Limiting current draw from Lead Acid Battery

Need to know nature of device...

They are both variable resistors.

Not wishing to sound rude but if you don't know basic electrical info you are best off not touching this with a barge pole.

12V batteries are not likely to kill you but when they push 90A then that is just over a kilowatt of power going somewhere. Enough to heat wires hot enough to melt insulation and flesh.

Maybe get an engineer to rip a motor speed controller from one of those invalid scooters? Though make sure you ask the invalid for it first :-)

Ho can someone do that when no one, including yourself, knows what you need? The only way to reduce the current draw from the battery is by developing a voltage to counter the battery voltage. Your device develops some voltage VD with 25 Amps through it, and VD may even be zero, so you will need to develop a counter voltage VC that makes BATT=VC + VD. This can be done with a resistance R so that 25Amps x R equals the required VC counter voltage. R has to sized to withstand a power dissipation of 25Amps x VC Volts which may be quite hefty. Going back to what you have said, the device and the battery resistance develop all the counter voltage at 90Amps, so that it must be

12/90=0.133 ohms. Then at 25Amps they will develop VD=25x0.133=3.33V. So the counter voltage from an added series resistance must be BATT-3.33=12-3.33=8.66V. The required resistance is R=8.66V/25Amps=0.346 ohms, and this must dissipate 25Amps x 8.66V=220Watts. Where you go from here depends on other information like is this battery being charged by an alternator so that the output voltage rises, how important is it to maintain 25Amps exactly or is this a ballpark figure, how is the device connected into the circuit, operating environment temperature range, etc...

What is the device? If it is a commercial product with a detailed Datasheet, maybe someone here can help you. If it is a simple device, then just tell us what it is.

--Mac

The "device" is a fictional entity in the troll's homework assignment. All of these losers will be washed out by December and we won't have to put up with their deceit then.

It's real hard to say, some devices can be damaged if underpowered.

Bye. Jasen

Well I guess this is the first time that someone has referred to me a Troll! lol........

Simple question. DC circuit, cold crank amps max to 300. Small DC motor will draw full rated amps if I let it. If I limit amps according to product line sheet I can slow the revolutions of the motor down. So what would I need to limit volt/amps to operate the motor at various speeds?

Thanks, Bob

Oh I almost forgot....I no longer am required to complete assignments by the semester's end since graduating some time ago with my MBA. I am very good connecting people in various serial or parallel circuits to achieve business objectives. However, when it comes to completing such circuits with electricity, I am rather at a loss to do so..... :)

Bob

There is insufficient information provided with your original question to suggest a meaningful answer.

Please be more specific - what are you trying to accomplish?

Thanks, Rich

I don't know what you can use- either something incredibly primitive like a lump of coal with a wedge wiper, or something modern and expensive.

I have my answer. Thanks for all the help.

The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved when the battery is fixed at 12 Volts?

The speed controller works by varying the average voltage sent to the motor. It could do this by simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to switch the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't notice it, it only notices the average effect.

Think about the motor, not the battery. You will get more constructive responses if you ask how to control the motor. Like, how do you keep it from drawing too much current when it stalls from a heavy load, or maybe you just want to learn about motor control in general, whatever. You could drive the motor from a battery or any other very low impedance voltage source and you would get pretty much the same answers to questions about the motor, I would think.