Posted by David Sherman on Friday, November 09, 2007 at 21:25:07 :
In Reply to: Re: Power in = power out, minus a bit posted by Todd Wilson on Friday, November 09, 2007 at 20:47:13 :
When they say a car battery is "900 amps", that usually means "900 cold cranking amps", which is basically a marketing gimmick along the lines of the exaggerated "Music Power Watts" that they used to claim for stereos. It purports to mean that the amp can put out 900 amps for a little while into a starting motor (which usually drops the battery down to 8 volts or so) while it's cranking. That has very little to do with the overall amp-hour rating of the battery, which is how many hours it will put out a reasonably modest amount of amps (say 10 or 20 or so). Deep cycle batteries, telephone batteries, solar batteries, and the like are designed to power moderate loads for a long time, whereas car/truck batteries are designed to power very heavy loads for a very short time. The construction is different (fancier plates with more surface area for the starting batteries), which gives them more short-circuit amps, but it gives them fewer charge/discharge cycles and less overall amp-hours. It's hard to find actual amp-hour ratings for car batteries. Personally, what I would do is if the whole arrangement draws less than 100 watts, pick the biggest 12 volt battery I have sitting around, give it a good solid charge (I like a constant-current charge at 5 amps for a day or so, rather than using an "automatic" charger that never fully charges an old or abused battery), and just go with it. It'll probably do fine for the duration of the parade. Or, if you think it might be marginal, hook up all your stuff and do a dry run and see if it lasts long enough. Turn it off as soon as the lights begin to dim or the blower begins to slow down, because if you run a battery all the way down, it'll never be the same again.
As for efficiency of inverters, 80-90% is a reasonable figure. Do make sure that everything you want will run off the inverter. Some motors or electronic devices don't like the standard "modified sine wave" inverters.
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