How a 72v@20ah battery would run differently than 60v@24ah.

[mef1975]

Questions regarding how a 72v @ 20ah battery would run differently than 60v @ 24ah (both 1,440 watts), for an electric motor...



The motor is 24-72v, 2,000 watt max, so I suspect one wouldn't want to go over 27 amps, correct?

Probably would be safer for the motor to keep it even cooler, like 1,440 watts?

Would both battery systems (72v @ 20ah and 60v @ 24ah) have the same top speed, or would the 72v be faster?

Would the 72v make the motor run hotter?

Would the 24ah battery run hotter?

Would the controller run any hotter with either battery system because of the v/a difference?

How would 72v @ 15ah be different?

 

[mef1975]

What are the pros and cons?

Which is safest?

 

[bionx 1954]

The battery is the amount of power that is available to the controller/motor. The controller is what send the motor the power (watts). If you have a controller rated at 72V@20 A then your max to motor is about 1440 W. If your controller is 72V@40A then the power to the motor is 2880W. This also depends on your battery C rating , voltage sag under WOT. Running at 72V will give you a higher top speed .

 

[arkmundi]

Its the amps running through the motor that will deliver higher performance. Also heat will be generated to degree of any component inefficiency, wires as resistance. Battery voltage limits max amperage, so 72V will deliver a higher potential maximum. Your controller, however, is designed to control amperage - that's its function, and again, to a maximum. Also the C-rate of the battery can limit amperage and its better to higher C-rate. For a 2000 watt motor, I'd go with a 40 amp controller and a 72V battery. Best!

 

[amberwolf]

Should I build a battery with 12ah or 3.3ah cells?

What are the pros and cons?

Which is safest?

I moved this separate thread into your first thread because presumably it is about the same battery pack build, and it'll be a lot easier for people to help you if you keep all the questions about it in one place. If it's a separate battery build, you'll have to post a whole lot more info to get any useful answers.

Without knowing *which* cells, and to what purpose you will use them, and what loading they will see, there is no way to answer either question.

Regarding both the original question and this one, I think you should do some reading around the wiki and the forum about batteries in general, because I think many of your questions would be answered even before asking. If you stll had questions after that, at least you might know better what info we will ahve to have from you to answer them.

 

[wesnewell]

What makes you think it's a 2000W max motor. Most motors are rated for a max continuous usage watt rating. IOW's my 1000W rated motor will run perfectly happy at up to 6000W for shorter time period although my 100V battery pack and 40A controller limits it to 4000W. Without knowing the motor you actually, I'd only be guessing at how many watts you could feed it without burning it up.
72V would be faster than 60V.
Voltage doesn't make a motor run hotter, watts do.
A larger ah battery would be less stressed than a smaller ah one of the same chemistry.
The more wattage you try and run through a controller the hotter it will get. VA=w.
"How would 72v @ 15ah be different?"
Different than what?

 

[dogman dan]

To start with, learn the difference between an amp, and an amp hour. Having more amp hours in your battery is good for the battery, but will not affect the wattage the motor gets. Volts times amp hours is watt hours. A watt hour is one watt, delivered for one hour.

But if controller amps remain the same, then increasing volts does increase watts. Because volts times amps is watts.

Also, your motor will have a faster maximum rpm with no load, if volts is increased.

As for the 2000w maximum, that may be the max for your motor to run efficiently. If it's a typical 28 mm magnet hubmotor, then they do heat up fairly fast if you run 3000w. At 4000w, they heat up very very fast. That is, if you run the motor at high wattage (fastest speed) while cruising. If you cruise 30 mph, it's only going to be about 1000w. So you can have a 4000w controller, and the motor will be fine if you generally only give it 1000-1500w. Typically, you will only see maximum wattage for a very short time as you start up. Or, if a hill is very steep, you may see some large wattage, but not the maximum.

So what I'm trying to say is, 3000w will be fine if you aren't climbing hills non stop, or cruising at 40 mph. Keep the high wattage draw time short, and cruise at 30-35 mph, and your motor should be fine. If you plan on punishing rides on steep single track trails and jeep roads, then limiting watts to 2000 makes more sense.

Motors with wider magnets and more copper can take a lot more than 2000w.

 

[r3volved]

Having the same wh per pack, does that equate to roughly the same available distance despite the different configurations?

...or I guess it would depend on your controller. Obviously pulling 20a @ 72v will drain faster than 20a @ 60v. But if you're pulling roughly the same wattage at both battery configurations, does that equate to similar distance on the same motor?

Reason would indicate to me that it would be similar, but I don't have the experience to know if voltage differences would affect other variables (such as heat, performance or efficiency) dramatically enough to make much difference.

 

[dogman dan]

It will be very similar in range, as long as the throttle position insures the same speed.

So postulate despite 40mph capability, he cruises 30 mph. Both configurations same watt hours, so both would go about the same distance at 30 mph.

But you might ride different with the higher voltage setup, resulting in different range. You might pull more watts up hills, or leaving stop signs. But you don't have to. If you need range, it's quite possible to ride just as efficient as a 300w bike. Just don't roll back the throttle past 300w.

 

[r3volved]

Awesome
Thanks for that. Clears up a bunch and opens up my options a bit more.

 

[arkmundi]

Obviously pulling 20a @ 72v will drain faster than 20a @ 60v. But if you're pulling roughly the same wattage at both battery configurations, does that equate to similar distance on the same motor?

Not so. The measure of capacity is amp-hours, hence the capacity of the two packs is identical. Voltage is a measure of POTENTIAL, also maximum draw. Delivering a constant 20a to the motor means draining the batteries at the same rate. And yes at the same rate, the same distance, because the amp-hours are the same. Similarly, if say, you deliver 40a from a 72v battery, hence theoretically twice the speed, you've gone twice as fast, but the same distance and consumed the same energy. Theoretically, because that does not account for inefficiencies. At the higher rate, more energy lost as heat, higher wind & rolling resistance, etc.

 

[dogman dan]

I can't agree with all of that.

Yes, if he's going faster, watts will be higher when he goes faster. If he's riding WOT, then 72v will be faster than 60v, and drain the battery quicker.

Only if watts is equal, for example wot at 60v, and less throttle with 72v ( or set controller for less max amps), will speed be close to the same as well as wattage. Then range will be the same.

There is an easy way to think about this. Just think that 20 mph will take the same watts at either voltage, 30 mph the same watts at either voltage, etc. In general, the same wattage is going to yield the same maximum speeds, provided both voltages can go that speed. 200w is about 15 mph, 400 about 20 , 1000w about 30mph.

It won't be perfectly equal of course, for complicated reasons. But it will be close enough to equal in most riding conditions to not worry about it. When I want to hit a certain wh/mi, to make a certain trip distance, I just dial down the amps of my controller with a CA, or if no CA, just use less throttle to maintain a steady speed that will yield the distance I need.

So what I'm saying is, whether I am on a 72v bike or a 48v bike, 15 mph is still my target speed to make a very long ride. My wh/mi will still be about the same. Because at 15 mph, my wattage will only be 200 or so. The higher voltage can still be efficient at low speed.