Eastwood said:
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.