Battery Voltage (42V) & Charging Voltage (42V) Questions.

[Better Peddler]

Hi Folks,

Around a week ago I completed the build of an Electric Bike Conversion for my wife which we are both pleased with the results, her more than me as she is the one who uses it, I am just the 'Engineer'.

The Battery Spec is 42V not sure about the Amp Hours.
After a 4 hour charge using a 42V 2A Li Ion Charger which has a fully charged Green LED fitted (Red while the battery is charging) the battery on a Voltmeter gave a reading of 41.6V.

While the output of the battery charger on the voltmeter read 41.9V.

I am not sure if this is on the low side as I get the impression from other threads that when the battery is fully charged the voltage is considerably higher to start with, but having only 41.9V coming out of this battery charger I think that would be a miracle!

I would also like to get a spare battery charger and wondered if I should get a 43.8V 2A or stick with the 42V 2A.

Any advice would be appreciated please.
Thanks

 

[trex]

your battery is probably standard 36V Lithium. Flat, 33V, fully charged: 42V.
The standard Lithium charger has output voltage 42V maxium 2A without fan, 2.5A to 4A with fan.
Stick with your 42V/2A charger.

 

[Better Peddler]

Hi trex,
Thank you for your reply and clarification.
As the bike has a 42V 500W Motor fitted am I correct in thinking that this will discharge quicker than having a 36V 500W Motor fitted?

 

[trex]

At 42V, the 48V 500W motor draws less Amps than 36V 500W motor. The difference depends on the motor design but generally, you lose about 25% power when you run 48V motor on 36V battery. The power consumption depends mainly on the speed you ride. As you cannot reach the same speed with a lower voltage battery, the consumption is usually less when running at lower voltage.

 

[Better Peddler]

500W/36V = 13.9Amps.
500W/42V = 11.9Amps
Seems to me that running at a lower voltage increases the power consumption from a battery.

 

[Alan Quay]

Both of you are getting confused here.

The motor rating has nothing to do with power, as the current, therefore the power, is governed by the controller.

If you increase the voltage, you increase the power. The current will will remain the same. It is regulated by the controller.

The rating on the motor is simply a maximum that the manufacturer has decided upon.

 

[trex]

I mean by power consumption WH/mile.

500W/36V = 13.9Amps.
500W/42V = 11.9Amps
Seems to me that running at a lower voltage increases the power consumption from a battery.

I don't understand what you are trying to say, but this article explains the motor constants Kv and Km. Enginneers use them to work out how the motor would behave in a bike.
https://en.wikipedia.org/wiki/Motor_constants
Remember a motor needs a controller to work. If the controller can give 14A max for example, your motor will take all 14A.

 

[Better Peddler]

Confused I think not, as I used Ohm's Law which has been around for a very long time to give me the figures above from the formula that was used in our Science classes.

If anyone else wants to have ago then here is a link that will do it for you....
http://www.ohmslawcalculator.com/ohms-law-calculator

I also understand that a 500W Motor is stronger than a 250W Motor so the rating is a bit like an Electric Fire as a 1200W fire is stronger than an 800W Electric Fire so consumes more power but warms you up faster if cold.

When using the 'Throttle' at Level 1 on a 500W 42V Motor will last longer than if you have a 500W 36V Motor.

 

[trex]

motors don't work like kettles, not much is static besides the motor constants. The coils have very low ohmic reistance, about 0.01 Ohm to 0.1 Ohm, you can confirm it with your meter. When the motor spins, the coils generate back EMF voltage which is proportional to the RPM. Without this back EMF, your controller and battery won't last one second before disconnecting.
The EMF voltage opposes the battery voltage, the difference is what is effectively used by the motor. However, the motor again does not work like a kettle here either. The current is used to create a magnetic flux in the coils, the Ohmic resistance of the coils is responsable for generating the heat in the motor. The phase of the motor current being 90 degrees behind the voltage, you need to measure the impedance of the coils to work out the current. The impedance is roughly static but increases near the maximum RPM of the motor.
http://electronics.stackexchange.com/questions/152024/brushless-3-phase-motor-bldc-in-generator-mode-dropping-current

 

[Better Peddler]

Not sure where you saw the word 'Kettle' as I was referring to an Electric Fire.

 

[amigafan2003]

The rating of the motor has nothing to do with it, it's the output of the controller that's relevant.

The power rating of the motor just suggests what power the motor can take without melting and the rated voltage is the voltage required to meet nominal rpm.

For example if a motor is rated at 48v500w @ 200rpm then that means the manufacturer suggests it can take 500w continuous without melting and it needs 48v to do 200rpm. If you fed it 36v then it would spin at 150rpm.

The actual power output by the motor is a function of the controller rating - i.e. if you have a 36v controller set to output 15a to the motor, then no matter what motor you connect to it, the motor will get 540watts (+ or - a few watts to allow for voltage drop of the batter discharge profile).

It's analogous to an engine in a car (motor) and a throttle body (controller). The engine is only capable of making the power the throttle body allows. Restrict the throttle body size, power goes down. Increase throttle body size, power goes up.

Confused I think not, as I used Ohm's Law which has been around for a very long time to give me the figures above from the formula that was used in our Science classes.

If anyone else wants to have ago then here is a link that will do it for you....
http://www.ohmslawcalculator.com/ohms-law-calculator

I was going to help you more but with that tone of condescension then I'll leave you to figure out for yourself.

 

[JohnCade]

D8veh hasn’t been around lately. He’s usually very patient.

 

[Alan Quay]

Confused I think not, as I used Ohm's Law which has been around for a very long time to give me the figures above from the formula that was used in our Science classes.

If anyone else wants to have ago then here is a link that will do it for you....
http://www.ohmslawcalculator.com/ohms-law-calculator

I also understand that a 500W Motor is stronger than a 250W Motor so the rating is a bit like an Electric Fire as a 1200W fire is stronger than an 800W Electric Fire so consumes more power but warms you up faster if cold.

When using the 'Throttle' at Level 1 on a 500W 42V Motor will last longer than if you have a 500W 36V Motor.

Yes, I am aware of ohms law, thanks!

But it does not apply in the way you are using it, because the load (motor) is not allowed to consume as much power (watts) as it likes, because the controller is limiting how much current (amps) it can have.

If you change to motor to a 10kw one, it will still only run at the max current that controller will let it have, normally around 15a or so.

These Brushless DC motors are very complex. In reality, they are 3 phase AC devices. They cannot be run without a microprocessor to control the current. It's not like an old DC motor.