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Lead Acid Battery Banks

Thread starter #1
How can I connect 30 x 12v 7Ah Batteries in parallel to form 1 bank and what kind of charger would I need to maintain full batteries from a wind charger.

mvs sarma

Well-Known Member
How can I connect 30 x 12v 7Ah Batteries in parallel to form 1 bank and what kind of charger would I need to maintain full batteries from a wind charger.
welcome to electro-tech-online.

what is capacity and specification of the windmill that have?
What sort of load do you want to drive?

I'd recommend a higher voltage, connect 15 series pairs in parallel for 24V.


Well-Known Member
How can I connect 30 x 12v 7Ah Batteries in parallel to form 1 bank and what kind of charger would I need to maintain full batteries from a wind charger.
It is NOT Practical to charge 30 Batteries in Parallel.
Trying to do that will result in some of the batteries charging at a much higher rate than others, causing them to overheat and get damaged.


Active Member
Yeah you can technically do it, but I wouldn't expect it to have very good long-term life. For one, lead-acid quality being what it is, the probability of a bad battery in that mix is already virtually 100%.

Get a better bank. In fact don't even mess with lead-acid anymore. LiFePO4 is better performance and actually vastly cheaper in the long run.


Well-Known Member
Most Helpful Member
Or just buy one 12V 210Ah lead-acid battery. No more parallel problems. THey use them in things like boats.
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This is one of the reasons they use HUGE 6V cells when building off-grid power supplies. Only use one bank in serial until you get the supply voltage you need. And if one goes bad, you only have to replace 3 cells, not 6.

I think they actually make 2 and 1 cell batteries for this purpose too.


Most Helpful Member
Paralleling or series connecting groups of LA batteries is very common. I have helped scrap a few multi 10's of KW commercial UPS systems that ran with over 100 4 volt 200 Ah LA storage batteries all in series before. They just used a higher voltage multi stage battery charger.
Whether you are charging 6 cell in series to make a 12 volt system or 200 cells in series to make a 400 volt system the concept stays the same.

Far to often I see the battery charging purists get too wound up over the number of cells charged in series on this site. Charging large banks of very large batteries is very common practice.
As long as common sense and proper battery maintenance is used any combination of series and parallel will work just fine.
You just need more equalization time the more cells you have if you want maximum life.

How about a system that charged each battery individually, but still allowed complex series-parallel configurations. :D


Well-Known Member
Well with series/parallel battery configurations, my 700ah 24 volt bank is made up of 28 6 volt 105ah in sets of 4. The batteries are AGM's and fully sealed so you cant add water to them. Over the last 3 years i've had 4 batteries fail and everyone was the first one in the string that takes the charge.

So by putting 30 of them in parallel would mean more than likely throwing away the first one in the string on a weekly basis, as it would hog all the charging, dry out then slowly ruin the rest.

If your serious about battery storage get some decent AH cells.

Just my 2.2 cents worth

Regards Bryan
Thread starter #11
Thanks for your posting on Electro Tech re connecting 30 batteries in parallel. Could you suggest a site where I might find a charger circuit for such a bank of cells.I am trying to build a system working through a inverter to energise my household system 220vac to enable me to save on our very expensive grid supply costs.
Reg regbrand@hotmail.com
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!



Well-Known Member
G'day Frank,
Well that was a long winded reply and also a good read. By the sounds you are well versed in the area of batteries for off grid situations. If you would like to do a decent writeup on the subject of batteries I'd be happy to include on the main RE forum in the sticky section. Just start a new thread in the main Re forum if you choose to do so.

Regards Bryan

And welcome to the forum Frank

Hello Bryan,

Thanks for your reply, and thanks for the welcome as well! I wish I had more time to browse this great site, and do more problem solvings here, but right now I'm in the process of a move to the East Coast and FL - the great Gulf Coast side that is!

You know - THE SUNSHINE STATE and the land of OFF THE GRID SUNSHINE as it were! There is SO much Green potential over there - that is - in so far as finding a new career that I'd rather be in right now. Obama's GREEN Dream as it were! We sure wasted the last 8 years with nitwits the likes of "bush/cheney" - as they were just like Ostriches with their heads in a hole in the ground when it came to getting rid of OIL DEPENDENCY - to put it mildly! NO CLUE at all.

Why I stayed in dreary CA as long as I have is beyond me! They are so far behind on OFF THE GRID Green Power usage and development that it isn't funny anymore! With a NITWIT here the likes of ARRRRRRNOLD THE DORKENNAGER for a Governor is it any wonder why? There you go…….!

I guess from your G'day greeting you are indeed living somewhere Down Under as it were?

I was kinda hoping to hear back from that original poster regb123, but I guess not - as I had seen that the original thread was first started way back in June, and who knows what ever happens sometimes to the org problem, and if it ever gets resolved by the org poster. Who knows?

As for the "decent write-up" on the subject of batteries - no problem there at all - as related to my day-to-day Electronics T/S work that I do - both on Marine projects and Home related projects as well. Just wish I was doing more of it these days is all.

I sometimes think I missed my true calling in life as a Tech Writer - beings as Tech "grammar savvy" as I am, and being very cmptr savvy and all that goes along with that as well. Being online LITERATE really helps out a lot as well - as some of the poorly written replies I see online always makes me wonder as to where this world is headed at times - esp when it relates to technology and all that goes along with it!

I had some great OFF THE GRID articles and write-ups saved on my other cmptr system that had its mobo crash on me late last year 2008, and unfortunately I was running a LINUX offshoot OS named Linspire on it at the time, so everything (DOC wise) saved on that hard drive (if that hard drive is still indeed good?) was in a different KERNEL format. I'll have to try and retrieve all the MY DOCS files one of these days, and then transfer them all over to my rebuilt (brand new full tower) WIN2000 OS system that I'm now running. I don't care for XP with all its glitches and driver problems! That goes double for VISTA as well - piece of crap OS! I may need to run OPEN OFFICE on that Linspire hard drive, and retrieve all the DOCS and great JPEGS that are saved there! Lets hope so anyway! In a way I liked the LINUX Linspire OS and Mozilla Browser a lot better then M$ WINDOWS OS anything, but it had it's own flaws as well. Oh well…….

If you have an outline as to what you'd like to see NEW in thread - as related to batts and anything OFF THE GRID. That is - in so far as a great thread on batts, and all related to them. Let me know what peaks your interest the most. I'd hate to start a NEW thread, and then rehash something OLD that already had been posted prior. Most forums frown on that and even DELETE reposts as it were.

As a rule I don't do any of those so-called Blogs, much less waste my time at any Forum type sites. I just see them to be a waste of my time except maybe if there is a Google'd link connection to one of them with some useful info (feedback) on a specific Google search I may be doing online at the time. The "wasted time" relates to all that "signing up" crap, and then not getting your comment posted in the first - as to the moderator's screening and all. Some online sites, and esp some online Newspaper sites as well are a real pain in the butt as it were!

Hopefully that regb123 poster will follow up on his post and update us as to what really happened with his "idea".

We'll see……

Best regards,


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