Re: Firefly batteries
The drawing is correct, and generates 24 volts. A full explanation is here: https://fetchinketch.net/boat_thoughts/battery-connections/(opens in a new tab)
The common connection across the "middle" of the bank allows a single battery balancer to keep the batteries high and low at the same voltage during charging. It also makes certain that load and charging current is evenly shared between every battery in the system. It also allows us to easily measure and detect a single badly performing battery by monitoring the voltages of the upper and lower halves of the bank. https://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf
In any multiple battery system the important thing is to be sure that the batteries share the load and the charging current and voltages as identically as possible. If this is not done, over time some batteries are chronically undercharged, and others are overcharged. This will shorten the lifespan of the bank as a whole, and increases the chance of a "shorted" battery to kill all the others.
For batteries like flooded lead acid that are regularly equalized this this isn't so much an issue, the equalization process balances the batteries. Gel and AGM batteries typically are NOT equalized, and therefore need more attention to this. It is possible that the regular "restoration charges" that Firefly recommends (and that we do) fully addresses this issue, but the addition of the balancer gives me an extra level of comfort that everything is charging evenly. Our measurements support that this has been effective.
It's useful to remember that with the system as installed by Amel, batteries were expect to be replaced OFTEN. In the Super Maramu owner's manual Amel says that for a boat that spends most of its time at anchor the lifespan of the batteries "will probably not exceed 18 months." As battery technology has improved, including much more sophisticated charging systems that let us keep our batteries closer to full more of the time, they last a lot longer.
If you install inexpensive batteries and change them often, you can get away with systems that are simpler than if you use more expensive batteries and more careful controls. Neither choice is better than the other, they are different and suit different styles of boat use and owner involvement in the details.