The Best Bluetooth for Your Boat (v0.1)

I'm in the process of rebuilding / refitting a 1968 cruising sailboat, which needs a total electrical refit as well. I was thinking of options for music, and decided to build a portable bluetooth speaker instead of hard wiring speakers and a typical marine stereo into the boat for a couple of reasons. Even when we're on the boat, we aren't always in the cabin where the stereo would sound best. I didn't want to mount speakers inside and out in the cockpit to keep it simple. But the best thing of a portable bluetooth speaker is that is can come to the beach, or be used around the shop when we aren't messing around on the boat. The is the prototype for a future (potential) refined version, which works amazingly well and sounds awesome.

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Of all places I started first with my components, it was the batteries. I found a closeout sale of 26650 LiIon batteries at my local Radio Shack, which let me get them for $4.99 for a two pack. I went to another Radio Shack in the area and got a total of 5 2-packs, and began to test them. For $25 total, it was a good price to experiment with them.

I also picked up the BMS, amp, marine speakers, dual USB / voltmeter combo, and 2.1mm female jacks from Amazon.

I found one battery of the bunch that wasn't performing as well, so I paired 8 of them into a 4S battery pack, and used a DROK 4S LiIon BMS board to manage the charging and prevent me from over charging any one cell. I don't know that this is the best solution, but in my experimenting, I also wired in a typical balance cable that I can plug into my Tenergy balance charger to periodically double check that they are staying well balanced.

The 26650 batteries are rated at 3.5Ah each at 3.7v (4.2 charged). By pairing two together in each cell, we've got 7Ah in each cell. Then putting those 4 pairs into series produces 14.8v (16.8v charged) at 7Ah.

This setup is more complicated and expensive then just buying a 12v 7Ah SLA battery, but this was also my first project building my own pack, and I wanted to give it a try. This 4S lithium pack does carry more total watt hours, and is lighter too. Plus with the battery protection, I'm less likely to run the pack until the batteries go low enough to sustain damage, like a sealed lead acid system would do.

Not shown here, I wrapped the pack in gaff tape when it was done to prevent any potential shorts in the case later.

Since I'm designing for use on a classic sailboat, I wanted something that sounded good, and looked like it fit in. I was going to make an enclosure that was build from strips of teak wood built around a form, kind of like construction of a wood boat. But, teak isn't cheap, and I didn't know if the speakers would sound any good in an enclosure that small. So I opted for this prototype version that would be about the same volume, but out of 1/2" birch plywood.

After roughly measuring the location where the speakers needed to fit (using some tape and paper because I forgot my tape measure at home), I used the Makercase website to help me get started with a basic plan for a wood box.

Using Adobe Illustrator, I edited the bottom to change the box joint seams to a clean base with just a piece of plywood that would sit inset the bottom. I designed an internal frame with the same dimensions, but the whole center cut out, so it could be glued in place and leave a nice flat lip for the above shown screws to attach to, which wouldn't be visible from the outside when the speakers were sitting normally upright. This is where I go in the box to install everything, and can get access later as I make changes.

I cut all the pieces on a 130w CO2 laser cutter from 1/2" 5ply birch plywood. I chose to use the side of the plywood that has more grain variation to make it look more interesting.

This is the first I've used one of these simple and cheap little Bluetooth amps, so I attached it directly to a speaker just to get a sense of the sound quality before wiring it in for good. The speakers didn't have much fullness without an enclosure, so I was a little nervous at first if the amp was lousy.

You can see some photos as I was figuring out how to extend access to the 5 buttons on the face of the amp to the exterior of the housing. These photos are from before I used slightly longer bolts that reach the buttons better, but you get the idea. As you press on the tip of the bolt (which are covered with cap nuts in the final enclosure) the head of the bolt presses the button, and you compress some foam which pops the bolt back to the resting position when you release the button. The bolts and cap nuts are stainless steel.

After I glued the box together and sanded it, I put on a coat of oil/poly finish and then later sprayed a few coats of gloss polyurethane finish. I screwed in the speakers, wired them to the amp, and mounted the amp to the inside so the bolts protruded through the 5 holes I previously had laser cut in the original design.

You can also see here the toggle switch that cuts power to the amp from the battery pack. At the bottom is a combination dual USB charge converter with digital voltage gauge in between the 2 USB ports. This takes 12-24v and outputs 5v DC, so it was perfect since my range with this pack is 14.8-16.8v. The voltage gauge will help me know how much juice is left so I don't unexpectedly lose my tunes. Having the USB charge ports right on the speakers assures the source of the music (my phone) isn't going to run out of juice unexpectedly either.

I also lined the interior of the box with some synthetic insulation scraps I had left from another project. Adding insulation improves acoustics, allowing the enclosure to perform like a larger enclosure. I decided not to port the enclosure in this initial prototype, since a closed design is easier to calculate (or ballpark) for good sound, where ported enclosures need to be just right for it to pay off with the best sound.

I'm already planning some changes in my next revision, but to start, I'm using a balance charger without the balance cable attached (since it's got the over charge protection inside the pack). I have a 2.1mm female power jack run to the exterior of the case from the internals, and I soldered together a cable to convert the XT60 connector on my balance charger to a 2.1mm power plug. This isn't the best solution, since it doesn't keep the cells balanced, but rather shuts the pack off if they get too far out of whack. I'll still open the case periodically to attach the XT60 connector and balance cable directly to my charger so I can see how quickly it loses balance. In initial tests, it didn't seem to get out of balance any noticeable amount, but that could drift over time.

In my next version, in addition to the external power input, I'll add a connection for the balance leads on the exterior. I haven't found a good BMS board for the internal pack that provides the protection like the current one does, AND balances during charging. I know they are out there, but I'm just new to building my own lithium packs, so feel free to suggest better options with URLs for buying for USA shipping. Once I have a board that does both inside, I could just use a DC to DC converter to supply 16.8v to the pack from my 12v boat battery.

So how does it work? AMAZING SOUND. This little amp is surprising clean and powerful with 50w output per channel, and the Polk Audio DB651 speakers are clean and bright at 60w RMS each, so no fear of blowing them out. A larger case would provide more low end bass, but there is good bass for all but the lowest range. The case fits perfectly in a little shelf in the main cabin of the boat, and is not too small a design for good sound.

In my next revision, I'm also going to wire a switch so I can also get power directly from the boat's 12v system or choose the internal battery. This way it doesn't use the internal lithium batteries at all when it's in the boat, and will prolong the life of the 4S pack. I'll also add a handle to the top, now that I confirmed there is sufficient clearance getting it in and out of the shelf space :)

If this project was useful, entertaining, or inspiring to you, please vote for it in the Wood contest (top right corner of your browser!) Thanks!!!