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Sorry if this is a amateur question, but I don't understand how to control power from a battery. If you just wire it up to the load, it seems it will draw the fastest possible current, possibly burning out the device.

A lead-acid battery is powering an electric motor. The question is, how to throttle the power so that I can run this motor at different speeds?

Example: 62 cells in the battery. Each cell is rated for 5,530 amps over 68 minutes, or 565 amps over 20 hours. Average voltage is 2.0 V.

I want to be able to choose the fast or slow discharge rate. What is the device that allows me to do this? Btw I took those numbers from a submarine example, Type XXI, but you have to go to the German version and use google translate to find the good data.

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  • \$\begingroup\$ I strongly doubt that there's any battery in existence that can handle 5.5 kA for over an hour straight.... \$\endgroup\$ – Hearth Aug 17 at 23:36
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    \$\begingroup\$ it seems it will draw the fastest possible current ... that is an incorrect assumption ... google ohm's law to learn the relationship between voltage, resistance and current \$\endgroup\$ – jsotola Aug 18 at 2:37
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    \$\begingroup\$ @jsotola So it's controlled by changing the resistance? \$\endgroup\$ – DrZ214 Aug 18 at 3:20
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    \$\begingroup\$ exactly ... the 2V cell may be able to deliver 5530A, but if you placed a 200 ohm resistor across the cell terminals, then you would be drawing 10mA through the resistor \$\endgroup\$ – jsotola Aug 18 at 4:16
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    \$\begingroup\$ 5500 A x 124 V = 680 kW without considering short-circuit current capability. This is not DIY (do it yourself) power. Are you considering working on this system for a project? Your question suggests that you are far from qualified to do so given the risks involved. \$\endgroup\$ – Transistor Aug 18 at 8:07
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The load will only draw the current it requires.

Taking something a bit more modest than a submarine, the starter in my car will draw a few hundred amps from the 12 volt battery when starting the engine, but the headlights will only draw 5 amps each, and the interior light will probably draw less than 1 amp from the same battery.

To control the speed of an electric motor, we often use Pulse Width Modulation (PWM) which turns the power off and on rapidly so the motor effectively sees a lower voltage when you want to run it slower.

Alternatively, the motor could have separate windings for high and low speed - I have windshield wiper motors on my boat that work that way.

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Power is seldom controlled. Power has two components. Electrical power from a battery is voltage multiplied by current. You can control voltage or current relatively easily, but it is difficult and generally not desirable to control both at the same time.

Mechanical power from a motor is speed multiplied by torque. Here again, you can control one or the other relatively easily, but controlling both is neither easy nor desirable.

In general, motor torque is controlled by controlling the supply current. If you control the torque, the motor run as fast as the necessary to get to the speed where that torque will no longer accelerate the load.

Motor speed is controlled by controlling the supply voltage. If you control the speed, the motor will supply as much torque as it can to get to the speed that is set. At that point, the torque will be whatever is required to sustain that speed.

From the standpoint of controlling the load, speed control is often desirable, but it also necessary to limit the current. The controller can be designed to allow a set maximum current, but reduce that current when the desired speed is reached. That is similar to the way we drive a car. We press the accelerator down to produce a comfortable rate of acceleration then back off the acceleration when the desired speed is reached and modulate the accelerator to maintain an even speed.

From the standpoint of electronically controlling a commutator-type DC motor, pulse width modulation of the battery voltage sets the voltage applied to the motor. The motor will draw current depending on the operating speed and the characteristics of the load. The controller needs to monitor the current and adjust the voltage modulation to keep the current within a safe limit.

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I = V/R

so the current is limited by the resistance, both internal (all batteries have some) and external: the wires and device or motor connected to the battery terminals (which all have a non-zero resistance, unless they are extremely cold superconductors).

Motors (inductors moving under a load) generate reverse EMF, which counteracts some of a batteries voltage across the motor, also reducing current.

You can add resistance. As well as rapidly open and close a switch, thus pulse-width modulating the average current over time.

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These could have been 500 lb Telco type 2V cells with 24 x for 48 V

or here 62x2V=124V x 6 banks

Type XXI Electric Boats 372 cells 44 MAL 740 (33900 Ah)

As in transformers, speed would be controlled by tap voltage changers to the battery packs.

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