Mixing battery chemistry

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  • NeilTheCop
    Member
    • Feb 2010
    • 53
    #1

    Mixing battery chemistry

    My small solar system is/was comprised of 4 * 6 volt golf cart batteries wired in series/parallel to give me about 200 amp hours at 12 volts. Two of the batteries have died simply due to old age and now I'm using the remaining 2 fairly new batteries in series to give me the 12 volts. Rather than just replacing the two failed batteries with more SLA ones I was going to upgrade to Lithium Iron Phosphate prismatic cells, and also upgrade my modified sine wave inverter to a pure sine wave 24 volt input.
    My question is can I mix and match the two chemistries and connect the SLA and Lithium batteries in series to give me the 24 volts until the remaining SLA's die and get replaced with more Lithium cells. Or will magic smoke and major damage ensue?
    Tags: None
    • inetdog
      Super Moderator
      • May 2012
      • 9909
      #2
      You are not absolutely guaranteed loss of magic smoke, but it would be very hard to avoid it. Batteries in series must carry the same current at all times unless you use two chargers, one for each set of batteries. The SLA and any Lithium chemistry will have far different reactions to overcharge. The SLA requires it (although not at high currents) while the Lithium simply cannot tolerate it without damage.
      It is a tempting thought, but I personally would not be comfortable with it, even with extensive custom BMS and close monitoring.
      SunnyBoy 3000 US, 18 BP Solar 175B panels.

        Comment

        • Mike90250
          Moderator
          • May 2009
          • 16020
          #3
          In short, I'd recommend AGAINST it. It would be very difficult to engineer it safely & well. The cost of catastrophic failure with Li cells is high.
          Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
          || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
          || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

          solar: http://tinyurl.com/LMR-Solar
          gen: http://tinyurl.com/LMR-Lister

            Comment

            • jflorey2
              Solar Fanatic
              • Aug 2015
              • 2331
              #4
              Originally posted by NeilTheCop
              My question is can I mix and match the two chemistries and connect the SLA and Lithium batteries in series to give me the 24 volts until the remaining SLA's die and get replaced with more Lithium cells. Or will magic smoke and major damage ensue?
              You could probably get away with it for a little while if you use a good BMS. If not, you'll get one or two charge cycles, tops, before you destroy one or the other.

                Comment

                • NeilTheCop
                  Member
                  • Feb 2010
                  • 53
                  #5
                  Thank you all. I decided to run two separate systems, one 12 volt SLA for light loads and a complete new 24 volt system with Lithium for the heavier loads. When the SLA's die everything gets switched to 24 volt Lithium. Unless of course one of the "Groundbreaking, new, wonderful, game changing" battery technologies ever actually comes to market

                    Comment

                    • Ampster
                      Solar Fanatic
                      • Jun 2017
                      • 3658
                      #6
                      Originally posted by NeilTheCop
                      Thank you all. I decided to run two separate systems, one 12 volt SLA for light loads and a complete new 24 volt system with Lithium for the heavier loads. When the SLA's die everything gets switched to 24 volt Lithium. Unless of course one of the "Groundbreaking, new, wonderful, game changing" battery technologies ever actually comes to market
                      That sounds like a good plan. What type of Lithium batteries are you using?
                      9 kW solar, 42kWh LFP storage. EV owner since 2012

                        Comment

                        • inetdog
                          Super Moderator
                          • May 2012
                          • 9909
                          #7
                          Originally posted by Ampster

                          That sounds like a good plan. What type of Lithium batteries are you using?
                          "... Lithium Iron Phosphate prismatic cells,...."
                          SunnyBoy 3000 US, 18 BP Solar 175B panels.

                            Comment

                            • Ampster
                              Solar Fanatic
                              • Jun 2017
                              • 3658
                              #8
                              I meant to ask what brand?
                              9 kW solar, 42kWh LFP storage. EV owner since 2012

                                Comment

                                • NeilTheCop
                                  Member
                                  • Feb 2010
                                  • 53
                                  #9
                                  Originally posted by Ampster
                                  I meant to ask what brand?
                                  Not sure yet, but if the tariffs with China go into effect the whole idea may be put on hold for a while as I'm sure the prices will spike

                                    Comment

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