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Old 04-22-2010, 01:21 PM
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Shaun315 Shaun315 is offline
TT Regular
Join Date: Jan 2010
Boat: Mastercraft
Location: USA
Posts: 136
The short answer: it depends on a lot of factors... the current draw of the system, temperature, battery age, battery type, internal battery resistance, wiring of the system, etc.

Most batteries are rated at 20hr A*hr capacities, which is capacity (current*time) of the battery when discharged over 20 hours. A*hr rating / current load = time or A*hr rating / time = current available. So a battery rated at 105Ahr (@20hr) / 20 hours = 5.25 A continuous output. Not really practical for a stereo system since 5.25A*12V is only ~60W.

A better gauge might be RC or reserve capacity of the battery, usually how many minutes a battery can provide 25A until reaching end of discharge (or max depth of discharge... whatever they use for pb-acid). 25A*12V is 300W more inline with stereo demands... I think the group 34 blue top is rated at 120minutes RC, so you should get at least 2 hours out of it with a 25A load, probably more because I doubt your stereo pulls 25A constantly...

If you want a more accurate answer you'll have to figure out your average system load on the batteries and go from there...
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