Paul,
Thanks for your answer, you are right, a straight connection to the batteries would be fine, but you'd need to rig up a relay (if one is already in the box, all the better) to connect the trickle charger on switching off the amps.
Batteries are very interesting devices. They are not quite what people imagine them to be; there are other issues when powering amplifiers. While noise is not an issue as it is with AC supplies, the source impedance of lead acid gel cels increases abruptly from 10KHz with frequency. For a Class A this is not a problem (though energy consumption is, of course), but it is anathema for a Class AB, where short, fast-moving stabs of current are extracted from the supply. The rising impedance with both current and frequency is an issue, resulting in current starvation at the output stage unless an inordinately large battery is used, and this forces an unpleasant situation. You have to use storage caps, just like an AC supply.
Of course, the storage caps are a major part of the total supply cost, almost as much as a transformer! And there's one further factor; earth return for the speaker. In a conventional supply, the filter caps serve as earth return for speaker currents, since all speaker drive is referenced to ground. This means the caps have to be passable quality, and no smaller than for an AC supply because it is primarily the speaker currents which determine the size of these items, as we are chasing a couple of ohms maximum at 10Hz. (This relates to about 8000uF, BTW). People are justly concerned about their speaker cables; how about an earth return via an electrolyte in a gelcel? Would this cut the mustard? The problem seems to be the mobility of the ions which carry the current in the battery; their weight and spacing in a liquid makes them sluggish and the audio quality suffers. So, for two good reasons, the optimum solution is NOT pointing at a battery supply.
Anecdotally, someone with a battery supply once told me that there was something very strange about the midrange in his rig. When he put the amp on an AC supply, the problem disappeared.
In closing, the inconvenience, bulk and weight (not to mention cost, though your find is stunningly cheap) makes battery power a dubious benefit for high end audio. I admit I've not tried it, but I believe that the engineering points to a less than optimal solution.
Paul, thanks for raising a very interesting issue, and I'm sorry to rain on your parade, but I have considered this option and these were my findings.
Cheers,
Hugh
