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Old 4th October 2012, 07:27 PM   #31
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The capacity is determined using a much lower current draw from the battery.
For example, a battery can actually provide 700mA for 10 hours. The capacity of that battery is 7Ah. If 7 amps are drawn, it can only provide that power for 38 minutes.
These numbers are hypothetical; I haven't referred to any battery datasheets. But they are somewhat realistic.
In fact, I believe the amp-hours standard is based on 20 hours of use. So a 7Ah capacity is determined by 350mA draw for 20 hours.
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Old 5th October 2012, 10:22 AM   #32
Kitax is offline Kitax  Latvia
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hmm kind of makes sense, couse thease batteries are made to last long, not to run out in 1h... so the real numbers are in the datasheets of batteries, right?

does that mean, that it would last for more, than 40h if 175mA are drawn from it, or does that only work one way?
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Old 5th October 2012, 10:36 AM   #33
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That would be the place to look, but not all datasheets are created equal. Hopefully, there would be a graph showing time and discharge rate. It does work both ways, within reason of course.
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Old 5th October 2012, 05:06 PM   #34
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then why not just build a test amp and see for your self, that it will give you hours of loud music?
seems reasonably simple, as a tda1557q amp takes roughléy 5 minutes to build.
then afterwards we can get back to this point if you like.
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Old 6th October 2012, 08:22 AM   #35
Kitax is offline Kitax  Latvia
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well i guess i could, but then i would have to go get the TDA1557q
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Old 6th October 2012, 11:06 AM   #36
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5 usd or so.
tda1557q, tda1557, tda 1558 and so on, there are at least 20 chips with close to same specs and pinout.
i used as a test nothing more than 3 packs of 4.5 alkaline batterypacs and it playd for least 12 hours at loud volume setting.
7AH is plenty for this job.
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Old 7th October 2012, 08:59 AM   #37
Kitax is offline Kitax  Latvia
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well that sounds awesome... also, how should i charge my batterry? should i just hook it up to a power source? and how big of a power source?
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Old 7th October 2012, 11:54 AM   #38
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grab the cheapest possible battery charger You can get from Your favourite mega-junk store complex, roughly a nother 5-6 $.
A simple relay can put the battery on charger when the unit is off.
and ta-da, you got plenty of power for reasonably sensitive speakers, no problems with main's voltage junk, and possibly the cleanest DC powersupply one can have.
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Old 7th October 2012, 06:08 PM   #39
Kitax is offline Kitax  Latvia
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dont i need a specific voltage?
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Old 7th October 2012, 07:06 PM   #40
balerit is offline balerit  South Africa
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A friend just dropped off an 18ah sealed battery and I see it says on the front slow charge @1.8A for 5 hours.
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