Correct way to connect 2 x 48v strings to the inverter

Lake_Man · July 2015

I want to connect 2 x 48v strings together and connect to the inverter. The batteries are Victron 12V 220 amp AGM's. I have 8 of these. So it would be 4 x 12v in series then parallel the two strings together then connect to inverter. So what is the best way to connect all these together.

Also, i have 2 strings now - new, just bought. Thinking of adding a 3rd string - if i do it, now is the time so as to keep them all the same age and matched. I know the less strings the better but is 3 strings acceptable or should i really not do this?

As you can see from the photo i have the two strings stacked one above the other. I dont have any choice on this as thats the only possible way they will fit the space they have to go into. I have just read here on another thread that this is not good because of temperature difference. Since there is nothing i can do about this i will have to push on regardless but how big a problem will this be? IS this a very good reason not to contemplate a 3rd string as it would be sitting above the second string.

Thanks for any heads up on this.

vtmaps · July 2015

Best way to connect parallel strings is a bus bar (use method 3 on this page).

Check out the picture of bus bars by 2manytoyz in this thread.

There are many reasons other than thermal not to add a third string of batteries. Parallel strings of batteries are intrinsically unstable. You will need fuses in each string for safety. The fuses and their connections add points of failure to the system. AGM batteries, with their lower internal resistance, are even more problematic than flooded batteries because the resistance of the wiring and interconnections becomes greater relative to the resistance of the batteries.

And then there are the stochastic reasons to avoid parallel batteries. Basically, the performance of the entire bank is NOT the average of the constituent cells... The performance is that of its weakest cell. Half your cells are below average. The more cells you have, the greater the chance that you will have an outlier weak cell.

Read this.

--vtMaps

Lake_Man · July 2015

OK thanks for the heads up.

Lake_Man · July 2015

Geez, after reading all that, not so sure parallel strings are such a good idea. If i could find big AH batteries here i would go that way, but i cant so........

I have the 2 strings so will use those. Not sure if i should add that 3rd string or not. Sounds like its really inviting trouble to do so, yet i need more AH's..... what to do?

inetdog · July 2015

Lake_Man wrote: »

Geez, after reading all that, not so sure parallel strings are such a good idea. If i could find big AH batteries here i would go that way, but i cant so........

I have the 2 strings so will use those. Not sure if i should add that 3rd string or not. Sounds like its really inviting trouble to do so, yet i need more AH's..... what to do?

Add the third string and buy a DC clamp-on ammeter to help you keep on top of string balance issues. Also make very sure that the cable interconnections are all clean and well torqued.
One potential problem would be adding the third string after you have put some miles on the first two. That will make balancing more difficult.
Definitely keep a close eye on individual battery voltages at rest and during charge/discharge until you are sure you have done all you can to keep the strings balanced.

PS: Note that the rule of keeping wire length the same for each string applies to the total length of a given wire size, not the individual +, -, and intercell connector sections.

You may want to put an air deflector between the top and bottom stacks to minimize any extra heating the top stack might get from the bottom stack. Measure temperatures during charging. If you have a CC that has a remote temperature probe, put it on the top stack.

new2PV · July 2015

Temperature variations of 5 degree F are acceptable. I dont like the batteries touching each other should leave 1/2" gap so they can dissipate heat between them.

Lake_Man · July 2015

Thanks for the advice.

Lake_Man · July 2015

What about this..... http://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf

At the bottom of the PDF it shows 3 strings of batteries connected to the battery balancer. Comments on this wiring schematic ?

vtmaps · July 2015

Lake_Man wrote: »

What about this..... http://www.victronenergy.com/upload/documents/Datasheet-Battery-Balancer-EN.pdf

At the bottom of the PDF it shows 3 strings of batteries connected to the battery balancer. Comments on this wiring schematic ?

What it shows is that only 12 volt batteries in series are balanced by the device. When 3 strings of batteries are used, three 12 volt batteries are first put in parallel to make a larger 12 volt battery, and then another three 12 volt batteries (in parallel to make a larger 12 volt battery) are put in series with the first three. There is no balancing of the three parallel batteries that make up the 'larger' battery.

As far as the series balancing is concerned: yes, it can help keep two 12 volt batteries in balance, but realize that each 12 volt battery is really six 2 volt batteries in series, and the device does nothing to keep the individual cells balanced.

--vtMaps

Lake_Man · July 2015

OK so i am confused (as you can already tell) - so if the battery balancer setup does nothing for the setup shown on the PDF - then what is the purpose of setting it all up like that? Why purchase 3 x battery balancers etc if this setup does nothing?

But i see what you are saying - 3 x 12v batts connected in parallel which are then series connected to another 3 x 12v batts in parallel which are series connected to another 3 x 12v batts in parallel.

So what would be be the purpose of this setup? I can see the shunt and the BMv 702 are useful but what is the point of the battery balancers in this setup?

Edit 2:

Why could one not look at this setup as 4 x 12v batts connected in series, paralleled with another 4 x 12v batts connected in series, paralleled with another 4 x 12v batts connected in series rather then;

as you put it,

3 x 12v batts connected in parallel which are then series connected to another 3 x 12v batts in parallel which are series connected to another 3 x 12v batts in parallel.

What am i failing to understand here ?

Edit 3:

I think i have answered my own question - its that black cable in the middle of the diagram that goes from neg to neg to neg post of each battery that has me confused.

If that confusing black cable was not there then it would indeed be 4 x 12v batts connected in series and then the 3 strings paralleled together - but with that black cable connecting each neg post of each battery - it changes it to something i dont seem to understand....... sheesh. Enough already - time for a beer, its almost 1.00pm here !

BB. · July 2015

Personally, I do not like "cross connecting" several series strings together. It makes if very difficult to find weak/problem cells/strings in a battery bank.

With a series string of cells/batteries--You can volt meter across each cell/battery and see if all is OK or not (you are looking for differences... A "different" voltage reading vs the rest of the cells, you need to investigate and see what is wrong).

When you cross connect between parallel battery strings--A weak cell/battery can be masked by the good cells in parallels with it.

Also, you can get relatively cheap DC current clamp meter (like this one from Sears). You can measure the string current to insure that current is properly being carried by each string. When you make cross connections--It is very difficult to measure string current (looking for corroded connections, open/shorted cells, etc.).

The cross connections is common for BMS systems and some Lithium battery banks. Lithium (in general) have very little tolerance for "over charging" (pumping more current through a cell to balance/equalize charge a "weak/low" cell in series). With a BMS system, they parallel cells so the BMS system can measure (and possibly charge) each cell (or parallel group of cells) individually.

With Lead Acid batteries, they tend to be more easy to get cells "out of balance" than Li... However, you can over charge (equalize) Lead Acid cells (by ~2.5 to 5% rate of charge maximum)--So per cell BMS/Charge control is not really needed.

And functionally, cross connecting strings does not help (current flow wise). If all is going well, there should be no current flowing on the cross connects. And if there is a problem where current will flow in the cross connections--It is probably masking a bad cell/connection (and not helpful to you when figuring out what is going wrong).

-Bill

Lake_Man · July 2015

Thanks for the explanation BB. The "cross connections" are confusing - at least to me - never seen them before. The simple answer is to run one sting .....if we could only find big AH batteries here, which we cannot.

I will keep reading and reviewing and try to educate myself on everything written above.

Thanks for the time everyone.

new2PV · September 2015

My 2 strings of batteries had many batteries undercharged, by cross connecting them it seemed to balance out the charge as they now a much closer voltages than before, so I do think it is a not a bad idea to do this, even though it maybe does make it hard to troubleshoot a problem.

Correct way to connect 2 x 48v strings to the inverter

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