Car Stereos: Battery Issues, deep cycle batteries, deep cycle battery

Question
Brian,
First off, you are clearly a genius because your name starts with "Br." Seriously though, I am building a waterproof stereo using a boat stereo and 4 x 45 watt speakers. The Stereos max pull is 15 Amps. Can you suggest a battery that would work for 6-10 hours of play at mid volume, low base? I also plan to include a solar panel for a trickle charge while not in use if that matters.
THANKS!

Answer
Hi Brad,

I have to admit that I haven't worked a lot with marine installations, or selecting marine batteries.  However, I'll try to come up with some useful information for you all the same.

First of all, while your head unit might be fused at 15 amps, it's very unlikely that the average current draw, under normal use, will come close to that level.  It's hard to pin down just what kind of current consumption you can expect.  The current draw of the system will mostly follow the output level of the amplifier; it uses more current during musical peaks than the quiet parts.  The fuse size is designed to contend with a very loud peak at maximum volume; but it's the average current consumption, not the peak, that will determine how long your battery will last.  At medium volume, with limited bass output, I don't think your system will use more than 5 amps on average--probably less.

When you're selecting a battery, you'll want to pick one made for deep cycle use.  This means that the battery is designed to be discharged and recharged, multiple times, without harm.  This isn't something you can do with a typical automotive "starter" battery, but most marine batteries are deep cycle batteries.  Even with a deep cycle battery, though, you should avoid draining it more than about 50% before recharging.  If you commonly drain the battery, you'll shorten its lifespan.

The battery spec you'll want to look at is "amp hours" (aH).  This is a rating of the battery's reserve capacity, so it directly relates to the way you're planning to use it in your case.  Most often, the amp hours will be shown at a "20-hour" rate.  For example, I looked up a mid-priced DieHard marine battery from Sears.  This particular model has a 20-hour amp hour rating of 115aH.  That tells us that if you apply a 5.75A load to the battery, it will be fully discharged in 20 hours (5.75 * 20 = 115).  If I'm correct that your audio system's current draw will generally stay below 5 amps, then this particular battery would easily allow you to reach your goal of 10 hours of playing time before the battery is 50% discharged.

Of course, a battery with a higher aH rating would give you more reserve, and you'll need to account for any other electrical devices that are going to be connected to the battery.

Hope this helps!

Brian