I need my motor to be powered with 12V, 5A for 1 hour continuously. How can i decide the Ah rate of the battery. Please suggest some lithium ion battery for the specification
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2 Answers
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The Ah rating is a measure of charge - how many amps the battery can supply, for how long.
In your case you need 5A for 1 hour, and that is simply 5 Ah. You multiply the current by the time required, to get the charge needed.
Next, The voltage rating is basically how many cells are used to make up that battery. You can't control that - you just have to get a battery that has the correct number of cells.
So the full specification you need is: something rated for approximately 5Ah at approximately 12V. (I can't advise on which one you should buy, as there are so many other factors that have to be taken into account.)
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$\begingroup$ The motors' stall current is 5A, I cant find any portable batteries with output current 5A, can i use the battery with output current 2.5A? $\endgroup$ Mar 17, 2016 at 9:59
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$\begingroup$ Ah that's a completely different question. You should build your robot, measure the current the motors actually take (possibly much less than 5A) and get a battery that can handle that. You might be able to get by with less than 5A, but don't let the robot run unattended just in case something nasty happens. $\endgroup$– AndyMar 17, 2016 at 10:23
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$\begingroup$ Thanks for you instant reply, well you want me to use some power supply unit initially to drive the motors and measure the average current drawn?? $\endgroup$ Mar 17, 2016 at 10:57
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$\begingroup$ Yes, if you can get hold of a 12V supply and an ammeter to measure the average and peak current draw while it's operating, that would be the safest way to be sure you don't exceed the battery's maximum current draw. $\endgroup$– AndyMar 17, 2016 at 11:16
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$\begingroup$ Another option would be operational testing (=trial and error), if a power supply is not within reach. Try a known battery and use it for as long as it gets. If it's more than required, try a smaller capacity. (Take into account that as battery capacity and battery weight changes, the power and energy needed to operate changes). $\endgroup$ Mar 19, 2016 at 14:15
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The best way to measure battery capacity is Watt Hours (WHr) because they are a measure of energy. Amp hour ratings are based on the batteries nominal voltage. This is especially important if you end up using switch mode regulators later.