I have a MTH Challenger 4-6-6-4 cab 5148 which is a PS 2 and a MTH Big Boy 4004 which is also a PS 2.  They are both G scale.  I have opened the Bid Boy tender where the speaker is, but there is no battery.  I have not opened the Challenger.  Where is the battery located on these two engines and is it hard to change out with the BCR unit?