Why wouldn't you keep them at maximum capacity all the time?
If your neighborhood only needs 50MW of power, but you want 1000 MWhr of storage (~20 hours at full power), a Redox flow battery would be able to do that. Or if you were building a Hospital (Lets say 5MW of power) and need to spec out 3-days of power (ie: 360 MWhrs of storage on 5MW Power), you just build a tank that can store a lot of the electrolyte.
I guess... the best way of putting it is that Lithium Ion batteries are like AAA batteries, while a Redox Flow battery could be configured as a larger D battery. D Batteries don't give any more power or voltage than AAA batteries, but are more efficient for energy storage.
It is very easy to imagine how to build a 3000MW-hr 50MW Redox Flow battery (just build a huge storage tank). It is almost impossible to imagine the same with Lithium Ion (I guess you can build massive Li-Ion cells, but note that the Tesla is powered by tiny 3.7V 18650 cells wired up together. So it really hasn't been done before)
Day/Night cycle storage is a low power (Megawatts) high-storage (Megawatt-hours) use case. Lithium Ion batteries are more useful for high-power / medium-storage situations (like frequency balancing, or maybe a couple hours-long storage).
Flywheels are exactly the opposite. They are high-power low-storage devices. Flywheels would be very useful for smoothing out power when birds / clouds come up in front of solar panels (for example). Its easy to build a 300MW flywheel with almost no capacity (only a few minutes of storage). Flywheels could be used when a power-source goes offline suddenly, while a few minutes need to be spent spinning up a new power-source. (ex: Main grid goes down, you need 30 seconds before your diesel generator fully turns on. The flywheel will keep very high-power for the next 2 minutes while things switch over). Although supercapacitors are also a promising technology for this problem.
Different storage technologies for different purposes.
http://www.npr.org/sections/parallels/2014/09/17/349223674/t...
Redox Flow batteries can be pumped into storage tanks and distributed (theoretically anyway). In contrast, the entire set off Lithium Ion battery needs to be connected to the grid to take advantage of its storage, you can "pump" Redox Flow batteries into storage tanks to increase capacity without increasing power.
For day/night cycle storage (or longer), I'd expect Redox Flow to be more cost efficient than Lithium Ion, despite LiIon having numerous advantages (density / power / etc. etc.). Mainly because you can save on "Power Output" and instead build larger and larger tanks to store the charged electrolyte.
LiIons get more and more expensive ($$$ per KW-hr storage) the "bigger" they get. Redox Flow batteries achieve scalability (cheaper $$$ per KW-hr the bigger they get). I do realize I'm imprecisely using the word "bigger", but I hope yall will forgive me on that :-)
My personal favorites:
1. Pumped Hydro -- Old technology, but a favorite of mine. Very simple design, just take advantage of lakes that already exist in your vicinity and use the water in the lake as a gravity storage device.
2. Flywheels -- Store energy by spinning a heavy object. Spin-spin-spin... spinning and gyroscopes are cool.
3. Vanadium Redox Flow Battery -- Liquid batteries, how cool is that?