You can't put 7812 when device is powered from accumulator. Because 7812 have voltage drop about 2V, so you need at least 14V on input to get 12Voutput, and 7812 is only 1A rated in to220 package, in to3 is 3A rated.
If you need all ways 12V on output, use buck/bust regulator, or use boost to get 15V and then use buck regulator to get 12V from 15V. This way your input voltage may be from 5-6V to 30-40V, depending on device. You also have simular PSU on ebay
http://www.ebay.com/itm/DC-Boost-Buc...item5af2b8bf04