Hello everybody new to this site , I have a question I bought this portable stereo amplifier and the input power is 12v 2amp so I have this battery which is 12v 18ah how do I lower the amps so that its 2a ? what would be the cheapest way?
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As Dennis already stated, this has nothing to do with NI hardware. I am curious what made you come here for this question.
As for your issue, that is what a fuse is for. Go put in a 2A fuse in series with your device and no more than 2A can go to it.
The amp hours is really useless here. All that means is that if you draw 1A, your battery will be dead in 18 hours.
A portable stereo should work just fine connected directy to a car battery. The voltage does not have to be very accurate.
You are confusing units. The 2A rating has nothing to do with the 18Ah rating of the battery, it just means that with that size battery you'll be able to run your stereo at max power for at least 9 hours if the device draws 2A.
That is only approximately correct. The ampere-hour product is valid for discharge at the nominal rate. For typical lead-acid batteries the nominal rate is for a 20 hour discharge. So 18 Ah/20 h = 0.9 A is the nominal discharge rate, often designated C. For discharge rates greater than C, the capacity is usually less. The image below is from the data sheet for a 12 V 18 Ah battery. At 2 A you would probably get 7-8 hours.