From 48v to 72v! Input please!

General Discussion about electric bicycles.
Post Reply
Eastwood   10 mW

10 mW
Posts: 32
Joined: Jan 13 2021 2:48pm

From 48v to 72v! Input please!

Post by Eastwood » Jan 21 2021 11:03pm

So I just ordered 72v20ah60aBMS. My current battery is a 48v30ah50aBMS,. So super excited to have the extra power now! Just have to wait a couple weeks :shock:

Can anyone share their experiences coming from 48 to 72??

So my current 48v30ah = 1440 watt hours
The 72v20ah = 1400 watts hours also.
So does that mean I would get similar range since the watt hours are the same? Also I’m thinking probably won’t need full throttle as much so hopefully that will help too. Going to miss the 30 amp hour but the 48V is not enough hahaha


My hub motor is rated for 48v1500w so hopefully it doesn’t get too hot 😅
If it does I’ll be purchasing 72v 2000w hub.
Also probably will buy heat sinks for the hub. My hub motor already gets hot at full throttle if I hold it for to long.

User avatar
amberwolf   100 GW

100 GW
Posts: 31693
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: From 48v to 72v! Input please!

Post by amberwolf » Jan 21 2021 11:50pm

Eastwood wrote:
Jan 21 2021 11:03pm
So I just ordered 72v20ah60aBMS.
Can we assume that this is a whole battery, and not just a BMS for your old battery?
So my current 48v30ah = 1440 watt hours
The 72v20ah = 1400 watts hours also.
So does that mean I would get similar range since the watt hours are the same? Also I’m thinking probably won’t need full throttle as much so hopefully that will help too. Going to miss the 30 amp hour but the 48V is not enough hahaha
You'll get less range, because you're going to go faster ;) which takes more power, and you're going to be harder on the startups and whatnot ;) which also takes more power.

How much more power, and thus how much less range, depends on how much faster you go, and how much harder you are accelerating. It also depends on your controller's capabilities. If the controller can't handle the power, current, etc., it's likely to either overheat and fail, or if it's better designed it'll overheat and begin limiting power to what it can actually handle.


Let's say you use full throttle all the time, and right now you're getting say, 20mph at 48v. At 72v, you could theoretically get 72/48= 1.5, so 1.5*20= 30mph. It's probably going to take more than 1.5 times the *power* to go 30 vs 20, though, because air resistance to speed is not linear. The simulator (linked at the end of this post) will help you see how this works.


The big thing about going lower in Ah is that this generally also means you cant' use as high a current from it without pushing the cells harder than they're meant to be, unless it has significantly better (at least half again as good!) cells than your 30Ah pack does. However, it's very likely that your usage is going to try to draw *more* current from it (assumign your controller can handle this, and that you increase the current limit of it to do so if it was previously hitting that limit).

My hub motor is rated for 48v1500w so hopefully it doesn’t get too hot 😅
If it does I’ll be purchasing 72v 2000w hub.
There's not really a "72v" or "48v" hub, in that there's no voltage limit (well, unless you go into the hundreds of volts, because of insulation limits) to the motors.

The difference is in how fast they spin for each volt applied without a load. (no-load). It's called kV, or Volts/RPM. You'll see terms like 3T vs 5T vs 10T, etc., and the more T the more turns of wire on the stator teeth, and the slower the motor will spin (but the more torque it has, for the same current flow).

If you did want to upgrade the motor to handle higher power levels, you probably want to get one that more closely matches your needs--not just one with a voltage and a wattage labelled on it, but one that specifically matches the speed you would like to reach at the voltage you'll be using it at, *and* that is more than capable of the power levels it is going to take to do all the things you want it to do, *and* that can generate the torque you need at the current levels your battery can support (or replace the battery, too).

Also probably will buy heat sinks for the hub. My hub motor already gets hot at full throttle if I hold it for to long.
You'll need ferrofluid too (also called statorade), or the heatsinks won't really do much. In fact, if you just add the FF, you'll probably find your motor is getting a lot hotter than you think it is (unless you have a temperature sensor inside that you're already monitoring).

I recommend going to https://ebikes.ca/tools/simulator.html and reading the entire page so you know how it works and what it does, then play with different systems under your conditions and change various things to see the results. This may help you understand better how all the stuff interacts, and help you better choose the upgrades to make the kind of system you need to do the job you want. :)

Eastwood   10 mW

10 mW
Posts: 32
Joined: Jan 13 2021 2:48pm

Re: From 48v to 72v! Input please!

Post by Eastwood » Jan 21 2021 11:59pm

amberwolf wrote:
Jan 21 2021 11:50pm
Can we assume that this is a whole battery, and not just a BMS for your old battery?
Yes it’s a new battery with a new bms.
amberwolf wrote:
Jan 21 2021 11:50pm


How much more power, and thus how much less range, depends on how much faster you go, and how much harder you are accelerating. It also depends on your controller's capabilities. If the controller can't handle the power, current, etc., it's likely to either overheat and fail, or if it's better designed it'll overheat and begin limiting power to what it can actually handle.
I have a sabvoton controller 80a Version.
amberwolf wrote:
Jan 21 2021 11:50pm
If you did want to upgrade the motor to handle higher power levels, you probably want to get one that more closely matches your needs--not just one with a voltage and a wattage labelled on it, but one that specifically matches the speed you would like to reach at the voltage you'll be using it at, *and* that is more than capable of the power levels it is going to take to do all the things you want it to do, *and* that can generate the torque you need at the current levels your battery can support (or replace the battery, too).
Yeah I’m pretty happy with the output of my 1500W motor but I would just like more power when needed. Like crossing an intersection or flowing with traffic for short periods of time. My bike as of now hits 39 mph with the 48v. That’s with the sabvoton controller because with the stock controller/kit it was only hitting 31 mph.
Last edited by Eastwood on Jan 22 2021 12:04am, edited 1 time in total.

markz   100 GW

100 GW
Posts: 12222
Joined: Jan 09 2014 11:38pm
Location: Alberta Canada

Re: From 48v to 72v! Input please!

Post by markz » Jan 22 2021 12:17am

Do you have a new controller to go along with your new 72V battery, does your 72V battery have a BMS?

48V to 72V in terms of speed is quite a bit for the same wheel diameter.

User avatar
amberwolf   100 GW

100 GW
Posts: 31693
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: From 48v to 72v! Input please!

Post by amberwolf » Jan 22 2021 12:21am

Eastwood wrote:
Jan 21 2021 11:59pm

I have a sabvoton controller 80a Version.
Is it already set to pull 80A? Or is it limited to not abuse the 50A battery you have now?

If it's limited to 50A to match the battery, then you could change its' settings to match the 60A one you're getting. If you don't (or can't), then this will limit your actual controller output power to about 72v * 50A, or about 3600w, rather than the about 5800w it could supply the motor at 80A.


If it's already set to pull 80A, then I would recommend getting a battery whose cells and BMS are both capable of easily handling more than that 80A. Otherwise, whenever you are using the system hard so it pulls it's full current of 80A (possibly more for short peaks), you're pushing the battery and BMS harder than they are designed to be used at, which depending on design and quality of parts (expect that these are both poor to sort-of-ok for many battery packs), could age the battery faster than it should, reducing it's performance and it's capacity, etc.

Also, a battery not really capable of the current demanded of it will sag in voltage more than one that is designed to handle that current, reducing the total actual power you get out of it at that time.

Eastwood   10 mW

10 mW
Posts: 32
Joined: Jan 13 2021 2:48pm

Re: From 48v to 72v! Input please!

Post by Eastwood » Jan 22 2021 12:38am

amberwolf wrote:
Jan 22 2021 12:21am
Is it already set to pull 80A? Or is it limited to not abuse the 50A battery you have now?
No no I have it set to 40a with my current 48v battery. I would be scared to run 80amps to a 50abms lol

But yeah even with my new battery that’s 60abms I probably won’t go past 50a for the constant current. But the new battery also has a peak discharge of 180 so I can play around with the boost :D
amberwolf wrote:
Jan 22 2021 12:21am

If it's already set to pull 80A, then I would recommend getting a battery whose cells and BMS are both capable of easily handling more than that 80A. Otherwise, whenever you are using the system hard so it pulls it's full current of 80A (possibly more for short peaks), you're pushing the battery and BMS harder than they are designed to be used at, which depending on design and quality of parts (expect that these are both poor to sort-of-ok for many battery packs), could age the battery faster than it should, reducing it's performance and it's capacity, etc.
Yeah I’ll never take it to 80a for the constant current with the battery

Right now I’m hitting 39 mph so I’m not too far away from the top speed I’m looking for. So I don’t plan on pushing this battery to its max.
Last edited by Eastwood on Jan 22 2021 12:46am, edited 1 time in total.

User avatar
amberwolf   100 GW

100 GW
Posts: 31693
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: From 48v to 72v! Input please!

Post by amberwolf » Jan 22 2021 12:46am

Eastwood wrote:
Jan 21 2021 11:59pm

Yeah I’m pretty happy with the output of my 1500W motor but I would just like more power when needed.
Well, right now, depending on what the actual current limit of the controller is set to (if it's set to it's max, or if it's set to not push the battery harder than it is designed for), you're actually getting peaks of 2400w (48v * 50A) to 3800w (48v * 80A).
My bike as of now hits 39 mph with the 48v. That’s with the sabvoton controller because with the stock controller/kit it was only hitting 31 mph.
Which explains why your motor is getting hot, if it's only meant for 1500w or so. ;)

It's around 1000-1400w (battery power, power to ground will be lower due to losses as heat in teh motor and other parts) to hit that original 31mph, depending on conditions and aero and bike, etc. Using a Crystalyte 3540, 48v battery, and 40A controller, the motor ends up at a bit over 90C, so not quite boiling hot, in the below simulation
https://ebikes.ca/tools/simulator.html? ... otor=M3540
39mph takes significantly more, closer to 2000-2500w, (same simulation except 72v battery) and leaves the motor a bit over 180C, so twice the temperature as the slower speed, in the below simulation
https://ebikes.ca/tools/simulator.html? ... otor=M3540

I used the higher voltage battery to reach the speed because the motor I picked wouldn't reach a higher speed without it. Using a faster wind of the same motor, 3548, then I can simulate something like your results with a lower current (25A) controller getting aobut 29mph with the same 48v battery as an 80A controller getting about 36mph (the fastest that motor will go on that voltage).
https://ebikes.ca/tools/simulator.html? ... otor=M3548
https://ebikes.ca/tools/simulator.html? ... otor=M3548
In that simulation, the power usage is less but that's because the speed is less.

You can play around with that for different setups, including using a custom controller for 80A (or whatever your actual current limit is) like i did in the above, and see about what you get out of it.

Eastwood   10 mW

10 mW
Posts: 32
Joined: Jan 13 2021 2:48pm

Re: From 48v to 72v! Input please!

Post by Eastwood » Jan 22 2021 12:53am

amberwolf wrote:
Jan 22 2021 12:46am
Which explains why your motor is getting hot, if it's only meant for 1500w or so. ;)
Yes exactly :lol: :lol:

But in all honesty the hub doesn’t get to hot compared with running the stock controller that came with the kit. Since my set up is new I stop every few miles to check the motor temperature just with my hand and it’s never been too hot but just warm you know. So I think it should hold up well with the new battery.

User avatar
amberwolf   100 GW

100 GW
Posts: 31693
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: From 48v to 72v! Input please!

Post by amberwolf » Jan 22 2021 1:00am

If the stock controller made the motor hotter doing less work than the new contorller does doing more work, then there's two common reasons:

--the hall/phase combo of the old controller/motor hookup was wrong, whcih creates more waste heat becuse the timing of the signals is wrong. Sometimes this can be so bad it actually melts things, but it depends on which wrong combo, and what kind of current/power limits the system has.

--the new controller uses a better control method than the old, like if the old used trapezoidal with poor timing, and the new uses FOC control with sinewave that more closely matches the motor output and uses better timing.

There are other possible reasons, too.

Post Reply