So yesterday I modded a GBP with an IPS screen from Funny Playing and noticed some flickering of the screen. I can't really tell where the flickering comes from. Sometimes it's there when I start a game but will stop after some time. And sometimes I start a game and there is no flickering. But either way, the flickering is always gone after
Since you said the green light flickers and nothing else it sounds like a battery issue to me. Like the battery is completely dead. Its not uncommon for micros and gba sps since their batteries are almost 20 years old at this point. I suggest getting a new battery or perhaps a cell battery replacement.
101 gba screen keeps flickering and having horzontal moving lines. So yeah the screen i added works fine, colors are nice, but the screen keeps flickering and the last few days more and more horizontal lines keep apearing. Those are not dead pixels but just lines that move up like on those old tvs as viewed from a camera.
It doesn't look like it, but the EZ flash carts require more power than a normal cart for boot up. It's common to need to turn the brightness down when powering up a game. Also, your battery light is red instead of green, indicating the batteries are low to begin with, which, coupled with the need for more power, is likely your main problem

- Try putting in a new battery, or try with someone else’s battery first to see if their battery will work in your GBA SP. Then you’ll know for-sure that you just need a cheap battery replacement bought online.

So I made a u-turn and came back home. I noticed the light goes out completely at idle. Nothing. It only flickers while driving. With the engine running I yanked out my multimeter real quick and checked the power at both battery terminals. Both showed 13.95 volts. Checked at the alternator, showed 14.12 volts at idle.

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gba sp battery light flickering