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I have read the other thread 'Best way to power Christmas lights without nearby outlet' and my question does differ.

We have a Christmas Tree which sits in the middle of our Village Green every year. The lights need to be on from about 4pm until midnight every day for almost a month. Until now, we have used multiple threads of battery powered Christmas lights (AA/AAA) but the results are not very good, with the lights being very dim.

We would now like to power normal (mains) bright LED Christmas Lights by some means. We have explored all options of connecting to a mains supply and they are prohibitively expensive. The current thinking is that we should use some sort of car battery. (Using a generator has been ruled out as being too noisy).

My question is what specification of car battery would we need to get to power about 600 lights for a month, hopefully without having to charge the battery too often. Is this even possible?

We have not yet purchased the lights, so we could buy ones that suit the battery. I appreciate from the other thread, we may need an inverter. The question in the previous thread related to powering lights for only a couple of hours, so this is a very different scenario. Also they already had the lights they wanted to use, whereas we can choose lights to suit the battery.

Thank you in advance for any help.

Suzanne

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  • \$\begingroup\$ It depends mainly on watt-hours of demand use, ability to recharge and budget. Auto batteries would be cheapest at say $60 and 600 Wh, but essentially you must give links to LED or power consumption and hours \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 4 '17 at 17:09
  • \$\begingroup\$ ..."with the lights being very dim." That's the key phrase that none of us can help you with. And it determines everything else. You need to figure out the exact light you want, first. The one that solves that problem you mentioned. Then how many of those, once you see them and approve them. Perhaps then we can help answer your question. \$\endgroup\$ – jonk Sep 4 '17 at 19:23
  • \$\begingroup\$ You need to provide more info as @Transistor shows in his answer it can be either 6 car batteries per day or with my comments with LEDs it can be one car battery per five days. So facts are key. \$\endgroup\$ – MatsK Sep 4 '17 at 20:26
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    \$\begingroup\$ My suggestion is that you buy two things. First, buy one string of lights that are bright enough. Second, by a kill-a-watt power measuring device. Plug the light string into the kill-a-watt measuring device and see how many watts of power the lights consume. Multiply this by the number of strings required to decorate your tree. This is your total power required. Multiply power by number of hours you want to run per day. This is the energy consumed. Pick a battery with anywhere from 2x (recharge daily) to 20x (recharge weekly) that energy. Choose an inverter capable of supplying the power. \$\endgroup\$ – mkeith Sep 4 '17 at 21:53
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My question is what specification of car battery would we need to get to power about 600 lights for a month, hopefully without having to charge the battery too often.

Here's how you work it out.

  1. Total the lamp power consumption in watts (W).
  2. Divide this by the voltage to find the current in amps (A).
  3. Multiply this by the run hours to calculate the daily ampere-hours (Ah).
  4. Multiply this by the number of days between recharges to get the total Ah battery capacity required.
  5. Get a battery with about 1.5 times that capacity so that you don't need to discharge it fully (never a good thing) and to de-rate it for low temperature operation.

e.g.

  1. 600 x 0.75 W lamps -> 450 W.
  2. 450 W / 12 V = 37.5 A.
  3. At 8 hours per day that's 8 x 37.5 = 300 Ah.

Let's do a reality check at this point. Here's a car battery:

enter image description here

Figure 1. A 12 V, 80 Ah car battery.

As you can see, with 600 of these lights things are well out of hand as you would need five or six of these.

Run the calculations yourself with specifications from the lamps and see how you get on. Don't forget that if you blink or fade the lights that you reduce power consumption proportionally.

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  • \$\begingroup\$ If the 600 lamps are LEDs it will be a different sum.: 1.8V/20mA per LED = 24W / 12V = 2A * 8h = 16A = 5 days. \$\endgroup\$ – MatsK Sep 4 '17 at 20:23
  • \$\begingroup\$ Thank you very much for this detailed response, which is a big help for me as a non-technical person. I now know how to work it all out, even if the news proves to be bad for the village christmas tree in the end. \$\endgroup\$ – Suzanne Smith Sep 5 '17 at 17:28

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