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Am I reading this block diagram right?
(thelemmy.club)
From the service manual for the Yamaha DX-100 FM synthesiser. The way this looks to me, both the 12v DC power supply, and 9v DC battery power get routed through exactly the same voltage regulator.
So it should be possible to just straight replace the 6x C-cells with 3x 18650s without needing to make any other changes, right?
I know exactly enough electronics to be dangerous, which is why I'm double checking here in case I've failed to account for something.
I'm aware that fresh 18650s put out a little over 4v, but I seriously doubt the vReg tops out at exactly 12v. I can disassemble the unit and check part numbers etc if necessary but I was really hoping to avoid that as it's quite a chore.
Lithium batteries are a bit finicky, they self-discharge, and the rate varies from cell to cell, so if you have three cells in series at 4V, some of them will drop in voltage faster than the others over time
When you then charge them in series like that, it'll charge until the three cells combined are at 12V, but that might mean that one is at 4V, the second is 3.9V, and the last is 4.1V
This drift keeps happening over time, and eventually one cell will end up at a dangerously high voltage, potentially catching fire
That's why with lithium batteries, you need a monitoring circuit that keeps the voltages of the cells balanced
So i would not advice you to simply swap them in like that, you could charge the cells in an external charger and then use them in the device, but remove or isolate the 12V supply (add a diode to the batteries so power can go out of the cells, but not go back in) so it doesn't charge them while installed
NiMH batteries might be a better fit, they basically self-balance i believe, but you'd need 10 cells in series to reach 12V
Edit: completely missed the switch there, yeah 18650 cells charged externally will likely be fine, they are usually charged to 4.2V, so 12.6V total, the regulator is likely rated higher than that
It looks to me like the 12v supply already isolates the batteries (some kind of barrel jack with integrated switch), but regardless I would definitely be charging them external to the device. I have a dedicated charger already so it would be quicker and easier to use that and just swap in a new set as needed. I'll be 3D printing adapter housings such as this, so adding a diode into that assembly should be easy enough. Thanks for your advice!