in a modified underfloor compartment next to the standard 4D battery box near the aft head.The water pumps that normally go down there are moved elsewhere. Most of us don't seem to order the 3rd battery (I wasn't even aware of it), and quickly find out that we should have. The best place for it then is usually under the aft bunk, on the starboard side. I've heard of a few going into the compartment next to the bilge (port side), but you've got to add a lift up floor section or dis-assemble the large screw down floor panel each time you want to check it.
I took the easy way out and used 5200 to bond one foot long wooden wedges to the curved hull under the bunk. Didn't want to drill into the outer hull (for an obvious reason) and also wanted the battery to be nearly horizontal - the 2 inch high wedges are tapered to almost zero at the back. A battery case (for a size 24 battery) was then screwed to the wedges, along with the strap to keep the lid closed. A larger case will fit, but you don't really need it. An Exide 1,000 MCA marine starting battery went into the case. 1,000 marine cranking amps should be more than enough to turn the Westerbeke over many times. It will never do anything else.
There are a number of ways of wiring something like this up. Some recommend a bunch of single throw switches to put each of the batteries in "line", eliminating the 3-way multi-switch installed at the factory. Others prefer to add a 2nd 3-way switch. I single hand a lot and did not want to run below to throw switches after the starter battery was charged to put the house banks on-line - I needed something that could do it on it's own. A battery combiner was my best way out - diodes tend to introduce a voltage drop. Heart Interface has a new "PathMaker" battery combiner in several flavors - 2 and 3 cells, 70, 100 and 200 amps. The unit is very interesting - you can control the low disconnect voltage, connect voltage and high disconnect voltage levels. In effect, it is a voltage regulator which can automatically put all of the cells in parallel, separate them, or totally disconnect them based on whether there is a charging condition present. I opted for the 3 cell 100 amp unit, which incidentally, can handle over 400 amp loads for short periods and installed it in the engine box near the existing 3-way switch. The photo shows the unit snuggled in there. A switch on the unit can force a disconnect, connect or "automatic" mode - and there is a remote control unit available which can do this from your nav table and show you what is happening down there. It connects to the combiner with a supplied phone wire. What will they think of next?To wire this up, I connected the new starter battery directly to the starter motor, and the wire connecting the "common" terminal of the 3-way switch (with the starter) was moved to one of the relays on the combiner, in effect connecting the starter battery with the combiner. The other relays are connected to "Battery 1" and "Battery 2" on the multi-switch. Because it is possible to have starter loads (300 amps or more) coming though this switch, use #2 battery cable or better on all of these leads. These are all RED, positive or "HOT" leads - you've also got to run a ground (black) wire from the battery to the engine. Take a look at the before and after schematics. Note that the "house" banks are NOT directly connected to the starter, and that the starter battery is NOT connected to the house loads. I can force this condition in an emergency by switching the combiner to "ON" to start the engine from all 3 cells. Speaking of house banks, I am still plural. I did not want to run the house cells as one bank as some experts suggest. My rationale was that any deep cell battery has a finite number of discharge / charge cycles. If always combined, I'd get maybe 500 cycles out of the pair. I've always done a lot of day sailing and short trips where either cell can easily handle the required load for 4 - 5 hours, and by switching back and forth I'll get 1,000 cycles out of the 2 banks. Sounds like double the life to me? If I do cruise around for much longer periods, moving the 3-way switch to "both" will take care of that. The factory 3 battery setup divides the system into starter battery as Battery 1 and the 2 4D cells as Battery 2, or visa versa. You may want to install a combiner in a similar manner to the above for more flexibility.
The other part of the project was to install something at the nav station that would monitor the 3 batteries for me. Most of the fancier products on the marker (Heart Link 10, 20, etc.) can do 1 or 2 cells at best. Guest is now making a "System Voltage Scanner" (SVS) which can handle 4 batteries and / or loads, scans them one at a time, and displays the results on an LCD graphic bar display along with a "fault" LCD. It's also about 1/3 the price of the other units too. The last photo has the Guest SVS on top and the Heart Interface Combiner remote on the bottom of the "stereo" panel above the nav station.Some tips - you'll have to lift several sections of floor, so have a power screwdriver handy. I lost count at about 100. It will be much easier to snake the cables through if you have a long thin wire to poke or "fish" with. The system works pretty good. When in "automatic", the relay's snap the batteries together about 2 - 3 seconds after the alternator or charger cuts in and bumps the voltage up past 13.3. When the charging device goes away, they disconnect about 1 minute later. It takes that long for the voltage to drop back down to the 12.9 range. All of this is totally adjustable in case you've got gel-cells or other types of batteries, or have different preferences for the connection levels. The voltage scanner lights up like a christmas tree as all of this happens - automatically scanning all 3 battery levels. All I do is sail the boat.