Hello,
I am trying to replace the battery in a battery operated motor with a direct AC source. So far i think i need to use an AC to DC power supply, but im not sure about the current/wattage specs. The battery specs are (Li-Po 37 V - 2.5 Ah - 92.5 Wh battery) and when i look at the manual for the product it lists the battery charger at (100 VAC - 240 VAC / 50 - 60 Hz / 42 VDC - 2A battery charger). The part that confuses me is the average motor power is 189 watts. So what should my system look like if i wanted to replace the battery with AC power, and what should be the specs for the power supply? I can answer some questions if needed.

 

Unfortunately, neither the battery or the charger specs are going to tell you much about what regulated power supply you need. The application, and its loading, will determine how much power you need. If it is 189W, then I would go with 250W or even possibly 300 just to be sure!

The problem is that the battery rating is supremely subjective. It kind of states how long it will last (in hours) if you use the rated current. For example, if the loading current is 2.5A, then it should be done in about an hour. If it is 1.25A, then it should last two hours. The reason why this logic is heavily flawed is because in all reality, a battery voltage will be decreasing as the internal energy is being utilized. So even if your current is one of these (2.5A or 1.25A), you will not see that voltage for the specified amount of time (1 hour or 2 hours respectively). To make it even more complex, hardly any application will always use the same current. Maybe it uses 1A now and 5A later. It depends on the application and what kind of loading is being employed.

The charger information is also kind of useless for this purpose as it only tells you what voltage and what kind of current it will use to charge the battery. Charging the battery is a completely different beast than running a motor, so this information can't be mixed.

Also remember that the power supply voltage will determine motor speed and the current how much torque it will be able to deliver. So you may not need a 37V power supply exclusively. Maybe you can do with 24V or 36V (depends on whether the speed you can get with these voltages is enough), or maybe you can use a 48V or even 60V unit.

I am running a very similar project where the battery is 56V but will go with a 60V power supply. I will try to get as much current as possible, since I already know these motors are quite hungry.

 

Unfortunately, neither the battery or the charger specs are going to tell you much about what regulated power supply you need. The application, and its loading, will determine how much power you need. If it is 189W, then I would go with 250W or even possibly 300 just to be sure!

The problem is that the battery rating is supremely subjective. It kind of states how long it will last (in hours) if you use the rated current. For example, if the loading current is 2.5A, then it should be done in about an hour. If it is 1.25A, then it should last two hours. The reason why this logic is heavily flawed is because in all reality, a battery voltage will be decreasing as the internal energy is being utilized. So even if your current is one of these (2.5A or 1.25A), you will not see that voltage for the specified amount of time (1 hour or 2 hours respectively). To make it even more complex, hardly any application will always use the same current. Maybe it uses 1A now and 5A later. It depends on the application and what kind of loading is being employed.

The charger information is also kind of useless for this purpose as it only tells you what voltage and what kind of current it will use to charge the battery. Charging the battery is a completely different beast than running a motor, so this information can't be mixed.

Also remember that the power supply voltage will determine motor speed and the current how much torque it will be able to deliver. So you may not need a 37V power supply exclusively. Maybe you can do with 24V or 36V (depends on whether the speed you can get with these voltages is enough), or maybe you can use a 48V or even 60V unit.

I am running a very similar project where the battery is 56V but will go with a 60V power supply. I will try to get as much current as possible, since I already know these motors are quite hungry.

Thanks alot for the quick response.
That's where i was getting confused especially since if the battery is only 96Wh than how can the motor be operated at 189Watts average. Unfortunately i dont have the product in front of me to do this testing, but i wanted to see what they overall system would look like theoretically. But i think all i need is a power supply that can atleast handle 189 Watts and i should be safe.

Thanks again!

 

You need to measure the current of the motor on the battery @37V, that way your AC adapter can be sized. Then you need to use a buck converter to drop it to 37V, or run the motor on 42v.

 

You need to measure the current of the motor on the battery @37V, that way your AC adapter can be sized. Then you need to use a buck converter to drop it to 37V, or run the motor on 42v.

Thanks for the advice! I was thinking of using something from Digikey like this. The output voltage would be 37V so i shouldnt need to drop it down. If you have other suggestions for power supplies i am all ears.

Regards.

 

I am looking at this one:

https://www.amazon.com/MEAN-WELL-SE...coding=UTF8&psc=1&refRID=TD1T00XWZSNGFD0WCTMT

Remember you do not need to step down the voltage at the power supply. Since you are driving a motor, chances are you have a power stage (either an H Bridge or 3 phase inverter) which will do the stepping down for you! I don't know if you were aware of this but motor drivers are in essence step down converters with the motor coil being the Buck Converter inductance. This is of course an over simplification, but they are very similar in that respect. In other words, if you have a 60V power supply and you apply a 50% duty cycle, the motor sees the equivalent of 30V. Of course in real life you need to take into consideration loses and such.

Good luck on your project!