTRIC-MOTOR-CONTROLLER/122976931745?hash=item1ca1ff0fa1:g:vBMAAOSwfdBal9eR

With a powerful 72v battery?

Tuning your controller from your phone sounds pretty darn sweet, if the performance is good and the app is user friendly of course..

 

[amberwolf]

"Maximum continous discharge" is what I've been looking at so far.

I'd cut whatever they say in half.

 

[markz]

You should just buy a rock solid Leaf Motor kit.
I'd suggest the 1500W in whatever size wheel with the KT display.
https://www.leafbike.com/

Then sort out your battery.

Or just buy their motor and ebay seller: hksunwin for the controller.
With the Leaf Motor, there is no guessing what the "chinese man" states because many people here on ES have bought the Leaf motor and love it.

 

[utsi]

Thanks for your suggestion, markz!
The Leaf Motor kit looks nice, but I've decided to try to overvolt my current motor before I go for a full upgrade.

Sadly, my controller is not pluggable, everything except the motor wires is soldered and shrink-tubed in place. I've looked for connectors to make a hot replacement setup but so far I've given up.

My current controller is just 41mm thick so it fits perfectly under my rear rack. When looking for 48v/72v variable controllers I have found them to be too beefy, sometimes 60-70mm thick, to fit my current configuration. I'm hoping that this controller can do what the seller says. The small size makes me a little suspicious, thought.

As for the battery, like amberwolf says, I will look for something that gives me twice of what I need on aliexpress, at least 50A continous discharge rating and 72v. I realize that it is a gamble because although all the batteries look the same, weigh the same and probably smell the same, there are huge variations in price and specs from the sellers. Ideally I hope that someone here has made a successful purchase and can recommend a seller/brand. :wink: