diff --git a/README.md b/README.md
index 184d1e7..28fbab0 100644
--- a/README.md
+++ b/README.md
@@ -3,12 +3,12 @@
 [![Greenkeeper badge](https://badges.greenkeeper.io/sbender9/signalk-derived-data.svg)](https://greenkeeper.io/)
 
 
-Generates deltas for values derived from  signalk values
+Generates deltas for values derived from SignalK values
 
 It currently calculates:
 
  * Outside air density (based on humidity, temperature and pressure)
- * Battery Power
+ * Battery Power and Time to Full / Time to Empty
  * Depth Below Keel (based on depth.belowSurface and design.draft.maximum)
  * Depth Below Keel (based on depth.belowTransducer and depth.transducerToKeel)
  * Depth Below Surface (based on depth.belowKeel and design.draft.maximum)
@@ -31,7 +31,7 @@ It currently calculates:
  * Moon illumination and times (based on time and navigation.position)
  * Sunlight Times: sunrise, sunriseEnd, goldenHourEnd, solarNoon, goldenHour, sunsetStart, sunset, dusk, nauticalDusk, night, nadir, nightEnd, nauticalDawn, dawn (based on time and navigation.position)
  * Outside Heat Index (based on temperature and humidity)
- * True Course Over Goround (based on courseOverGroundMagnetic and magneticVariation)
+ * True Course Over Ground (based on courseOverGroundMagnetic and magneticVariation)
  * Magnetic Course Over Ground (based on courseOverGroundTrue and magneticVariation)
 
 To add new calculations, just create a new file under the `./calcs` directory.
diff --git a/calcs/batteryTime.js b/calcs/batteryTime.js
new file mode 100644
index 0000000..0519301
--- /dev/null
+++ b/calcs/batteryTime.js
@@ -0,0 +1,75 @@
+const _ = require('lodash')
+
+module.exports = function (app, plugin) {
+  return plugin.batteries.map(instance => {
+    return {
+      group: 'electrical',
+      optionKey: 'batteryTime' + instance,
+      title: 'Battery ' + instance + ' Time to Full / Empty',
+      properties: {
+        ['capacity.' + instance]: {
+          type: 'number',
+          title: 'Capacity of battery bank in Wh',
+          default: 5120
+        },
+        ['socHigh.' + instance]: {
+          type: 'number',
+          title: 'State of Charge at Full (Percentage)',
+          default: 100
+        },
+        ['socLow.' + instance]: {
+          type: 'number',
+          title: 'State of Charge at Empty (Percentage)',
+          default: 20
+        }
+      },
+      derivedFrom: function () {
+        return [
+          'electrical.batteries.' + instance + '.power',
+          'electrical.batteries.' + instance + '.capacity.stateOfCharge'
+        ]
+      },
+      calculator: function (p, soc) {
+        let capacity = parseFloat(
+          plugin.properties.electrical['capacity.' + instance]
+        )
+        let socLow =
+          parseFloat(plugin.properties.electrical['socLow.' + instance]) / 100
+        let socHigh =
+          parseFloat(plugin.properties.electrical['socHigh.' + instance]) / 100
+
+        // app.debug('Capacity: ' + capacity)
+        // app.debug('SocLow: ' + socLow)
+        // app.debug('SocHigh: ' + socHigh)
+
+        output = []
+
+        // how long til full?
+        let time_to_full = 0
+        if (p > 0 && soc < 0.99) {
+          time_to_full = Math.round(capacity * 60 * 60 * (socHigh - soc) / p)
+        }
+        output.push({
+          path: 'electrical.batteries.' + instance + '.capacity.timeToFull',
+          value: time_to_full
+        })
+
+        // how long til empty?
+        let soc_remaining = soc - socLow
+        let time_to_empty = 0
+        if (soc_remaining > 0 && p < 0) {
+          p = Math.abs(p)
+          time_to_empty = Math.round(capacity * 60 * 60 * soc_remaining / p)
+        }
+        output.push({
+          path: 'electrical.batteries.' + instance + '.capacity.timeToEmpty',
+          value: time_to_empty
+        })
+
+        // app.debug(output)
+
+        return output
+      }
+    }
+  })
+}