Battery Calibration
Last updated
Last updated
The battery connected to the Edge Controller is monitored by a process known as coloumn counting. In simple terms, we count the amps that go in and out of the battery, and by knowing the maximum capacity of the battery, we can accurate report the state of charge.
The current status of the battery can be seen on the main device overview page.
The batteries charging state is shown in the top left: Idle, Charging or Discharging. The amount of power going into or out of the battery is show in the top right. A positive number means the battery is currently discharging, and a negative number means it is charging.
The percentage state of charge is shown in the middle of the card. At the bottom we show the battery voltage and the current going into or out of the battery.
Due to measurement inaccuracies and temperature variation, it is not possible to track the batteries capacity with 100% accuracy. Therefore the Edge Controller has a mechanism to calibrate the battery to 100% when it knows the battery is fully charged. This process will continually happen and is expected behaviour.
The Edge Controller will automatically calibrate the batteries state of charge to 100% if the following criteria are true.
the battery voltage must be greater than 13.5V
the current going into the battery must be less than 2% of the batteries total amp hour capacity
mains power or auxiliary power must be connector or the solar charger must be in absorption or float mode
If the above criteria are true for more than 3 minutes, the battery will be assumed to be at 100%. A calibration event will be shown on the graph as a sudden jump to 100%.
The battery will sometimes have more capacity in it than we are expecting. This can happen for various reason described below. When this happens, the battery state of charge will go negative.
When the Edge Controller initially boots up, if the battery state of charge is negative (ie. -4%), it will be automatically set back to 0%.
We do this because we make the assumption that if the tower has a negative battery level when it boots up, it is likely that it was caused by the battery going flat. The sequence of events is likely:
The battery had slightly more capacity than expected, and it went negative (ie. we got more power out of it than we thought we could).
The battery then went flat and the Edge Controller shutdown (the low voltage cutoff kicked in).
The battery was then charged again (for instance, by solar the next morning). Eventually the voltage will increase enough for the Edge Controller to boot again.
At this point, the Edge Controller believes the battery is still at the same capacity as when it was shutdown (because the Edge Controller cannot monitor the battery whilst it is switched off). However, in reality, the battery has charged slightly before the Edge Controller booted (and it cannot possibly be negative).
Therefore the Edge Controller resets the battery capacity to 0% if it sees the capacity negative during the first boot.
The state of charge of the battery can be manually adjusted. This is useful if you have swapped or charged the batteries. To manually adjust the batteries, use the control on the device settings page:
When an automatic or user initiated calibration takes place, an event will be sent to the Giraffe API. This event can be viewed by filtering for 'Battery' events.
The event will show the calculated amp hour capacity prior to the update, the calculated amp hour capacity after the update and the configured total capacity of the battery.
Note that the values in the event log for calibration events are in amp hours rather than percentages.
You can tell whether the event was an automatic calibration or a manual calibration based on whether there is a user name next to it.
The state of charge estimation can never go over 100%, but it is possible for it to go into negative territory. If it goes negative, it essentially means that the battery delivered more capacity than the Edge Controller expected it to.
The most common reasons for this are:
the battery is misconfigured in the device definition. It is important that all devices with the same definition have the same battery capacity physically attached.
the battery is over performing compared to the manufacturers datasheet. This can happen with lithium chemistry batteries, where the manufacturer rates the capacity based on a higher rate of discharge. Batteries that are discharged very slowly will generally deliver more total energy.
the battery has been swapped when it was discharged, and the person who swapped the battery forgot to reset it to 100%. The system has no way of knowing the battery has been swapped.
the battery is being charged from a charge source that the Edge Controller is not able to measure. This can happen if a battery charger is connected directly to the battery terminals.
If the battery state of charge is - (undefined), it means that the battery has never been calibrated. This can only happen when an Edge Controller is new or has been reset.
In order to set the battery capacity and get the Edge Controller to start tracking it, you have a couple of options:
If you know the battery capacity (ie. that it's fully charged) you can manually set the capacity.
If you don't know the capacity, you can fully charge the battery and wait for it to calibrate at the top of it's charge.
