Open Software Solutions

Source: Microsoft Support

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Consider the following scenario. You have two battery modules attached to the same computer and keep detaching and attaching one of them. You find that the battery status is randomly reported incorrectly in some programs, such as Windows Mobility Center.

 

This discrepancy is due to how battery capacity is monitored and the timing of queries relative to the timing of the battery detachment/attachment cycles.

 

The battery percentage is calculated by using the following method:

 

(Current capacity of first battery + current capacity of second battery)/(full capacity of first battery + full capacity of second battery)

 

When the problem occurs, the battery percentage displayed on UI is calculated by using the following incorrect method:

 

(Current capacity of first battery)/(full capacity of first battery + full capacity of second battery)

 

If the first battery is 90% charged and the second battery is also 90% charged and the full capacity of these two batteries is the same, when you detach the second battery and then attach it again, the battery percentage displayed on UI might be calculated as 90/(100+100) =  45%.

 

 

You can work around this problem by waiting a few seconds for the battery percentage to be updated automatically.

In Windows Vista, the full charged battery capacity and the current battery capacity are stored in two different structures inside the system kernel and are maintained by two different approaches. The full charged battery capacity is updated in real time, but there might be a short delay when updating the current battery capacity. Due to this, sometimes when you attach the second battery, the full charged battery capacity will be updated immediately to include two batteries, but the current battery capacity might not be updated immediately. As a result, the battery percentage displayed might be incorrect.

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