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I want to power Christmas lights (without a nearby power outlet) for at least 2 hours and was wondering how to go about it. The lights run on 120V AC and draw .34A per strip (about 40 watts) and I want to use 5 strips. I could buy a 12V battery and power inverter to supply 120VAC, but I would like to see if there's a cheaper or easier way.

I could rig up some AA batteries in series and parallel to supply 12V to a power inverter, but since batteries lose voltage as they drain, would I need to buy some sort of "voltage booster" to maintain the 12V? Could I even use a single portable phone charger battery (3.6V) and a voltage booster instead? Or is there some other easier way to do all of this?

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  • \$\begingroup\$ Buy a digital timer relay with 2 prong AC outlet. set start stop times. it remembers in case of power failure. \$\endgroup\$ – Sunnyskyguy EE75 Sep 4 '16 at 22:55
  • \$\begingroup\$ What do you mean by power ... locally? You want 200W of mobile power for at least 2 hours? Please clarify your question. From the sounds of it, you would want an inverter and 12V lead-acid batteries. AA batteries and small lithium batteries cannot effectively operate in your application. \$\endgroup\$ – user2943160 Sep 4 '16 at 23:18
  • \$\begingroup\$ @user2943160 Yes, you are correct. 200W of AC power. I will be away from a power outlet. \$\endgroup\$ – Ryan Sep 4 '16 at 23:54
  • \$\begingroup\$ @Ryan please click the edit button to incorporate this information into your question text. \$\endgroup\$ – user2943160 Sep 5 '16 at 0:04
  • \$\begingroup\$ A 50Ah car battery which is 600Wh would only last 3h from 12.5 to 11.5 ... get an extension cord. or better than your Leds get a laser moving garden light RGB with hundreds of beams from 10W \$\endgroup\$ – Sunnyskyguy EE75 Sep 5 '16 at 0:12
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If you want a quiet, convenient way to power your lights, your idea of using an DC/AC inverter with a conventional 12V lead-acid battery is probably the best one. You will not be able to supply 200W in a cost-effective way using either AA batteries or a miniature portable 'power bank' for cell phones.

Doing some figures, using an approximate energy density of 35 Wh/kg and a battery mass of around 20kg, this gives you a total capacity from a single automotive-sized battery of 700 Wh. You will need to verify this capacity when you choose a battery, which may be difficult since many automotive batteries do not have watt-hour or amp-hour ratings advertised. However, this means you can meet your requirement (ca. 400 Wh) with a single (fairly expensive, but reusable) battery connected to an appropriately rated inverter. Since you will be operating the battery without immediately charging it, look for a deep-cycle type battery with a discharge count rating in addition to its energy capacity rating.

Another option which is louder and requires a different investment is a hydrocarbon-fueled generator. These are available in a variety of capacities and fuel capabilities. Many options would greatly exceed your duration requirement, but would still be usable. Be aware of the exhaust of the generator and only operate it outdoors with enough ventilation.

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The other answer is correct. For this kind of power, a 12V SLA is the only practical battery based solution, while a generator of some kind (a stand alone or a car) is the other solution. Old car batteries could be used, instead of buying new ones. Old batteries won't start a car, but can easily power this.

That said, you should look into a more efficient setup. Depending on the lights, there may be a diode full wave bridge rectifier that converts the AC to DC. If the lights have a small bulge near the power connector it likely does. You could measure the voltage at one of the bulbs.

If this is the case, you could use a simpler step-up boost regulator, instead of a bigger inverter. No need to invert to AC just to rectify it again.

You could also use LED strings instead of incandescent if you haven't already switched. Less power needed.

Finally, you could bring the voltage down a bit. At 100V, it should still light, just dimmer. Depends on what you think is acceptable. But it would cut out say 40W from your five string setup. Like buy 4 get 1 free deal.

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  • \$\begingroup\$ What do you mean by "step up boost regulator"? What would that do? \$\endgroup\$ – Ryan Sep 5 '16 at 3:52
  • \$\begingroup\$ What it says on the tin. It will step up the 12V DC in to 120V DC in this case. These switching regulators tend to be 80~95% efficient. \$\endgroup\$ – Passerby Sep 5 '16 at 3:57
  • \$\begingroup\$ Thank you, I did not know that you can use 120V DC in replacement for AC. I will look into this... \$\endgroup\$ – Ryan Sep 5 '16 at 3:58

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