Camping Battery/inverter question

[theuserformallyknownasd00d]

Hi ya'll! I have a small 350w power inverter with a household power socket I got for out camping. Now I gotta match a battery or two for it but am having trouble working out how long they'll run for. Say we had a string or two of led fairy lights and music thru a small Logitech computer speaker, how long would you expect it to run off a car battery, caravan battery etc.. Is it that simple? I've read and read but can't wrap my head around it! Is there a formula or something to work it out?

Thanks

[Dreamwalker.]

leds are probably best for pencil/solar rechargeable batts...............they last a long time.........................a laptop on an inverter.................will last a very long time 5 hours? or longer from a good newish car batt...............but you don't won't to deplete the car batt too much...................because you'll kill it....................so you want to recharge it regularly.......................or better still........go to a batt shop and buy their old batts (the ones they take out of the cars and replace)...you can give them a few dollars per batt to cover the amount they would get from recycling it as scape lead......................these batts won't start a car ....but will run lights/music/computers for hours and hours ........start your car with the good batt....take it out and put in the old one to recharge ...if no power outlets available........................will save your good batt.........and for a few $ you can get a few of them....heaps of power and spares too.....................

[theuserformallyknownasd00d]

Thanks heaps mate, yep that's the plan buy a few second hand and fully charge em, maybe alternate em and charge em off some running cars. 5 hours a batt sounds ok if they cost sfa

[altenate]

I set up a dual battery thing in my ute. Cost a little bit the battery was the most but well worth it.

You can make it pretty simply or as complicated as you like depending on how much you want to spend. Made a solar panel pretty cheaply for day charger when we aren't driving. Chinese cells but basic soldering will suffice & a few odds & ends.,

The regulator was approx $120 but will never drain the main battery.,we run lights( again cheap Chinese strip led lighting), an iPod & speakers , watch the odd laptop thing & most importantly keep the fridge with cold beers going.... Lasts longer than I thought!

Get a good battery or look into reconditioning old batteries...it's supposedly doable...

[ballzac]

Is there a formula or something to work it out?

First you need to work out how much power you will be drawing. Then you need to look at your inverter's efficiency curve for that power usage. For example, if you are going to be drawing 100W, and your inverter efficiency is 95% at 100W (not unreasonable for a 350W inverter), you will be drawing about 105W from your battery. Then you need to know your battery capacity. Let's say it's 100Ah for simplicity. If you multiply it by the voltage (12V?) you get a value of Watt-hours (1200) then divide it by the power you will be drawing (105) and you'll get about 11.4 hours out of it. There are peculiarities of battery behaviour not taken into account here, so you want to give yourself a wide error margin and give yourself a conservative estimate of maybe 7 or 8 hours.

So

t = C*V*E/P

where t is the time you get, C is the capacity, V is the voltage of the battery, P is the power you are going to be drawing, and E = E(P) is the inverter efficiency when you are drawing P watts, expressed as a fraction (i.e. 95% = 0.95).

[theuserformallyknownasd00d]

Thanks for that Balzac, understood quite easily actually !

Thanks for everyone's replies, just gotta find a good deal on some batteries and jump in by the sounds

Edited December 12, 2013 by prioritise