Special Project, need a constant hot feed

I'm building a "portable" stereo for my 4 wheelers. It's basically a wooden box with a stereo and 2 speakers in it. I have a cigarete lighter plug coming out of it to supply power. This makes it portable between my bikes. My issue is I don't have a constant hot without running another wire. Does anyone know of a small battery that I could put inside the box to supply a hot so I don't lose my presets and other memory functions? I can't imagine it draws much and would think a small battery would last a long time. It works great other than this one issue. Any ideas???? thanks

It depends on the radio. On SOME radios, the constant (yellow) is the only source of power, with the acc (red) really behaving as nothing more than a remote wire, like the blue one run to an amplifier. If your constant (yellow) has a large value fuse (like 10 or 15A) then you're going to have to make certain your radio is OFF, everytime you unplug the cigarette lighter plug, because this is where ALL the current for the radio is supplied. Inside the wooden box, why dont you have both of the leads hooked to the +12 volt of the cigarette lighter plug? (If you do, my bad, and I just misunderstood your post.) All you have to do is get the smallest sealed lead/calcium battery you can find - something like what you might find in a computer UPS, and connect it in parallel inside.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I currently have both feeds, yellow and red connected to the "switched" output from the cigarette plug. My goal is to make this thing as portable as possible and ONLY require a cigarette plug for it to operate. It will then be portable so that I can put it on either one of my 4 wheelers or even on a friends. In reality it could go anywhere I have a cigarete plug. I beleive I would need a 12 volt battery, I don't think PC batteries (that keep cmos information) are 12 volts, correct? I guess I can just look into the smallest 12 volt battery I can find. My problem is the box is only 18" wide by 18" deep and about 3 inches tall, so it needs to be very small.

Here's one... And another... Those both came from this page. Follow some of the other links there as well, looking for your battery size needs. Pretty much any of those 12 volt batteries would do it, and SOME might give a couple minutes of low-volume listening time as a stan-alone supply, as well! Size and weight are going to be your important factors, I think. For the weights listed in grams or kilograms, 454g is one pound, and 2.2 pounds is one kilogram. With your space restrictions, I think you'll be looking at nothing larger than around 2.2Ah, which will be small enough to possibly fit, but will require you to turn the thing off every time, before you unplug it. Here's the link I got from Yahoo! to find that particular company. You might locate something else there.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

I would go with the smallest marine battery you can find, and wire it so that it charges while it's plugged into the cigarette lighter.  This way, you can listen to it on or off the bike, but you don't have to do anything special to charge it.  Depending on the radio, it shouldn't be too much of a draw.