The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of usable energy it can store at a nominal voltage. Storage capacity is additive when batteries are wired in parallel but not if they are wired 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 with fewer cells (and hence lower voltage) in series can provide the same storage capacity yet be portable.
Two 6V, 100Ah batteries wired in Parallel will have a total storage capacity of 200Ah at 6V (or 1200 Watt-hours)
Battery banks wired in Series-Parallel are even more complicated. Here, four 6V cells are wired in two "strings" of 12VDC that were wired in parallel. Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah at 12V or 2,400Wh.
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Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down and increased as temperature goes up
This is why your boat battery dies on a cold winter morning, even though it worked fine the previous afternoon
If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature -25 degrees C (about 77 F)
Battery TIPS
Stay with one battery chemistry (flooded, gel, or AGM) Each battery type requires specific charging voltages. Mixing battery types can result in under or over-charging. This may mean replacing all batteries on board at the same time.
Keep batteries clean, cool, and dry.
Check terminal connectors regularly to avoid loss of conductivity
Add battery electrolytes to flooded lead acid batteries when needed. Keep them charged.
Clean corrosion with a paste of baking soda and water.
In the following video, you can expand the information on this topic.
Get here the Marine Electricity Handbook where you can find all the information on this and many other topics.
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 with fewer cells (and hence lower voltage) in series can provide the same storage capacity yet be portable. 
Battery banks wired in Series-Parallel are even more complicated. Here, four 6V cells are wired in two "strings" of 12VDC that were wired in parallel. Using 6V, 100Ah batteries, this system will have a storage capacity of 200Ah at 12V or 2,400Wh.
