When my buddy showed me the "add-on", i immediately looked up some photos of the thing, out of sheer curiosity. Since lithium-polymer batteries put out 4.2-4.3V fully-charged (3.6-3.7V "nominal"), and the active pickups require at least 9V, it was quite obvious there would be some sort of small boost-converter in the device itself. This is the real world, so there's no magic involved. Much to my surprise though, the circuit board visible on the back turned out to be far more complex than i would've otherwise thought. Well, considering the (at least seemingly) trivial tasks it had to do, anyway.
Before going ahead with the install, we gotta take a quick peek at it, don't we? Easiest thing first, though - the battery.
It's encased in a plastic "clamshell", and the LiPo cell itself is wrapped in a layer of double-sided tape, keeping the thing closed. Not ideal, but plenty fit for purpose. The cell is a "chubby" 103450 (ie. 10mm thick, 34mm wide and 50mm long), rated at 1800mAh, made by Howell Energy. The official specs claim an operating time north of 200 hours or so; pickup specs claim around 2-3mA current draw each, so call it 6mA all in all. That works out to 1200mAh-ish at 9V, so around 11Wh of energy, while the battery markings say 6.6Wh. Quite the fudge-factor they've got there (and that's even without considering quiescent currents, not to mention conversion efficiency, which most definitely will not be 100%). Best case scenario, with 4mA total draw, gives out "only" 8Wh of energy, which is still ever so slightly on the optimistic side.
And as one would hope and expect from a half-decent battery, it does include overcharge / undervoltage / overcurrent protection on the little circuit board connecting the cell tabs to the output cable. Couldn't quite make out the 6-pin chip's markings, but it's most likely some form of DW01, which controls the 8205 dual N-channel MOSFET next to it.
Now for the good stuff - there sure is a lot going on here. My first observation is that, especially since this is, out of sheer necessity, some form of switchmode supply, why would you NOT strive to get most of the circuitry on the other side of the board, and use the bottom side (the one facing the rest of the electronics inside the guitar's control cavity) as a shielding groundplane? Your guess is as good as mine...
The brains of the operation seems to be that tiny speck of a BGA chip. That little bugger turns out to be a Maxim Semiconductor MAX8895X. Which, as it turns out, is "only" a USB charge controller. Ever so slight overkill though, in my humble opinion at least; one could achieve a similar-enough result with something like a lowly TP4056, set to a "safe" 500mA charge current (as per the oldest USB power delivery standards), but what do i know?
But if that's only handling the battery charging, then... The only other complex-enough-looking component on the board is a SOT23-5 packaged part. I would've figured the "lesser" device to be the one supervising the battery charging, but i stand corrected. In hindsight, it DOES make more sense, especially with the diode and tiny inductor right next to it (D2 and L2 respectively). The "PHPI" marking shows this to be a Texas Instruments TPS61041 step-up converter (as more or less expected).
But that only leads me to wonder what exactly is it with all the "extra" transistors and passives. Ok, you obviously want some filter caps to clean up the power, but everything else..? Ah well, i might leave the reverse-engineering for another time. Onwards to the install - step one, removing the now extraneous 9V battery and associated clip and wire.
Oh, great. Good thing i tried a quick test-fit; turns out the LiPo battery, at least in its casing, is juuuuust a couple milimeters too long to fit. Hmmm... Well, i guess turning the tone potentiometer about 180deg (to move the tone capacitor more into the corner and make some room in the middle) seems to be worth a shot. And while i'm in, might as well correct a minor brainfart from last time, and use a shielded wire for the volume pot to output jack wiring.
Aaaaaaand nope. Or at least not quite, just yet. Even had to rotate the output jack, because the tip contact was getting in the way. But it's starting to look like i'll have to shave off some plastic off of the battery casing's lips, to get it to juuuust-about-fit.
Whew, finally - we're halfway there, and i'd say the worst is behind already. Now, to mark off the back of the guitar, so i can then overlay the cover plate and figure out the right place to drill the hole for the charger.
Speaking of which, i'll also drill some smaller holes in its own "faceplate" as well, for some extra mounting screws. That's largely because i don't really trust the sticky-tape it comes with (can't really know how well it sticks to the back of the cover plate, and i have a whole bunch of M2.5 screws and nuts that'll be perfect for this.
3mm pilot hole, and then widened it with imperial step-drills up to the half-inch the manual says. Made it all pretty, as well - deburred both sides of the hole. Now, i'll admit the included double-sided tape will come in handy. Specifically, to hold the faceplate in place, while i drill the extra mounting holes. Speaking of which, the injection-pin marks come in really handy for getting the future screw placements nice and symmetrical.
Even with a bit of countersinking for the screw heads, i've still got at least 1mm clearance before touching the PCB. That's the way i like it...
If you didn't know, you'd say it was factory installed. Yeah, right... But either way, i'm happy with how it came out. Nothing left but to solder in the output wire harness that came in the box (to the 9V input of the pickups, the volume pot's casing for the ground, and the switch-on wire to the ring of the output jack. And whaddya know, it fits just fine depth-wise too!
Before you ask, yes, it also works just fine. Battery came in at around 3.8v, just about halfway charged, so even as it is, it should give a good few dozen hours of lifetime.
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