This project doesn’t use a microcontroller, nor even the slightest specialized integrated circuit (IC). In spite of this, it will look after your batteries, especially if you are reasonable about the charging time. Whether they are old nicads (NiCd) – becoming extinct these days because of their many shortcomings and toxicity – or the omnipresent nickel-metal-hydrides (NiMH), these types of battery have to be charged at constant current.