This time, module tiercide is occurring on shield power relays, shield rechargers and shield flux coils.
There are continuing issues with these three modules, because you only ever use the shield power relays (SPR's).In essence, the way passive tanking works is to create as large a pool of hitpoints as possible, then create a reasonably large recharge rate, and have decent EHP per second tank.
Therefore, the key is to distinguish between raw hitpoints per second (RHP) and effective hitpoints per second (EHP). You can have high EHP/s and RHP, or both can be low (ie; a piss-poor passive tank).
The influence of sig radius on passive tanks is also of concern, obviously, given the sig radius drawback of rigs, and is mentioned in passing in reference to T3 destroyers which get a sig bonus. You may have decent EHP/s tank, but if your sig radius is gigantic (as it will be) you cop larger weapon damage a lot and low raw hitpoints risk being alpha'd off.
There are therefore six module types which can create good passive tanks, if you include power diagnostics, all types of resistance amplifiers, and shield extenders.
Extenders (and extender rigs) just increase raw hitpoints. Pretty obvious. You use this to increase buffer and hitpoint pool, and keep recharge rate constant, thus increasing RHP and EHP tank.
Power diagnostics can be used to help with passive tanking, given they increase shield hitpoints and recharge, but also because their capacitor bonuses offset negatives gained from SPR's. For large enough pools of shield hitpoints, PDS's themselves can effectively replace SPR's. PDS's can therefore have a massive effect on the efficiency of buffer tanked ships - say, Rattlesnakes and the like.
Resistance modifying modules and rigs, obviously, increase resistance in the shield. So, for a given pool of shield hitpoints, with a fixed recharge rate of X hp/s, if you increase the shield resistance enough the effective hitpoints per second becomes an efficient EHP passive tank for a low RHP.
Rechargers give you a smaller, flat percentage bonus to shield recharge rate (ie; reduces the recharge time and hence, your pool of shield hitpoints regenerates faster) but you are giving up a midslot which could be used to increase hitpoints via a shield extender, increase effective hitpoints via better resists thereby making a smaller recharge rate more effective than it otherwise would, or fit utility modules (scram, web, etc).
Purger rigs are worth a mention, as they increase recharge rate at the expense of signature radius. This is nominally not a problem outside of, sometimes, Pulsar wormholes, where you stack extra sig radius onto your ship. For small ships it's worth considering the sig radius with passive fits might end up being so gigantic it counteracts the benefits entirely - especially with MWD's on.
Shield flux coils nerf your overall buffer but increase recharge rate.So you lose raw and effective hitpoints, but gain back recharge rate. The nett result is a nett benefit to recharge rate.
Shield power relays nerf your capacitor recharge rate (ie; make it slower to regen) but increase shield recharge rate (shorten).
Now, the whole passive tanking philosophy is one of trade-offs. The only real modules to use, unfortunately, are the SPR's, PDS's, extenders and extender rigs, and resistance modifiers.
The reason you don't use a recharger in almost any fit which isn't utterly CPU constrained, is that you are trading off either a resist buff or hitpoint buff in order to get higher recharge per second. here's why;
Assume you have 10,000 raw hitpoints and 600s recharge, with 0% across the board resists, and are deciding on an invul field (eg; 30% resist bonus for simplicity's sakes) or a recharger with 15% resist bonus. The passive tank raw is 16 hp/s. With 30% resists the effective hitpoint tank goes to 20.8. 15% recharge bonus goes to 19.6. Therefore you will choose better resists to give a better EHP tank with a lower RHP tank.
Thereafter, the choice between a recharger and extra extenders is never made in favour of a recharger, which is pretty obvious because the recharger merely adds small amounts of RHP and smaller EHP/s, whereas an extender would add more RHP and even more EHP....and because you already have a recharge rate, the extra raw hitpoints also therefore increase the RHP and EHP per second passive tank.
The reason you don't use a set of flux coils instead of SPR's is the nerf to raw hitpoints (ie; buffer). Nominally, yes, you are 10% better off using a T2 flux coil because the recharge rate bonus is so much higher, and compensates for the loss in buffer. However, as shown above, RHP/s tank is nearly meaningless, and SFC's also work counter to adding any buffer in the mids, excpet on small EHP ships with oversized extenders (LSE Svipul).
For example, if you have added extenders and resists in the mids (and obviously forsworn rechargers because they are not as efficient as adding more extenders or resists) why would you work against that choice by adding flux coils?
These are the reasons why passive tanking in all situations relies exclusively on SPR's, extenders, resists and shoe-ins from the DCU and occasionally the PDS. Capacitor stability is an issue, but you can get around that with passive resistance modules versus invuls, or by adding a small or medium nosferatu in a utility high.
This is tried and tested in eleven years of EVE, and yet CCP seems to think that in tierciding these modules, there's any use at all for rechargers and flux coils. The problem is purely mechanical insofar as the mechanism of creating a passive tank is simply: add buffer, then add resists, then add recharge. You don't ever choose to reduce buffer at any point.
Therefore, flux coils need to change, and rechargers can be discarded. I'm yet to see a fit where a recharger actually makes sense except perhaps a rattlesnake fit up to take on weak rats - ie; you can sacrifice midslot resists and extenders for a low buffer, high recharge setup and devote lows to DPS buffs. A pretty narrow and dangerous choice to make.