In this photo you can see how the charger is feeding the bank. With banks in parallel or series parallel it is important that the charger supplies it current across the bank. If you look you'll see that the positive feed and the negative return pull off opposite sides of the battery bank.
Wiring this way forces the current to flow through the entire bank and helps to minimize any intrabank imbalances. This is one of the most often violated rules of charging I witness on boats. It is important to note that this is not just for charging sources such as chargers, alternators, wind or solar but also for the DC loads. Always connect across your bank to keep intrabank imbalances to a minimum. As banks get larger there are more precise ways of wiring that can lead to better balancing but doing it this was gets you a lot further ahead than pulling everything off one end.
You will also note the three bussed ANL fuses on the left which protect the ALTERNATOR, HOUSE BANK and CHARGER wiring. The ABYC requires that any device connected directly to a battery be fused within 7" of the + battery post to protect the wiring. These fuses are not intended to protect the devices but rather the wiring in the case of a dead short to ground. While the 7" rule is often very tough to meet always try to get the fuses as close as you can to the battery + post. The wire marked HOUSE BANK + FEED is about 16" long but runs in a conduit for about 9", under the quarter berth, then comes out at the fuse.
Because the alternator and charger do not use the same size wiring as the house bank feed to the battery switch, they each need their own fuse to protect the wire.
When you parallel banks you add or combine the amperage as in Ah's, cranking amps or short circuit amperage. These group 31 wet cell batteries can pump out in excess of 1200 cranking amps at 70F. For just two of these batteries that is 2400+ amps of cranking current at 70F. The short circuit current is always slightly higher than CA at 70F. Marine Cranking Amps (MCA) are rated at 32F and Cold Cranking Amps (CCA) are rated at 0F. As temps drop you have less available cranking amps and as the temp climbs the more amps you have to fry things. 2400 amps is enough to weld metal with and that is not even the short circuit rating.
I have a customer with 4 Odyssey thin plate pure lead (TPPL) batteries with over 20,000 short circuit amps as a bank. If you short this bank the wires insulation will catch fire in mere seconds. Always fuse devices connected directly to the battery and fuse it for the WIRES ampacity rating.