In order to install 4@6V batteries to provide 12V with double the capacity of one of them, you would need to install them as shown in the final drawing (series/parallel). Was the starter battery in one bay and the house batteries in the other bay or are there still two separate bays with cables
laying in there? Do any of the cables
go from one bay to the other or do you have a red and black running to each bay from a copper bus bar or terminal block near the alternator
? Is the engine alternator
the sole source of charging for the batteries at the moment?
The following excerpt is from How Lead Acid Batteries Work
Most marine, automotive, and RV applications use 12V DC. You have the choice to either buy a 12V battery or to create a 12V system by wiring
several lower-voltage batteries/cells in Series
When two 6V, 100Ah batteries are wired in Series
, the voltage is doubled but the amp-hour capacity remains 100Ah (Total Power = 1200 Watt-hours).
You may decide to wire batteries in series because a single
12V battery with the right storage
capacity is simply too heavy, unwieldy, or awkward to lift
into place. Batteries consisting of fewer cells (and hence lower voltage) in series can provide the same storage
capacity yet be portable. It is not unusual to see solar power installations where the battery bank consists of a sea of 2V batteries that have been wired in series.
Two 6V, 100Ah batteries wired in Parallel
will have a total storage capacity of 200Ah at 6V (or 1200 Watt-hours).
Battery banks consisting of 12V batteries wired in parallel are often seen on OEM installations in boats and RVs alike. Such banks are simple to wire up and require a minimum of cabling. However, the wiring must have the capacity to deal with a full battery bank.
You should fuse each battery individually in such a bank to ensure that a battery gone bad will not affect the rest of the bank.
Battery banks wired in Series-Parallel
are even more complicated. Here, four 6V cells are wired in two "strings" of 12VDC that were then wired in parallel. Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah at 12V or 2,400Wh.
Since such a system has more wiring, it is very important to group "strings" logically and to label everything. Furthermore, it is a very good idea to fuse every "string" of series-wired batteries to ensure that a problem in one part of the battery bank does not take the whole bank down.