DIY BMS or do it yourself battery management system.
I really want to move to Lithium iron phosphate batteries onboard (not gonna explain the pros and cons) and drop-in solutions are overpriced and do have issues anyway. Also it needs a lot of different controll compared to lead acid batteries. So just not to be too bored with volunteered isolation while moving between countries I found out about DIY BMS (You can find a lot of details on youtube and while googling). People are using it mainly for powerwalls but I thought why not to give a try for a boat as well?
Another challenge was to install something like ammeter. As I was completely new at Arduino world, I made some mistakes and bought not the most suited chips, and had some try and fail days when connecting resistors and capacitors and reprogramming (can I say firmware? :) So here it is - Arduino Nano sending Amps from ADS1115 through EPS8266 wifi chip to Raspberry PI.
And there you see the result - tiny 12V 12Ah battery bank! (each cell is 6000mAh). It still needs proper wiring and fusing and a voltmeter for total bank voltage. Also I really want to install MPU9250 gyro sensor to send more data to the autopilot onboard. But it seems that Sail Map app needs more of my attention for now.