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Correct me if my wrong - DMG (as well as any other GB) has switching power supply, therefore if batteries and adapter are both in place battery supply is switched off.
Now if we replace batteries with a LiPo cell and wire adapter jack directly to charging PCB it should be fine. It will be powering the boy and charging the cell. No current will be sucked from the cell.
Or am I missing something?
Cheeers!
The switching supply doesn't isolate the batteries from the socket, the socket has a contact inside which isolates the battery when a plug is inserted (plus a diode to prevent the battery supply bleeding back into the socket)
Yes, you can add a LiPo, a charging circuit, an undervoltage protection circuit and you'll be good to go! Except you'll be running the GameBoy from 4.2 down to 3.7volts which may cause more switching noise, contrast flicker etc... I think there was a test done with the gameboy's output voltage vs input voltage, Nitro2K01 did it AFAIK.
You shouldn't run a LiPo down to its protection circuit cutoff (usually 3.0-3.1volts) this reduces its life a LOT. A custom circuit that isolates at 3.6 is a far better idea though will add complexity.
Alternately, buy a 'battery bank' for charging a mobile phone and use that directly. Pull it apart to use its PCB and if the larger LiPo cell won't fit, replace it with one from hobbyking that will. That has your charging circuit, a boost converter to generate 5v (you can modify it to output 6v easily), undervoltage protection and at the current draw the GB operates at, will be very quiet electrically.
I was thinking of taking the battery bar route, hoping it would be simple enough to pull off. Is there anything specific to look for in the battery bar?
Size I suppose. Or go the cheapest option knowing that you'll need to buy your own LiPo cell.
Also, most have an on/off switch or button. This will need to be either accessible or if you are handy with a soldering iron, you could cut a trace near the power switch and re-route some traces and use the switch to control the battery bar.
If you are happy posting pics of the build, I can talk you through any of the tricky bits