Volts of the battery need to match the Volts of the device - 12v device needs 12v battery.
To figure out what you need, you need to find out the "draw" of your device, or how many amps it uses. If your device has a draw of 100amps, a 100amp/hour battery will power it at full voltage for 1hour.
In a typical case, the Tascam HD-P2 has a "Power Consumption" of 6W (watts). 6W = 12V x ___Amps, or 0.5Amps, or 500mA.
A battery pack like this one:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4011 will power the HD-P2 for 14hours (7000mAh/500mA = 14h).
What are you trying to power???
Terry