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FOLKS, am a learner. This is the problem am trying to get around.

  1. what would happen if I use a 200amp 12V battery to run a dc standing fan of say 2amp power draw or a piano of same rating or any other dc load? Will the circuitry of the dc load have problem since the amperage from the battery is not regulated or will the load circuitry control the amp draw hence everything will be fine. Remember the dc load used to be operated via mains power pack but in situation where there is no power supply this comes handy.
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  • \$\begingroup\$ Don’t shout at us... \$\endgroup\$ – Solar Mike Apr 8 '18 at 14:25
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A battery behaves somewhat like an ideal voltage source. That is, it will maintain a reasonably constant voltage between its terminals over some range of current.

A battery (or other power supply) that is rated "12V, 200A" does not force 200A to flow in a circuit. It forces 12V across the circuit, and the circuit will draw however many amperes it wants from the battery when 12V is forced upon it. The "200A" rating of your battery means that if the current drawn by the circuit is 200A or less, then nothing bad will happen.

If a fan or a piano or any other device that uses electric power is rated "12V, 2A" then that usually means that it needs 12V in order to function correctly and, it will not draw any more than 2A when it is functioning correctly.


Beware the word "usually!" Most electronic devices expect a constant-voltage power supply, and most power supplies are designed to supply a constant voltage.

Some applications (e.g., high power light emitting diode lamps, arc welding) need a regulated current, and some power supplies are designed to adjust their voltage output as needed in order to force a certain current through the circuit. These types of power supply usually will be labelled for a very specific application, and you won't see the typical howevermany volts at somenumberof amps rating.

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Actually the battery ratings are 12V and 200Ah and not 200A. 200Ah means the charge stored in that battery. ie. a 200Ah battery can provide 12V 200A for 1hour.

If you connect a 12V 1A load across such a battery then the load will only draw 12V 1A current from it and the Battery can provide it for 200hours.

By using charge stored (Ah) and current rating of your load you can easily measure how long the battery will last for your load.

Hope this will help you.

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The load will draw only what it requires to satisfy Ohms Law, which states that the current into a load is equal to the applied voltage divided by the load resistance.

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Think: What will happen if you connect a small 12 V lamp such as a car tail lamp to the battery?

The resistance of the load resists the current flow. Ohm stated the relationship as \$ I = \frac {V}{R} \$. This means that the current that flows will be proportional to the voltage (which is fixed at 12 V in your case) and inversely proportional to the resistance.

Small loads have higher resistance than large loads so less current will flow.

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Voltage determines how much current will flow through a load, the current rating determines the maximum that you can take before the supply (battery in this case) can't maintain it's specified voltage.

Some loads will only take the current they need, but very often a load will take more current as the provided voltage is increased. So the most important parameter is the voltage of the supply.

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