Hobostubs Ashlock
1818 posts
Jun 10, 2012
8:46 PM
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Could I use a power converter and a car battery,or maybe even 2 car batteries to run a 60 watt pa and a rp 155 foot pedal for a few hours,I was looing at them on the net and ther not that exspensive,And I have a new car battery here thats laying around.I was wondering how long it would work if ,In fact it would,I was also wondering ,If it would couldnt you run a couple batteries In line para el or somehow and increase the time?Also If one would work what size of one?any help greatly apreaciated
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Hobostubs
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Greg Heumann
1644 posts
Jun 10, 2012
10:00 PM
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ANSWER: DO THE MATH
Suppose your amp has a 2 amp fuse. Because Power (in watts) is equal to Volts * Amps, then we can determine that your amp draws less than 240 watts of power. (If it drew more than that it would blow the fuse.) We can use this to figure out how many amps of current you need to make the same power from a 12V source.
Use the same equation and solve for amps - 240w/12v= 20 amps. If you have a car battery rated for 100 ampere hours, that means it can deliver 20 amps for 5 hours, (or 10 amps for 10 hours), etc. (Don't pay any attention to the battery's "CCA" or "Cold Cranking Amps" specification - that is about sucking the max amount of power out of it in a hurry only.)
So theoretically you could play this rig for 5 hours. Analysis so far: That means you're in the ball park.
But WAIT. The above is theoretical.
Your inverter will not be 100% efficient. The inverter's spec should tell you something about its efficiency and you need to figure that into your math. The battery's voltage might not hold up high enough toward the end of the cycle. Although we call these 12V batteries they really put out about 12.8V in normal operation so use that number to do your math/
The good news is that your amp's steady state draw is probably much LESS than 240 watts. If you have an ammeter you can actually measure how much current (amps) is being drawn and therefore determine how many watts it uses, which will allow you to proceed with the above math.
Just a quick google search - this inverter, according to the specs (http://www.dcacpowerinverters.com/go.asp?ic=PW200-12) would probably do the job. But it says "up to 90% efficiency". Let's just assume its real efficiency is 80% - use that in your math too. (It also says "modified sine wave" output - your amp's rectifier may not like this - it may not be able to produce the power the amp needs internally, or it may create noise. So don't go too cheap on the inverter!
Note - You're going to want a "deep cycle" type battery as normal car batteries don't last well at all when you discharge them a long way between charges.
Finally - when you put two batteries in parallel, you get twice the capacity. The batteries should be of similar size and health though, or one could spend a lot of energy trying to charge the other.
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/Greg
Last Edited by on Jun 10, 2012 10:08 PM
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Hobostubs Ashlock
1819 posts
Jun 10, 2012
10:54 PM
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thanks Greg.Ill have to see if i can firgure that out;-)
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laurent2015
256 posts
Jun 11, 2012
4:25 AM
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Never went to USA but can I conclude that your electrical tension is 120 volts? (we have 220)
Thanks to your explanation, I found a trick to remember
the formula W=VxA: I think WAV files (no link, but...)
Last Edited by on Jun 11, 2012 4:26 AM
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markdc70
117 posts
Jun 11, 2012
6:39 AM
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I'm from the states, and it blows my mind that other countries use 220 volts as standard wall outlet voltage! I can't imagine how many deaths that must cause every year.
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Tuckster
1063 posts
Jun 11, 2012
7:35 AM
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120 volts will kill you just as good as 220 v will. It's the current that kills you. I've accidentally grabbed the business end of a 6ooov ignition transformer. It hurt like hell,but not enough current to kill me or cause any real harm.
Last Edited by on Jun 11, 2012 7:42 AM
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mandowhacker
153 posts
Jun 11, 2012
4:25 PM
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I've used amps with several different inverters in several different big trucks. They all made noise enough that I ended up using battery powered amps every time.
It could be that they were not isolated from the truck's electronics even with everything shut off and that an invertor connected to a "stand alone" battery would work fine. I've never done that.
Greg
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Just when I got a paddle, they added more water to the creek.
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Jim Rumbaugh
733 posts
Jun 11, 2012
5:08 PM
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The Harmonica Club uses an inverter for our parades. Here's a few tips.
1) For best results, look for a "pure sine wave" inverter, not "modified sine wave". Economical inverters put out square wave or modified sine. Those are the ones that give you noise, We've used them and had to turn down the treble to get rid of the buzz. No problems with "pure sine"
2)Our newest 600 watt inverter has a rare feature, a power used meter. Even though we use two Peavy 200 heads (100 wats rms each), the power gauge barely moves. Because music "pulses", and is rarely at full volume for an extended time, A 100 watt sound system "may" be drawing 25 to 50 watts. I would be hesitant to make any predictions on battery life, though you do need to start somewhere with making a decision.
3) I have not tried this, but, a while back, someone suggested getting a computer UPS (uninterupted power supply). This "might" be an economical and portable answer.
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theharmonicaclub.com (of Huntington, WV)
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