Hello regb123,
Your 30 AGM batt bank of 7ah batts "all in parallel" problem is nothing new to me seeings I deal with batt issues every day in my consulting business.
I must say - I've never in my life come across someone trying to take your approach to such a simple grid solution. Regardless if you are running off 110VAC or 220VAC (Edison or whoever's) grid power.
Having repaired many a 12-volt Inverter, as well as 110VAC UPS units past to present - I first have to wonder why you would even consider a parallel bank of such low Ah rated AGM batts to begin with. Forget whose brand name is on them, as that 12-volt parallel bank capacity of 210ah using 30 batts is just plain ridiculous to begin with. At least from a jumper wire standpoint if nothing else!! If they are USED batts to start with then it gets even more ridiculous as from a MATCHED BATT SET to start with.
Even if each batt cost $10 each - that's $300 right there regardless of actual unit price per AGM batt. For that same $300 I can get a couple nice 12-volt 100ah AGM batts, and then only have to deal with 4 cable connections total - as far as the power cables go. That is - 2 parallel jumper cables between the 2 batts, and 2 more cables for the Inverter connections. That's it! What could be simpler?
I could even buy a single "good quality" 12-volt 225ah rated AGM batt for $300, and then I'd only have to deal with making 2 cable connections for the single AGM batt. Even better! All batt warranties aside as well - as those bigger capacity AGM batts usually carry a pretty good warranty program behind them straight from the batt Mfr! Even if it's extra - for a few extra bucks it's well worth the added cost! Peace of mind.
With 30 small Ah capacity batts in parallel you are constantly going to have multiple batts failing on you periodically regardless if they are all matched from the start or not. It's just the nature of the beast is all. I should know as I seen it happen all the time with UPS units that used an 8 batt DC power scheme. Toshiba brand 1400 VA UPS units were notorious for doing this.
There were a few other brands I dealt with as well that used 8 batts each, but I forget the brand names now, as they were all older and out of production even years ago. The UPS systems that utilize 4 AGM batts make way more sense to me, as they are much easier to maintain for starters, AND the larger batts last far longer then with those smaller undersized 7ah AGM batts. HINT HINT!!
Though it is true - like some have stated here in posting - that only having to replace one bad batt at any specific failed time is less costly then having to replace 1 bigger (single 12-volt) batt costing $300, but how often does a single $300 AGM 225ah batt go bad? NOT VERY OFTEN AT ALL. They have a very good track record as from a mfg standpoint - all batt abuse aside that is!
If I were you I'd definitely stay away from buying any 6-volt type AGM or GEL cell batts - esp those TROJAN brand T-105 DEEP CYCLE FLA (Flooded Lead Acid) batts, as Trojan Batt Co has had production run problems in the past with them, and they had to issue a few batt RECALLS as well. Had something to do with loose internal connections from a + / - mounting post standpoint. Buyer beware is all I can say!!
I never cared for having (2 or 4) 6-volt Marine app batts connected in series/parallel anyway - much less having any 12-volt marine app batts connected in series and/or parallel as well. Just way too many connection related weak points is all. Not to mention salt-h2o related connection problems as well.
FYI - One or more batt(s) is/are always prone to failure just from the constant LOADS or OVERLOADS (as it were) being applied, and then the recharging part as well - esp if the batts are severely drained down voltage/capacity wise. Then - if constantly recharged or overcharged at excessively HIGH charge rates right afterwards that too will cause batts to overheat even worse and prematurely fail early on.
That and the fact that the older the batts get and the less preventive cable maint that is done over time will again aid in premature failure just from having poor (dirty) cable connections between those banked batts.
I've seen it time and time again to no end! Those high resistance cable connection points will cause excessive heat to build up there at the terminal post, and then transfer that heat inside the batt through the batt posts, and THAT is what causes batts to fail 9 times out of 10 every time. When I worked at 2 SEARS AUTO STORES years ago as a Batt Tech there I used to see it all the time!
More recently I've gone back to doing Marine related Electronics repair work to fill in the money shortage gap - as my main Bldg Consulting business is seeing hard times these days and it's really slow these days work wise.
On one such service repair job I actually found a brand new (FLA) Marine Deep Cycle batt with one of the 2 Wing Nut posts completely melted off because the cable connection was loose, and the boats DC Inverter current draw got the Lead so hot that it actually melted it away, and it flowed like liquid volcanic lava over the top of the batt case until it cooled to a solid state again!
Wish I had had my digital camera along with me at the time to take a pix, as people I tell that to still can't believe that one even after I swear on the bible that it's totally true! I'm sure that was just short of an Engine compartment FIRE as well.
See my point here about cable connections and high resistance points? I rest my case as to why batts fail as well - as I think by now you get the point! That batt - btw - was still OK even after the melted Wing Nut post melted away. The regular clamp post was still intact and rock solid at the case entry point, so it was caught just in the nick of time otherwise it would have been a batt FIRE at sea!!
What someone had to say in post about the lead (first) batt always being the one to fail first in a series string holds no water as it were! Though it might at first always seem that way - there is no real scientific/Physics proof as to the why - how - and for what reason! My educated reasoning is what I stated above, and then it's just left up to the batt(s) itself/themselves.
I'd also have to ask a couple more ??'s here similar to those already asked - as to what you plan to run off this batt bank and Inverter system to start with?
Since you are saying your house AC Grid Service line is 220VAC then I take it you are also configuring your Inverter as a 220VAC Output Inverter as well - but with the DC Batt Input only being 12 volts? Are you sure? I would think you want the DC Batt Input at 24 volts at the MINIMUM, and better still 48 DC volts Input being more logical - as related to the Batt DC LOADING effects, and the LOAD Transfers to the Inverter OUTPUT.
The Wind Gennie part as well, as it would seem you haven't done any research at all into setting this "Off the Grid" system up!
My guess is that you either live somewhere in Europe or else in Australia seeings you are saying you run 220VAC from your existing grid now? If that is the case then there is a TON of OFF THE GRID info online as related to living in Australia OFF THE GRID as it were.
Going back to the original ? about connecting (30) 7ah AGM batts in parallel, and then using a Wind Gennie OR charger as it were.
Here's another quick synopsis just in closing -
(30) small undersized 7ah AGM batts (new or used condition - unknown?) with all tab connected jumper wires (29 jumper wires + 2 main + / - delivery wires/cables, or else solidly point to point soldered "wire to tab" if really doing a better electrical job of it) connected to some sort of Wind Gennie charger (with or without proper rated PV / Wind Gennie Charge Voltage Controller - unknown?) running some kind of Inverter at some unknown house LOADS value (Wattage - unknown) - that is all we're being given here.
End scenario - probably a HOUSE FIRE worst case!
Hopefully no loss of life associated with the jury-rigged "Off the Grid" system?
Good luck!
Frank