Device Battery Saver

/*
This function checks the battery level of a device and turns on/off a
corresponding charger in order to maintain optimal battery life.
The function relies on a get entities node set to find the naming convention of each device to get the
battery level and outputs to an empty service call node.
*/

// High-low battery range
const high_battery = 90  //Stop charging at or above this level
const low_battery = 30  //Start charging at or below this level

/*
These are the variables used to transform the tablet's entity ID
into the charger's entity ID. The two entity names need to be
relatively similiar. For example,
sensor.fire_living_room_battery_level
switch.fire_living_room_charger
*/
const domain_from = "sensor"
const domain_to = "switch"
const name_segment_from = "battery_level"
const name_segment_to = "charger"

//Get charger name and status
const charger_entity = msg.payload.entity_id.replace(domain_from, domain_to).replace(name_segment_from, name_segment_to);
const ha = global.get('homeassistant').homeAssistant.states[charger_entity]
const charger_state = ha.state
const friendly_name = ha.attributes["friendly_name"]
const battery_level = parseInt(msg.payload.state)
var srv = ""
node.status({ text: "No actions at this time." });

//Tablet is not available
if (msg.payload.state == "unavailable") {
    return
}

//Tablet is being charged
if (battery_level <= high_battery && battery_level >= low_battery && charger_state === "on") {
    return
}

//Tablet is on battery and does not need to be charged
if (battery_level <= high_battery && battery_level >= low_battery && charger_state === "off") {
    return
}

//Turn off charger
if (battery_level >= high_battery) {
    if (charger_state === "on") {
        srv = "off";
    } else {
        return
    }
}

//Turn on charger
if (battery_level <= low_battery) {
    if (charger_state === "off") {
        srv = "on";
    } else {
        return
    }
}

//Catches unnecessary service calls
if (charger_state === "on" && srv === "on") { return }
if (charger_state === "off" && srv === "off") { return }

node.status({ text: friendly_name + " turned " + srv + "(" + battery_level + ")" });
msg.payload = {domain: "switch", service: "turn_" + srv, data: {
    entity_id: charger_entity,
    }
};
return msg;