The question was "additional" not replacement.
The battery is built in and cannot be replaced
No...it doesn't work
The valve stem adapter for toys is loose under the cover on top of the pump.
No, the battery is not replaceable, and the unit can fill about 3 average tires on one charge, however a very long charging cord is included. Suggestion, plug this cord in while the unit is still fully charged as you can use it while it charges and this will extend the run time.
Yes, this item comes with a rechargeable battery.
This unit is an air compressor. The most efficient way to inflate an air mattress is with a high volume Inflator. Air compressors inflate pressurized items like tires and sports balls. High Volume inflators fill air mattresses pool floats and other large volume low pressure items holding air.
There are many conditions that will contribute to how quickly the tire can be inflated, from ambient temperature to battery charge level and many other factors. However in optimum conditions a typical vehicle tire could take 5 to 10 minutes to fully inflate. Please be sure to read the instruction manual before use.
The maximum PSI is 120.
Yes, this item has a DC power adapter.