Parallel Battery Connections

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  • Sunking
    Solar Fanatic
    • Feb 2010
    • 23301
    #1

    Parallel Battery Connections

    There are three ways to parallel batteries for Cycle Use. One is the Conventional Method seen below, two is Diagonally as seen in the link , three seen below as Alternate if you are using single cells. Example 2-Volt Lead Acid or 3.2 volt Lithium cells

    First a little background. When you parallel batteries, it is almost impossible to balance Battery Internal Resistances, Cable and Connector Resistances. This forces one string to do most of the work. It weakens the strings and results in early failure.

    There are only two reasons to parallel batteries.

    1. To achieve a desired AH capacity. Problem is a lot of folks who come here fail to do homework. Rather they go and buy a bunch of 12 volt 100 AH batteries. Example 16 of them to make a 48 volt 200 AH battery. Parallel batteries should be avoided. Folks are stuck in 12 volt boxes. In this example all they had to do is buy 8 6-Volt 200 AH batteries resulting in a single 48 volt battery string. Well here is a fact unless you you need more than 4000 AH there is no reason to parallel battery strings as they make 2 volt cells up to 4000 AH

    2. Mission Critical Applications to allow you to take one string off-line for maintenance. In this approach, no more than two parallel strings. Note it takes a special disconnect switch and a extra charger to do this. You must equalize the string voltage before connecting. Otherwise if you have more than 1/4 volt difference between strings, you will weld the disconnect contacts together.


    Edit Note:

    The Alternative method below I stumbled onto about 10 years ago when I started building my own racing golf carts and helping friends build EV's. Every Commercial EV manufacture and DIY EV guys if they Parallel Batteries use the Alternative method below. Caution it can only be done with Single Cell Batteries like 2-Volt Lead Acid Cells, and Lithium Cells. It will not work 4,6,8 0r 12 volt batteries made from more than 1-cell. So any of you with Lithium should be wiring as below for many reasons.

    OK the PROs are many, and few CONs: On the Pro side it gives you full redundancy as conventional. If any one cell fails, you can still make it home or keep the lights on until repairs can be made. Internal Resistance is Automatically Balanced at the cell level, so there is no fancy cable arrangements, or buss bars to fuss with to balance current and resistance. If you happen to using Lithium batteries with BMS would require only 4 Circuit Boards as opposed to 8 circuit boards in this example. The only Con I have come up with is maintenance could be more labor intensive if you have to replace a cell with the extra wires paralleling the cells. Myself I would use buss-bars for all Inter-cell connections.









    Last edited by Sunking; 08-29-2016, 07:57 PM.
    MSEE, PE
    Tags: None
    • Stuck
    • Sunking
      Solar Fanatic
      • Feb 2010
      • 23301
      #2
      Something must be wrong with my puter, I can never get the Smart Gauge Link to work, but can on my cell phone.
      MSEE, PE

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        • SunEagle
          Super Moderator
          • Oct 2012
          • 15126
          #3
          Originally posted by Sunking
          Something must be wrong with my puter, I can never get the Smart Gauge Link to work, but can on my cell phone.
          The Smart Gauge link in your first post works for me. Maybe you have some type of website block set up on your computer.

            Comment

            • Sunking
              Solar Fanatic
              • Feb 2010
              • 23301
              #4
              Originally posted by SunEagle
              The Smart Gauge link in your first post works for me. Maybe you have some type of website block set up on your computer.
              Not my Link, Mike put it there. Still does not work for me. Just my home puter will not go there. Laptop, work puter, and cell phone all OK.

              EDIT

              Fixed it. Had to change DNS servers.
              MSEE, PE

                Comment

                • PNjunction
                  Solar Fanatic
                  • Jul 2012
                  • 2179
                  #5
                  Bravo! Those pics are excellent.

                  The main point being that if you are dealing with individual cells, and not entire batteries, you should change your setup from conventional to alternative and smartguage only shows the conventional wiring designed for a battery with no access to individual cells, ie most batteries we deal with today.

                  If I had to put it into words, I'd say that when dealing with individual cells, and if you have no choice but to parallel them, then "parallel to build your capacity first in groups, and the series-connect those groups together to reach your target voltage". Ideally use the largest capacities you can to cut down on cell-count.

                  Case in point on a much smaller scale - I destroyed a Shorai lifepo4 powersports battery by wiring two of them together the conventional way - despite both of them being balanced together through the external balancing ports and using a cross-connect instead of a ladder-type connection. In time, one battery started to diverge and take the brunt of the load. No amount of tender loving care brought it back to the condition of the first one. And of course merely connecting the balance ports together won't work as the tiny connections won't support the load - the ports are not designed to do that.

                  Still, on a larger scale, and regardless of chemistry, the alternative connection is preferred when dealing with individual cells.

                    Comment

                    • Sunking
                      Solar Fanatic
                      • Feb 2010
                      • 23301
                      #6
                      Originally posted by PNjunction
                      Bravo! Those pics are excellent.

                      The main point being that if you are dealing with individual cells, and not entire batteries,
                      It can be done with 2, 4, 6, 8, or 12 volt batteries, but some of the the advantages are lost when you go to 4 volt or higher batteries. In the EV world pretty much strictly uses Lithium cells. Stil a few around tight on money will use AGM. Lithium has one characteristic not other battery type has. They cannot have an EQ charge applied. Once they are charged, current stops. If you still have low cells in a strings, they will remain low, thus a BMS is required.

                      One argument I have not been able to get across to my peers in the EV world, is they are notorious using parallel cells to achieve a required AH. Ladder eliminates some of the issues, but not all. The manufactures are guilty, and Tesla is the biggest offender. One example is the Leaf battery is a 60 AH battery. It is constructed using 3 20-AH cells in parallel in a Ladder as pictured 3-cells in parallel in series with another group of 3 and so on. What baffles me is why? No one can come up with a good answer other than it is just the way it is done which is ignorant. Same thing we see here with 12 volt batteries in parallel. Today Lithium cells come from 1 AH up to 300 AH and everything in between. Parallel just complicates things that need not be and it drives me nucking futs. Well not really as I like a good debate as I usually learn something.
                      MSEE, PE

                        Comment

                        • inetdog
                          Super Moderator
                          • May 2012
                          • 9909
                          #7
                          I can see the advantages of the ladder connection for high current, low internal resistance cells like Li chemistry. I am not as convinced that there are particular benefits for FLA batteries, and would like to see more of the arguments in favor of the ladder connection.
                          It can make it easier/safer to replace a single bad cell (or battery) without taking the whole system down, but it has some severe problems if you try to actually operate for any length of time with one failed cell (open, not shorted) and do not keep above 50% SOC for the good battery alone.

                          IMHO using the ladder requires more careful monitoring of the condition of individual cells, even if it makes it less likely that they will go out of balance in the first place.
                          SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                            • Sunking
                              Solar Fanatic
                              • Feb 2010
                              • 23301
                              #8
                              Originally posted by inetdog
                              I can see the advantages of the ladder connection for high current, low internal resistance cells like Li chemistry. I am not as convinced that there are particular benefits for FLA batteries, and would like to see more of the arguments in favor of the ladder connection.
                              Been there done that for 5 years before passing it along. In EV's there is quite a few advantages like redundancy. Example if one cell fails open circuit, you can still make it home instead of being stuck.. Lithium does not fail short circuit, and with FLA 90% or more failures are open circuit.

                              • Cuts the cost of BMS Balance boards. One Balance Board can be used for 1, 2, 3, 4...... cells rather than 1 for each cell in every string. Balance boards cost are $10 to $20 each which ad up real fast on a 144 volt system.
                              • But here is the big one for EV's using Lithium, at least on the DIY side of the coin using LFP. Internal Resistance. Perfect real life working example, lets say you want 180 AH and will use CALB CA series cells. You have two options you can use either 3-60 AH cells in parallel to get 180 AH or single 180 AH cells. I will let you figure this out for yourself. Here is your Clue Batman. A Calb CA180 Ri = .66 milli-ohms, and a Calb CA60 Ri = 1 milli-ohm. Which combination yields the lowest Ri Batman? That ought to settle that argument for DIY EV because the difference is significant.



                              Originally posted by inetdog
                              It can make it easier/safer to replace a single bad cell (or battery) without taking the whole system down.
                              Not so much as it is easier and safer using conventional method. Conventional method you would or should have a Battery Disconnect Switch to take the string under test off line allowing you to work on it and not take any chances disturbing the set on line. Not possible with Ladder method. In addition before you close in the off-line cell or string, it has to be equalized within a 1/4 volt of less, or else you weld your contacts together. That can be done a couple of different ways with either a resistor or a charger of appropriate size.

                              The biggest advantage of the ladder method using Lead Acid is it eliminates most of the problems and guess work of string resistance mis-matches. I certainly do not recommend it from a design point of view initially. Only for the Knuckle Heads who screwed up from the start when the went out and bought a bunch 12 volt batteries to make a 24 volt or higher voltage system, and now trying to make the best of their mistake work. In that context it works. And for the Knuckle Heads who go out and buy a bunch of 12 volt batteries for a 12 volt system, will to dang bad, no better teacher than wasting your money.
                              MSEE, PE

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