Help charging a LARGE bank of AGM batteries.

[bidaw]

I have recently acquired gift (because I could not come close to affording) of 2 large banks of 'new' AGM batteries. They are (36) C&D Technologies Dynasty UPS12-210MR 12v 53.4AH Batteries and (36) UPS12-300MR 12v 77.8AH Batts. My thoughts are to have (2) 48v Banks with their own inverter/charger. 9 parallel strings of 4 series tied to 48v in each battery set. This giving me ~480ah and ~700ah respectively.

My question is, do you see anything wrong in having banks of this size wired as such? Wiring the banks in this way, any recomendations to a inexpensive standalone AC battery charger? What amperage should I look for in charging? Would a 3 stage charger that does AGM be the way to go??

I also have a solar array currently charging a 24v bank that I am thinking of decommishioning to move to the UPS12-300MR bank to go with the AC charger (wanting to keep a float on the bank with solar)...its 840 watts of panel wired at 115v into an MPPT charger that can do 48v. I'm not tied to this idea, but it was a thought.

Anyway...any suggestions would be appreciated, because I have 99 problems, but lack of batteries isn't one!

[thastinger]

I might look at bartering some of those batteries for other equipment you could use to build your new system. Maybe you could trade some for some FLA batteries and some more panels, or just sell them and use the money for more panels and an FLA battery bank of the proper size because you are destined for failure with 9 parallel strings of AGM batteries.

Sizing a charger is going to depend on the final size of the new battery bank, if you were to end up with a 48V 200Ah battery bank, you would need a 48V 20A charger.

[bidaw]

[QUOTE=thastinger;92415]I might look at bartering some of those batteries for other equipment you could use to build your new system. Maybe you could trade some for some FLA batteries and some more panels, or just sell them and use the money for more panels and an FLA battery bank of the proper size because you are destined for failure with 9 parallel strings of AGM batteries.

Sizing a charger is going to depend on the final size of the new battery bank, if you were to end up with a 48V 200Ah battery bank, you would need a 48V 20A charger.

So are you trying to say that I shouldn't go 9 strings parallel? What is a reccomended size? 700Ah bank was what I am looking at...if I used the same charger (20A)...wouldnt it just take longer vs. say a 50A charger...or should I find a 70A?

Really...im not being difficult...im just trying to understand. Thanks for your input.

[thastinger]

Yes, I'm saying don't parallel 9 strings of batteries, it will be nearly impossible to have them balanced since there are so many charge and discharge paths of least resistance. A much better setup would be to size your battery string to one series string. If you need a 450Ah bank and want 48V, then buy 8 of these http://rollsbattery.com/public/specsheets/S-600.pdf and wire them in one series string.

You should always determine your loads then build the system from there. Just going from what you already have, you do not have enough panel wattage to charge anything near that size battery bank anyway. For FLA batteries, you want a C/10 charge rate (C/8-C/12 works as well). AGMs can take a C/4 rate but you can't come close to suplying either with the panel wattage you presently have...which is why I said to sell what you have and buy the right stuff with the money you get.

[bidaw]

Yes, I'm saying don't parallel 9 strings of batteries, it will be nearly impossible to have them balanced since there are so many charge and discharge paths of least resistance. A much better setup would be to size your battery string to one series string. If you need a 450Ah bank and want 48V, then buy 8 of these http://rollsbattery.com/public/specsheets/S-600.pdf and wire them in one series string.

You should always determine your loads then build the system from there. Just going from what you already have, you do not have enough panel wattage to charge anything near that size battery bank anyway. For FLA batteries, you want a C/10 charge rate (C/8-C/12 works as well). AGMs can take a C/4 rate but you can't come close to suplying either with the panel wattage you presently have...which is why I said to sell what you have and buy the right stuff with the money you get.

I think that you are mis-understanding what I was meaning (when you say I don't have enough panel watttage). I want to use AC ie. 'the grid' to charge the batteries. The solar part was just a wish list item to keep the batteries to float. Another thing that I probably didn't make clear was that these 2 banks were going to be for backup purposes only. To keep the house powered up until I can move the house over to generator power (I have a fast transfer switch already). Full disclosure is that I have a small 24v 450Ah battery setup now...this is being charged by the 840w I previously mentioned. I also am running my house grid tied to 6kw of solar as well. The reason I was given the AGM batteries was that I was known as "that guy that does solar" and a local company gave me the batteries because they heard that I was looking for something to ensure I wouldn't have loss of power in case of brown/blackouts. Thing is, they didn't hook me up with a charger. This is why I came here to see if I could get some help for identifying a proper charger and ways to hook the batteries up. I have the cables...all cut to exact measurements for consistant flow to minimize resistance problems. With this additional information, does this help my situation any?

Thanks again.

[Mike90250]

read this article about parallel batteries. 2 in parallel is possible. more becomes much more difficult.

http://www.smartgauge.co.uk/batt_con.html

If all you are doing is keeping them on float, that's fine, but cycling them will require buss bars and matched cables.