Esp8266 with RTC and battery measurement

STARTING SITUATION: Gas boiler with 2 rechargeable batteries type D GOALS: - replacing batteries with a (lithium?) battery pack and connect it to a solar panel. - monitor battery status and send data on thingspeak every 24h - solar charging status monitor (not necessary for now) HW for the project ESP8266 in deepsleep(0) that will be woke up with RTC - push notifications on the cellphone for battery low - OTA (will be the same for other sensors) - vcc + battery % + wifi % NOTE: Boiler and ESP have to be same battery in order to simplify the system