Superbender has a constant problem with his camper bus batteries. Normally camper is left over the winter and so batteries dies without proper care. In camper there are two batteries – main starting and auxiliary. So he decided to build a dual battery charger to keep them alive over the winter season.
He already had a 15V 5A power supply which seemed to be just right. Since batteries will be connected to charger over all winter time, charging current doesn’t play a significant role. Charging circuit is pretty simple. Control is based on Attiny85 microcontroller which controls both charging channels. Two ADC channels are used to read voltages of both batteries Resistor dividers scale voltage down to microcontroller level. Charging circuit is controlled by transistors that are optoisolated from microcontroller. Battery charger works in charging and monitoring mode. It compares voltages of both batteries to upper 14V and lover 12V thresholds. When lower limit is reached – charging of this battery begins, when it’s been charged to 14V, then charging is stopped. During long time, normally there will be one battery charged overtime.