What's the supply voltage? Do you have a breakdown of which parts of the board consume how much? I feel like one should be able to use way less than that, but there are plenty of ways to waste power on a board that isn't designed to be battery powered.
One project I've been thinking about recently (if I only I could find the time..) is to build a tiny server around a microcontroller coupled to a small NiMH pack (just standard AA/AAA rechargeable cells) and a small solar panel, running by the (not particularly well lit) window of my home office.
I don't have a breakdown, it was cobbled together from whatever was around, an STM32 devboard, some regulators, some half-dead li-fe-pos, almost useless BMS from ali, this sort of stuff. Obviously that ate significantly less than an RPi, but I can't say how much, don't actually remember since that wasn't the problem.
Another take was a weather station, basically an ESP on a ~2500mAh 18650 li-ion.
That lived for like a week on a charge, stuff being fetched from it every minute over wifi.
I would recommend against using NiMH for that, just get some 18650s and work them between 80-40% charge, they'll last forever.
One project I've been thinking about recently (if I only I could find the time..) is to build a tiny server around a microcontroller coupled to a small NiMH pack (just standard AA/AAA rechargeable cells) and a small solar panel, running by the (not particularly well lit) window of my home office.