I installed dual batteries and a selector switch over the weekend in my 2002 1800 SR. This was actually much more simple than I had anticipated.
My boat came from the owner with one 24 series battery strapped into a 27 series tray. So i decided to buy a 27 series battery and a 24 series try. So I could have a proper tray and battery for each.
One 27 series interstate marine deep cycle battery was $90.
The 24 series battery tray was $5
Perko off-1-2-all battery selector switch was $40.
6ft of 2 gauge marine power wire from west marine $?.
6 2 gauge battery terminal ring ends $?.
I started by pulling out the existing battery, tray and master kill switch.
I unscrewed the blower fan as it needed to be slightly relocated.
I then configured the two tray's inside the battery area so they both fit snug. I felt the best fit was to have the 27 series against the back wall and the 24 series in front of it.
I screwed down the back tray first and then the front tray.
I placed both batteries in their trays.
I decided to mount my switch on the inside wall just below the master kill switch.
I made up two new positive wires that could reach from each battery to the slector switch. I used vise grips to secure the terminal ends to the 2 gauge wire.
I then took the remaining wire and wrapped it in black electrical tape and used it as my ground across each negative terminal.
I unhooked the bilge and stereo memory from the master kill switch and attached it to the battery selector switch common.
I also hooked the trim power wire to the selector switch common.
I decided to wire the 24 series battery as #1 and the 27 series battery as #2.
So with my master switch off my bilge pump, trim and stereo memory has power if needed. However there is no power to start the motor or any other electronics.
With my master switch on all power is available from Batt 1, 2 or both at 12v. So I can trim and start the motor on either battery and the alternator can charge either batt 1, 2 or both.
Some pics of the final install: