A resitor will only blow your device .. The Raspberry current may change, and thus the voltage over the resitor.. Please do not attempt this!! A quick fix qould be to use an al cheapo LM317 (on a headsink) and two resitors.. just download from Fairchild /On Semi or Digikey a data sheet and build the regulator. Be warned the the LM317 will disipate about 7 to 8 Watts and shall be very hot, thus buy a large heatsink. Heatsinks does have a degrees per watt value, thus a 7deg/Watt will reach 7*8+25 ==> very hot, so go for a 1,5 to 2 Degree/Watt heatsink.
Calculate the resitor values for 5.1V, (R2 = 220ohm (between out and adjust pins (votage accross=1.25v), R1 = 680ohm to ground wil deliver 5.11V (fine);
NS A lead acid battery if fully charged , could be between 13.6 and 14.5V, so it is not 12V at all.