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I'm trying to replace my AC adapter with a battery. The AC adapter output is 12v @ 500mA DC. I can get a 12v battery, but I can't find one that outputs exactly 500mA. I'm trying to power a strip of LED lights. So I guess the real question is, do I need to reduce the amperage if the battery is the correct forward voltage for the strip? And if so is there a formula to use to reduce the amps? Lets say I get a 12v battery that outputs 7900mA; what type of resistor do i need to get 7900mA down to 500mA without reducing the voltage?

Thanks.

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    \$\begingroup\$ Linked question is about power supplies, but the answer applies here too. \$\endgroup\$
    – The Photon
    Jun 18, 2019 at 5:38
  • \$\begingroup\$ the term is current, not amperage or amps \$\endgroup\$
    – jsotola
    Jun 18, 2019 at 6:44

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A circuit pulls an amount of current from the source. The source does not feed its rated current into the circuit.

So you don't need to do anything to 'reduce amps'. When selecting your power source you need the correct voltage and at least the required current. The circuit will only draw as much as it needs.

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  • \$\begingroup\$ Great thanks. So All I need is 12 volts that output at least 500mA? \$\endgroup\$
    – John
    Jun 18, 2019 at 14:03
  • \$\begingroup\$ That is correct. More available current wont hurt, but obviously if you don't have enough it wont work. \$\endgroup\$
    – hekete
    Jun 18, 2019 at 14:05
  • \$\begingroup\$ @John though a battery doesn't actually work like a DC transformer. The rating should be in mAh or Ah, which is its total capacity. The actual amount of current you can draw from a battery depends on its internal resistance. Any way 500mA isn't that much, I would expect most batteries to handle it. It's just slightly more complicated, the battery will also not produce a clean 12v, it will start out at like ~14v and drop down as you use it. \$\endgroup\$
    – hekete
    Jun 18, 2019 at 14:11
  • \$\begingroup\$ The voltage changing could be undesirable for some circuits. I'm going to say it will probably be fine for your LEDs, but without knowing exactly how they are wired up I can't say that using a battery wont break something. \$\endgroup\$
    – hekete
    Jun 18, 2019 at 14:20
  • \$\begingroup\$ So the details are that I have a strip of LEDs that is powered by an AC adapter. The adapter reads "output: 12v DC 500mA", I want to use batteries to power the strip instead of plugging it in. So I want to make a bank of 8 AA batteries (rechargeable) to power the strip. 8 x 1.5v = 12v The batteries output 1700 mAh each. So I want to simply attach a 2.1 mm dc connector to the end and plug them in. The batteries will be connected in series. I don't want to overpower the strip. I realize as the batteries run down, the voltage will drop but hopefully they will last for a few hours. \$\endgroup\$
    – John
    Jun 18, 2019 at 15:00

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