It seems to me that there are two different ways to look at battery bank size:
- How big is the greatest load?
- How much depth of discharge are we willing to tolerate?
How big is the greatest Load?
The first concept to grasp is that Lithium Batteries have a recommended continuous discharge current and a maximum continuous discharge current and that the batteries should if possible be sized so that the recommended continuous discharge current is not exceeded on a regular basis. This is to preserve the life of the battery. As you can see in the table below for Victron 12.8 Volt batteries the maximum continuous discharge current is twice the recommended continuous discharge current.
In the table above if we were to plan on having a continuous current of 300 Amps (which is the max current of a 12 volt MultiPlus inverter/charger) then we should plan on having a battery bank of 300 Amp Hours. This is just common sense of course, the 300 Amp load would run the battery from full to empty in one hour, which seems to be an appropriate minimum size for a battery.
How much depth of discharge are we willing to tolerate?
The next concept to grasp is the like conventional batteries, Lithium Ion batteries that are discharged more deeply have a shorter lifespan. In this table below you can see the variation of depth of discharge with cycle life.
Discharging a battery to 50% makes it last 5000 cycles but discharging a battery to 80% makes it only last 2500 cycles, This may end up being the most important attribute to selecting a battery size. I go into the cost implications of this in more depth in my Blog post on price comparisons for battery types but the summary goes like this: If you have a bigger battery bank it costs more up front but the lifetime cost is less. If you have a smaller bank it costs less up front but wont last as long and will cost more per cycle of use.
One way of looking at this is to consider is lifespan of the project. How long will you own your boat or RV. Will you ever get to do 2500 cycles. At one cycle per day that is more than 9 years off the grid. Do you really need that much lifespan?
There is no one answer to how much batteries you need, you have to make the call yourself. My personal feeling is that part of the advantage of Lithium Batteries is their reduced size and weight so for my limited budget I would prefer the lower up front cost, which would mean a shorter life, but in the meantime would give me the most benefits of reduced size and weight. Either way the performance is the same because unlike conventional batteries Lithium Batteries deliver power in a similar fashion throughout their capacity range.