This diagram is my attempt at showing how all the S-CAB components are to be connected. If you have, or could potentially have track power, you can use the locomotives track power pick-up method for connecting to the BPS. This would allow the BPS to charge the battery whenever it detects 6 volts or more of AC, DC, or DCC power. The DCC decoder is powered by the battery via the BPS (note: the latest version of the BPS also allows for bypassing the battery and having the locomotive be powered by available track power whenever the battery runs empty). The DCC decoder then controls the motor, lights, and sounds, as normal. The "off" momentary push-button shown in the upper-right corner was supplied with the older version of the BPS; the newer version has a second reed switch mounted on the BPS board itself (so, both the on and the off reed switches are on the board).
I have fully documented the installation of the S-CAB system with battery power in my three engines:
S-scale NW2 (installed December 2012)
S-scale RS-1 (installed April 2013)
S-scale FA-2 (installed September 2013)
If you do not feel comfortable doing your own electronics installations in your locomotives, there are people who can do it for you, for a fee. My recommendation is to first contact Neil Stanton. If he doesn't have the time to do it, he will be able to refer you to someone who is qualified to do it. However, if you have ever installed a DCC decoder yourself, especially a hard-wired one (as opposed to a simple plug-n-play one), this is only slightly more challenging. It is not so much the wiring (which is well-documented), but rather the fitting of the components that is the challenge. As you can see from my own installations above, a little creativity goes a long way.
Other installation examples:
Robert Langlois' Installation in a O-Scale 0-4-0 (PDF)
Neil Erickson's Exploring Dead Rail
Installing S-CAB in HO-scale
Installing S-CAB in HO-scale Plymouth Switcher
Installing S-CAB in HO-scale S4 Switcher
Installing S-CAB in N-scale
Stan Ferris' Duration Test