As has been said before, boosters and cells are not the same thing.
Shield boosters are installed in Utility slots and multiply your maximal shield strength by a factor. If you want to see the effects I suggest you play around with different configurations at
http://www.edshipyard.com/ . For example the standard 2E shield of the sidey has 52MJ shield strength. If you install an A0 shield booster the strength goes up to 62,40 MJ. You can install multiple shield boosters and the effects stack.
Shield cells are installed in internal compartment slots. Think of shield cells as "health potions" for your shields. You have a number of charges and can activate them (via fire group or hotkey). Each charge restores a fixed MJ value of your shields. But they take time to activate (about 5 sec before they kick in) and they are useless if your shields are down.. or go down before the cells take effect. Someone recently made some tests to see how much MJ which shield cell replenishes.. need to go look for the thread.
Edit: found it:
https://forums.frontier.co.uk/showthread.php?t=137188