Trying to wrap my head around the dual battery setups used and the pros and cons.

1. BEP Supported Option: http://www.bepmarine.com/Single-Engi...-180-1460.html Includes Voltage Sensitive Relay and 4 Position Battery Switch(1, 2, Both, and Off) All load is wired to the COM terminal on the Switch which I think means all load and charging is passed to the sensed battery at all times. When the volts get high enough on the sense battery the relay is closed and charging is passed to both batteries.
The switch in this option appears to have 2 purposes:
A: Change the sensed battery which means the load and charging goes to that battery selected first and once volts are above the set limit then both batteries.
B: In the event that you run down the sensed battery while sitting(not charging the batteries due to engine running) you can turn the switch to the other battery or the both setting and start the engine(the new battery is the sensed battery the VSR will not charge the other battery until volts reach the level set on the relay)

2. Factory Option: Same device as the first but load is split between batteries (example: starter and engine on first battery and stereo and accessories placed on 2nd battery) and only the charging is connected to the COM terminal of the VSR. So in this option the Switch and VSR operate the same but the difference is the load does not move based upon the switch setting of 1 or 2. If you run down your second battery using the stereo and accessories while the engine is off then there is no need to turn the switch setting, just start the engine and Once running the VSR will run and once volts are high enough charge the stereo/accessories battery.

3. Another Option: http://bluesea.com/category/2/productline/overview/329 Includes Automatic Charging Relay and 3 Position Switch (Off, On, Combined) In this setup you would place starting load and charging on the Starting Battery and Stereo or Accessory load on the 2nd battery. ACR monitors the starting battery similar to the VSR and allows charging to the 2nd battery when the starting battery reaches a certain voltage threshold.
The switch in this case operates in the on position and send both respective loads only to the respective battery. When in the combined position both batteries are fully combined for emergency start or other purposes.

Considering these options that I have seen I would like some feedback as to the pros and cons of them.

Also, I wonder in option 1 and 3 what are the pros to using the switch in either set up(briefly is appears the ability to combine both batteries for emergency use without moving cables for both and for 1 it appears you could change the sense battery but I think the VSR would sense both batteries without the switch due to the Dual Sense feature (see more here: http://www.bepmarine.com/store/web/C...-125A_Inst.pdf).