Batteries In Parallel

[svencool]

I was reading a post that was over 120days old. The user said this,

"with AGM, you do NOT want any parallel battery strings, they have very low internal resistance, and the subtle wire resistance differences will cause un-equal loading of the batteries.

If you need more battery amps, look into 4V or 2V cells, and use more in series.  Of course, the quantities are lower, and the price goes way up."

So am I reading this right, He is saying not to parallel batteries?

Mike

[Vic]

Hi Mike,

Generally,  parallel battery strings are not good,  especially for off-grid systems,  where the Solar day is relatively short,  compared to Gird-connected systems.

AGM batteries have very low internal resistance,  so the battery with the lowest resistance can HOG almost all of the available charge current,  starving other parallel strings from much charge at all.

For gird-connected systems,  that can charge/Float essentially forever,  the lowest resistance string will become fully charged,  become higher resistance,  and allow the lagging strings to eventually get fully-charger  --  often over a number of days after a discharge event.

There are significant differences in the behavior of off-grid and grid-connected systems.

Strings of batteries age differently,  and any balance in current sharing that was achieved initially  will diverge with time,  even for systems that had a perfect initial balance,  causing  damage to batteries that are chronically under-charged.

Then there are the temperature differentials in strings that do not share currents equally,  and so on  -- there are many,  many reasons why any parallel battery strings are not a good idea,  especially for off-grid systems.

My take,   Vic

[svencool]

Ok my system is less than a month old or so. I have 4 100ah batteries in parallel. Is it possible to make 2 strings in series and then parallel them in the end? Or would it end up being the same as you put what would be 2 banks of batteries back in parallel? I have a 12v system with a 12v DC - AC Inverter. Batteries were not cheap and I really don't wanna kill them before there time. I have a 4 year warranty that isn't prorated but.... Its an off grid system as I rent where I live at the moment. I use it to power the living room at night as that is where my son and myself spend most of our time once we get home from work.

One day I hope to move back to Wis where I have a home that was left to me by my dad which I wouldn't mind doing a grid-tie system. But for now I'm just playing around with this and want to learn as much as I can....

Mike

[Vic]

   ...   Is it possible to make 2 strings in series and then parallel them in the end? Or would it end up being the same as you put what would be 2 banks of batteries back in parallel?    ...   
Mike

Yes that would be (or would have been) a better thing to do,   BUT,  then,  it becomes a 24 V battery bank,  and you know what that would mean!

This is the system that you now have.  If you do not want to change the inverter,  and perhaps the string configuration of your PVs (forget just how your PVs are strung),  you will need to run what you have,  and perhaps get an inexpensive DC Clamp Ammeter from Sears,  or perhaps Radio Shack,  if they still have them.

The Clamp DC meter will allow you to measure the DC current in each battery string.   You will be looking for very similar currents flowing in each string during steady high current charging or discharges.  The balance need not be perfect,  but divergent currents in each string will generally become worse over time.

There are configurations of parallel battery strings that will help improve current sharing:
http://www.smartgauge.co.uk/batt_con.html

Using high quality battery cable lugs,  and large high quality battery cable will help with balance.

But,  you should expect that balance will change over time,  and you will probably need to separate strings and perhaps charge the lagging batteries separately,  either on of off of your power system.

In addition,  fuses for each string are really needed for safety.   These will introduce a bit more voltage drop,  will help protect against a fire should a battery develop a fault,  etc.

Opinions.     All of the above issues/concerns are manageable.   They just require some attention.   Batteries,  like relationships,  really need attention.   After all,  they are a part of your family!   Vic

[svencool]

Well I think for now that I wanna keep it 12v and I have my batteries hooked like Method 2. But i think after reading this Method 4 would make the most sense. If I'm thinking wrong please step in and tell me so I'm still learning and wanna be safe and keep my learning costs to a min Thanks Vic!!!!

Mike

[Westbranch]

consider selling the 12  inverter and getting a 24 volt version if you are not using 12v lights appliances etc, you wont regret it a....  and consider in the cost of replacing those 4 batteries in a year or to vs the cost of a new inverter...

[svencool]

Anyone have any suggestions on a good 24v DC - AC Inverter?

[mike90045]

Anyone have any suggestions on a good 24v DC - AC Inverter?    

What power range ?
Do you want integral charger with it ?

[svencool]

I'm looking for around 1000w continuous to power my TV and Home receiver and couple led lights. and charge my cell phones and laptop. I've heard you don't want to but to big of an invert for your needs as they will just suck up your stored power faster than if you have one size right for your system requirements....

Mike

[Westbranch]

look into these...  http://www.cotek.com.tw/product/18.html

Do you plan on this being a permanently wired unit or just use an extension cord.  The ST is the only one for permanent wiring AFAIK.

You probably don't need a 1000W model something a bit smaller , that you will use at about 75% of capacity consistently would suffice

[svencool]

Well I'm renting a house so I have a room off the living room that use to be one of my kids rooms but they have grown up and moved out. So I use it for my solar setup now. I have an extension chord running to my entertainment center powering my tv and receiver.

Mike