Max number of Series strings?

Farmerboyz1721Farmerboyz1721 Registered Users Posts: 39 ✭✭
I have been told that you should not have more then 3 parallel strings in a battery bank. 

What about the series side? Is there a maximum amount of batteries you can have in series?

I have 6 volt 210 Amp/hr batteries and would like to wire them to 48 volt.

That would be 8 series string.



  • DConlyGuyDConlyGuy Registered Users Posts: 80 ✭✭
    people make up 48 volt banks all the time this is a good place to learn how to wire them right
    600 watts of solar panels,Epever 30 mppt , 2 PWHR12500W4FR battery's in 24 volt setup
  • BB.BB. Super Moderators, Administrators Posts: 33,281 admin
    edited July 2016 #3
    The 3 parallel string "limit" is my suggestion... 1x is ideal. 2x has been used to have "backup" (if one string fails, you can limp along on the 2nd string). 3x is my personal recommendation (maximum parallel string limit) that the various issues start to add up and make massively parallel strings something to really avoid if at all possible. (lots of batteries to fill, issues of current sharing between strings, each string should be fused/breakered, secondary effects from temperature, problems monitoring/debugging issues, etc.). People have put more 3x strings in parallel and have been happy with their banks--But I would suggest avoiding if you can.

    Batteries in series... In reality, a "battery" is already a series of cells (3 in series for 6 volt, 6 in series for 12 volt, etc.). There is no basic reason you cannot put XX cells in series. There are some secondary issues that make this an issue:
    1. You need to check / monitor cell/battery voltage. A shorted cell in a 6 volt battery means 4 volt operation--Easy to see. A 48 cell battery bank (96 volts nominal) with a shorted cell means 94 volts--Not obvious in a "working range" of 84 volts to 120 volts (near dead to equalization voltage).
    2. High voltages--The battery case may not be rated for a >> high voltage bus voltage (12 volt battery in a 96 volt string would need to be tested to ~360 VDC to ground (~3x max operational voltage "highpot" test).
    3. A lot of cells to check water levels...
    4. More "devices", is more chances for failure.
    5. Higher voltages, more chances that a ARC can form and cause a fire.
    6. Higher voltages need higher voltage rated fuses/breakers--High voltage/high current fuses/breakers are not cheap and sometimes not easy to find
    7. Higher voltages are more dangerous (electrocution hazard)--And any voltage over ~60 VDC is considered hazardous and needs to be in a locked cabinet or room (for commercial equipment).
    So--To your question... 48 VDC nominal lead acid battery bank is about the highest working voltage a day to day type residential off grid power system is going to be designed for/installed. There are higher voltage AC inverters available (typically used in computer room installations and such)--But, not going to be "standard issue" for our use.

    I also like to keep cell count down... I would not go with a 24 cell (48 volt) battery bank to power a smaller system. For example, a 1,200 Watt or less system can work very nicely on a 12 volt battery bank (with relatively large AH batteries--nominally a ~480 AH @ 12 volt battery bank minimum).

    I would not usually suggest, for example, a 1,200 Watt inverter on a 48 volt battery bank. That is ~120 AH batteries (cell) minimum. It can  work--But you now have 24 cells to "water" and inspect vs 6 cells (and interconnects) on a 12 volt bank. But if all you can get are 12 volt @ 100 AH deep cycle batteries in your area--perhaps that is a "good enough" reason to go 48 volts (and if you can find a 1,200 Watt @ 48 volt input AC inverter, etc.).

    Remember, Power=Voltage*Current. So, it is a balancing act... Perhaps for some reasons higher voltage is "useful" (i.e., long wire runs), or perhaps higher current with low voltage batteries is more cost effective (i.e., using 6 volt @ 200 AH golf cart batteries). Many of the rules of thumbs we use around here are "biases" towards certain choices/suggestions that usually work out for the best. They are starting points and can be "adjusted" for specific reasons/applications and when the advantages outweigh the drawbacks in your installation.

    So, back to the basics... What are your load requirements? Watts, Watt*Hours per day, Amps at what voltage, Amp*Hours per day, etc.... What equipment do you already have (suggest paper design before buying anything).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    The lifespan of a multi-cell battery is NOT the average of the individual cell lifespans.   The lifespan of a multi-cell battery is that of the shortest-lived cell in the battery.

    If you build a 2 volt system and buy a single cell to operate it, you have a 50% chance of getting an above average cell.

    If you build a 6 volt system, you have a 1 in 8 chance of getting 3 above average cells.  That means there is a 87.5% chance that you will have at least 1 below average cell.

    The lifespan of cells forms a normal, gaussian distribution (bell-shaped curve).  A few percent of the cells will be far above or far below average.  The more cells you have in your system, the greater the chance that you will get a cell that is far below average.

    Above average cells don't help you, but below average cells do hurt you (as mentioned: The lifespan of a multi-cell battery is that of the shortest-lived cell in the battery.)

    The most reliable system is designed with as few cells as is practical.  That's what Bill was writing about... if your needs can be met by a 6 cell, 12 volt system, that is probably what you should build. 

    Regarding parallel batteries... I differ with Bill a bit...  Parallel batteries are electrically unstable, and I do not recommend any parallel batteries.  I consider parallel batteries to be a great compromise in a design.   Designing a system always involves compromises, but parallel batteries is very low on the list of compromises that I am willing to make.

    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Farmerboyz1721Farmerboyz1721 Registered Users Posts: 39 ✭✭
    Thanks for the feed back. 

    Yes i am trying to save money by using the 6 volt golf cart batteries at Costco. 

    I am putting together a 'large' system. 

    I was hoping to wire 24 batteries together- 8 in series with 3 parallel strings. For a total of 30,000 watts.  
  • PhotowhitPhotowhit Solar Expert Posts: 5,996 ✭✭✭✭✭

    I don't know the cost of batteries at Costco, Sam's was up near $90 + $20 core charge, suspect Costco would be close. $110 x 24 = $2640, You might consider a forklift battery. They have some different characteristics, but also have a likely longer life.

    A 48 volt 660 amp hour would cost about $4725 delivered.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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