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Adding batteries - How much is too much... batteries that is...


KnottyBuoy
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Hi All the clever guys out there,  

Here is a question for you... a good one if you ask me. (you can be the judge though) 

How big of a battery bank is too big?  

I have 5 Axpert 4KW / 5kva inverters and 45 x 295W Yingli pannels feeding them.  (3 pannels in series and 3 strings in parallel X 5)  One bank wired to each inverter. 

For storage, I have at this stage) 16 Omnipower 260-12 batteries.  Wired in a 48V config 4 in series and 4 strings in parallel.  (see diag).  I want to add storage capacity to the system and want to do it sooner rather than (later.  The system has only been online for about 2 months and adding new batteries now is way better than adding them a year from now.  

Charging the batteries to SOC = 100% happens fairly quick.  By late morning they are full and are being maintained throughout the rest of the day (while the sun is up) 

The supplier seems hesitant and recommend I don't add more than 4 extra, (at a push a max of 8 extra), quoting the internal resistance of the batteries getting too high to grow it even bigger. 

Now with my limited understanding of electricity and circuits, if one add resistance to a parallel circuit the overall resistance of the circuit decreases.

ie. Lets say the internal resistance of a 4 series battery set is 5 ohm and I have 4 strings in parallel.  I apply the formula 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4  .  

R1 - R4 @ 5ohm each , then the Rt value is 1,25ohm.  

Adding 2 more strings we get R1-R6 @ 5ohm each 

   1/5 + 1/5 + 1/5 + 1/5 + 1/5 + 1/5

= 0,2 + 0,2 + 0,2 + 0,2 + 0,2 + 0,2

= 1,2

and 1/1,2  = 0,833333 ohm

 

In the end, I am switching to the grid more than what I would like to, and therefor the need for an increase in capacity.  

Some preemptive answers to questions that may come up

the inverters are connected in parallel and feed into the house - single phase. 

The batteries are all connected into one big bank of 48V 

Battery balancers and monitor omitted from the discussion

 

 

 

 

Battery bank layout.docx

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34 minutes ago, KnottyBuoy said:

How big of a battery bank is too big?  

The supplier seems hesitant and recommend I don't add more than 4 extra, (at a push a max of 8 extra), quoting the internal resistance of the batteries getting too high to grow it even bigger. 

The inverter / charge controller needs to be able to charge the battery bank at a recommended minimum current of 10% of the Ah rating of the batteries. In your case all the inverters are linked and each inverter can charge at up to 120A (60A solar and 60A grid) which gives a total charging current of 600A. That means that you can effectively have a battery bank of up to 6000Ah in your installation.

Currently you have a 1040Ah (4 x 260) battery bank, so effectively you can almost add 80 more batteries.

Ideally you do not want any parallel battery strings because of the dangers if one or more cells in any of the batteries fail and becomes shorted and because of the complexity of balancing and monitoring and the challenge of having exactly the same length of cable per string in order to ensure that each string is charged and discharged equally.

Having said this I would not recommend adding any more of these batteries in parallel to the existing 4 strings.

If you do want to increase the capacity, I would suggest to rather sell the 16 Omnipower batteries and replace them with one string of 2V cells with a higher Ah rating.

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+1 on what SuperDIY said. You are in a corner. Sell and get large AH batteries. The less batteries and connections there are, the better.

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