Some dude has shown off these mods on reddit. There is decent stuff on the GBC battery mod but almost nothing online on the Neo geo pocket battery mod. Basically I just want to stick a rechargable battery in the NGP. Anyone have resources?
holy moly that sounds complicated. I could follow a YT tutorial. Ive modded IPS gameboys with video guides, but there doesnt seem to be one for NGP. One option I could try is to just copy what the video did on a GBC to a NGP , howevber I imagine the poewr draw is completely different on a NGP/Rechargeable battery is basically the same in concept whatever device you are doing. Voltage is voltage and as long as it delivers the amps (which if you are comparing to anything previously powered by AA disposable batteries is no big ask) then you are good. Too much voltage and you blow stuff up, too little and it won't power.
The device might have rated power limits, though for batteries then the simple start is take 1.5V for each AA cell and multiply it up to how many there are. Now AA cells don't stay at 1.5V throughout their runtime and drop quite quickly ( https://data.energizer.com/PDFs/E91.pdf ) so the device will then have a cutoff voltage below which I won't work. This is your range you want to hit, though you might be able to go a bit higher at the risk of maybe cooking the device in the longer term.
If you have an external power supply (preferably a nice bench supply) then you can figure out the voltage.
You might also want to know how much current it draws (naturally high action game/flash cart, max volume, bright screen...) while it does it so if you have an ammeter/current sensor on your supply or a multimeter the find that out too; when batteries on the seller have nice little mAh rating that means how many milliamps it will supply for an hour, simple division to then get the upper limit on runtime as practically it will be a bit lower. You then get to choose how much to spend, what to sacrifice to make it (and whatever management it needs) fit, what you want for a lifetime (having it last a week of solid play is great and all but if you are rarely more than a few metres from a wall socket then... yeah) and other such things. Management is necessary for lithium based batteries, though some might have their own onboard, but they do provide the most power for the size.
Hopefully the battery itself provides the voltage you want but if not then you get to figure out how to make it the right one.
Stepping down is usually no big deal -- resistors, diodes, zener diodes, boost convertors, regulators... all potential options here with varying sizes, costs, efficiencies and the like.
Stepping up with DC voltage is harder. Boost convertors however have got pretty good in recent years so many will opt for that. You can also series wind batteries (makes management harder but still done all the time -- if you have ever used a battery powered hand tool then it will likely be this, also 95% of the time when the battery pack dies it is only one or two cells within it that have died because management hard) and get to the required or step down again if you want to go that path.
Ultimately then you fit rechargeable battery plus any necessary management in along with charger and any voltage management if you need to step it up or down to get to a suitable voltage. Fitting it and getting the charger port in there then tends to be the hard part and limits the size of battery you can wedge in there, or dictates how much of the case you slice out to get that bigger battery in there.
If the device has external power source that might charge an original battery then either put a diode in there to stop poorly handled reverse charging, figure out how to embrace it (free power and probably in a place that is nice to plug in on the original device), delete it (if the wires going to the charge port are cut...) or otherwise make sure some user you made promise up, down, sideways and on their pets that they would not do it do not in turn burn their house down when "I just thought".
In some newer devices you can get particularly unlucky and there will be a nice secret handshake that happens when you put the battery in and the management chip has a chat with the device proper. If so replicate this, disable this or otherwise cannibalise an existing chip (and hope it does not have a built in "I have been charged 1000 times and now I am declaring myself dead despite being nowhere close because marketing needs a "user has a too fat wallet and we want some of that'" deal, or sort that too).