From the onset, I focused on three things. Battery life, portability/weight and sound quality.
I researched more than a few existing products on the market and while some of them looked promising, they missed the mark with one of two things. Either the battery life was short or the sound quality was sub-par.
Step 1: Battery Life
I decided on a sealed lead acid battery (SLA). SLA batteries, when used properly, can provide years of service. They are sealed and maintenance free. They are very popular in many low tech applications such as motorized scooters, wheelchairs, uninterruptable power supplies, etc .
Choosing an SLA battery is as simple as picking one based on the amp hour rating. The amp hour rating is generally expressed as nnAH where nn is a number designating hours. In my case I used a 20AH battery. What this means is that the battery, under ideal conditions, would be able to provide 12 volts for 20 hours, to a device drawing one amp of current. I'm not an electrician, so I may not be wording this technically correct.
Choosing a lower amp hour battery would reduce battery size and weight, at the expense of run time.
I chose a Battery Tender Junior battery charger which both charges and maintains the battery when not in use. SLA batteries should be stored fully charged and if possible attached to a charger that can maintain the charge. Another option is to use an onboard charger which is meant to be installed into whatever the application may be. The Schumacher 1.5A Onboard Charger gets great reviews.
Step 2: Speakers + Amplifier
I researched quite a few amplifiers and decided that a full size car audio amplifier would require too much current from the battery. I went with a small Class T amplifier rated at 20W per channel. The 120v wall power supply provided with the amplifier delivered two amps of 12 volt power power to the amplifier. I cut the cord off the power supply and wired the ends up with crimp on connectors which would be hooked to the battery terminals. Using a multimeter I was able to measure the true current draw (in amps) of the amplifier. At 75% volume the amplifier pulled between 0.5 and 0.9 amps. I could easily expect 15+ hours of runtime from this system, and that's a conservative estimate.
The amplifier accepts input via two methods. RCA or 1/8" stereo line. I bought a 12 foot cable with an 1/8" stereo plug on one end and RCA outputs on the other. The 1/8" end plugs into my mp3 player and the RCA ends connect to the amplifier.
Step 3: Containment + Wiring + Conclusions
As shown in the more than basic wiring diagram photo, I decided to take advantage of the 12v power and wire up a cigarette lighter socket to the battery. This allows me to use a USB car charger to charge my mp3 player or cellphone.
For use on a bicycle, I run the cables (audio + 12v socket) up from the trailer to the top tube of the bicycle using velcro straps. On the handlebars I've mounted a universal electronics mount (bar mount, universal clamp) to hold an mp3 player or cellphone. This puts music selection, volume control and charging right at one's fingertips.
How does it sound? Awesome, better than I had hoped. I used it continuously for 7 hours last weekend. Nice dynamic range, plenty of volume, and excellent battery life. I multimetered the battery at the end of the night and it was at 12.4 volts. Using this voltage / charge chart indicates the battery was still at approximately 80% charge.
I'm going to build a version 2 for direct mounting on top of a rear pannier rack. My plan is to halve the battery size, use the same amplifier, and use a set of 4" Boston Acoustics component speakers I have in my parts bin. I'll build a wood box to house everything and it shouldn't be much bigger than a trunk bag.
Step 4: Noisy USB Charger Issues
There are two solutions. Use a second/separate power source to charge the mp3 player. A 12v battery with a separate cigarette lighter plug or a re-chargeable battery pack that has a USB output works well.
The second solution is to buy a UBEC (Universal Battery Eliminator Circuit) device intended for use in hobby aircraft. This little device is an Andes Mint size voltage regulator that takes up to 18 volts in, and supplies a clean 5v output, delivering as much as 3 amps. I wired one of these up to an inline switch + female USB port and then plugged the mp3 charging cord into said port. Completely silent.