Spare current is a method of battery charging in portable electronics which maintains a constant charging current to the battery independently of device usage, by additionally drawing the current needed to power the other components from the power supply, rather than subtracting it from the power arriving at the devices battery.
The benefit of spare current changing is enabling the user to utilize the device without reducing the charging power arriving at the battery. In contrary, some other mobile phones even throttle the total (battery + components) power throughput when used while charging, which discourages usage of the device during charging.
However, spare current charging demands a power supply that is able to cover the power demands for both the battery and the device's components simultaneously. If the power supply's performance is exhausted, the device will subtract power arriving at the battery supply the components with power needed to run at the highest speed.
This exemplary bar chart represents where in the device the electrical power arrives with different charging modes.
In this example, the battery's highest charging speed is 10 watts, and the device's momentary power consumption is 2 watts.
- Device unused
- In stand-by mode or powered off while charging.
- Spare current
- Power needed for device components adds to the total throughput.
- Current subtraction
- Power needed for device components becomes subtracted from the total power throughput.
- Current subtraction with throttled total power throughput.
- Side-by-side comparison
Spare current charging is supported by most, if not all laptop computers released since the early 2000s.
For many laptop computers, manufacturers offer a higher performance power supply (e.g. 90W instead of 65W for mid-range laptop computers) and a larger battery variant which is able to handle more charging power.
The additional performance offered by the stronger power supply variant (90W on mid-range laptops) is usually able to supply the laptop's components with sufficient power during high processing speeds while still being able to supply the laptop's battery at its highest supported charging speed simultaneously.
The normal power supply variant (65W on mid-range laptops) may not be able to maintain the full battery charging speed while other components are demanding hugh wattages.
Some mobile phones such as the UleFone Armor 2 utilise the spare current charging method while other mobile phones subtract the power needed for the device's components from the charging current.
While the battery is being filled, the charging current declines because the idle voltage (which the cell would have without current flow) comes closer to the supplied terminal voltage, making it demand less power and allowing the cleared wattage demand to be allocated to the device's components while still charging at the highest momentarily supported wattage of the battery.
Other mobile phones such as the Samsung Galaxy S7 (see Samsung Galaxy S7 charging behaviour) not only subtract the power needed for the components from the total power throughput, but significantly reduce the total power throughput.