Battery Current Help

[coastergeekrtc]

I have an R/C car that runs on 2 Ni-Cd Batteries (Series of 6 1700mAh 1.2V cells). The car has two 14.4V DC motors. I want to use one Ni-Cd battery to power one motor for onother project. The problem is that the current for the battery is too high to test. Does anyone know the current or can someone help me measure it?

 

[Nigel Goodwin]

What are you trying to do?, the current is determeined by the load, NOT the battery, so your question is meaningless!.

 

[coastergeekrtc]

Its that launch project I was working on with the PIC Microcontroller controlling the motor with PWM. I want to make an H-Bridge...requiring that I supply the correct power to the motor. Basically, I want to replace the battery with a power supply (like the pic bellow) from a wall socket. I'm sorry if all this is really simple to calculate, I'm just not the electronics type.
**broken link removed**

 

[akg]

Its that launch project I was working on with the PIC Microcontroller controlling the motor with PWM. I want to make an H-Bridge...requiring that I supply the correct power to the motor. Basically, I want to replace the battery with a power supply (like the pic bellow) from a wall socket. I'm sorry if all this is really simple to calculate, I'm just not the electronics type.

so u want the values.?
a transformer with 9-0-9/0-18v secondary,2200mfd/25v+ smoothing cap, 7815 regulator IC, a heatsink

 

[stevez]

What appears to be missing is the current requirement for the motors. You indicated that the motors are 14.4 volts and if that's where you want to run them at then we've established the voltage. If you want to be able to run them at higher voltages (some people do for very short periods) you'll need to account for that.

You may not have access to the data for the motor but what you can do is make up a crude test fixture where the motor can run - and you can apply some braking - just something to add a load to the motor. You would then apply power and measure the current going to the motor and the voltage at the motor. The voltage at the motor ought to be maintained close to the voltage you've chosen to work at. Make note of the maximum current - this is a guide to use in designing your circuit and power supply. Many DVMs have a current capability of 10 amps or so - maybe enough for what you are doing. You might need to make a shunt if the current exceeds your meter's capability.

A very crude alternative, if you are willing to risk blowing some fuses - arrange the motor so you can run it but also apply a load (a rag or something) so you can take it from no load to fully stopped. Start with a fuse that's really high - maybe 20 amps. Make sure the wiring will support this. Bring the motor to a stop and if the fuse doesn't blow then work your way down until you blow a fuse. This is crude but it will give you some indication of current. Note that your application may never cause a high current situation but unless you know that for sure then plan on the high current.

A hands-off way might be to look at manufacturer's info for typical motors of this kind and maybe that will give you sufficient guidance.

I hope this gives you a start. Ask more questions if you need to.

 

[Optikon]

Its that launch project I was working on with the PIC Microcontroller controlling the motor with PWM. I want to make an H-Bridge...requiring that I supply the correct power to the motor. Basically, I want to replace the battery with a power supply (like the pic bellow) from a wall socket. I'm sorry if all this is really simple to calculate, I'm just not the electronics type.
**broken link removed**

I'd like to offer you some safety advice.

Since you are not the electronics type, I would reccommend NOT fooling with transformers connected to mains. IMO, your money is better spent on a meter that can measure the current you need. The capacity of the types and arrangement of batteries you have indicated is not outside of the realm of measurement. You just need a shunt (or a few paralleled) and a meter capabable of measuring the range. If you dont have that meter, it would be a good investment for future projects as well. The dangerous mains power supply would be a persistent risk.