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    Question about using slightly mixed capacity cells

    Hi folks,

    I am working on building up another battery or two, using castoff laptop batteries that would normally just get sent off to the electronics recycling bin (which costs us money). I am saving the batteries, and recycling all the plastic, so it's a win-win!

    So then, we tend to buy from single sources when possible. That means I have a good amount of Dell packs to work with. Most packs have Samsung ICR18650-26D batteries, but some have LG DBB1865 cells. Both those are 2600mA capacity, with 5A discharge.

    I also came across Samsung ICR18650-30B cells. Those are still 5A discharge, but 2950mA capacity. A third cell in the mix is a Sanyo ZTA. Those are 3000mA capacity, and 5.8A discharge.

    The DBB's and 26D's are lower capacity, but flow the same current as the 30B, and ZTA cells. I will pretty much never fully charge, and the BMS balances voltage as we go. From what I read, it seems like it should not be a problem to use all these cells together. I would just need to be sure I set my controller so it doesn't pull more than 20A for a 4P build, or 30A for a 6P build.

    I searched for threads talking about this kind of issue, but didn't find anything, so I am putting this thread here for discussion/feedback.

    Jose

    #2
    I am no expert. But I would stick with same capacity cells grouped up. Imagine you try to fill a 3000 liter bottle connected to a 2600 liter. I would think their may be some strain on the 2600.

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      #3
      Ok,

      So the more I look around, the better I think I am understanding how these large e-bike battery packs work.

      From what I have gathered, the important thing to understand is that as each series group discharges, they balance amongst themselves first, but also try to do so up/down the series, which is the benefit of the BMS adjusting voltage across parallel groups of the entire pack. If you don't run a BMS, then voltage is constantly running back/forth across all your parallel groups. For the discussion below, let's assume a 16S6P battery pack is the goal, all batteries are rated to at least 5A constant draw, and it is mostly 3000mA cells, but mixes in 2600mA cells. First, I'll describe what I understand is a very unsafe way to build a battery pack within the given constraints.

      If most parallel groups are built from 3000mA cells, and only a couple of parallel groups are using just 2600 mA cells, you increase the risks of draining them too much, or too fast. You could also end up charging them too fast, or too much. All conditions can lead to early failure, or possibly fire. The reason this condition is so bad, is the balancing that is going on. When the batteries are discharging, the voltage drops quicker in the groups that are only 2600mA. The neighboring groups of higher resistance 3000mA batteries then try to back-flow voltage to balance out that more rapidly draining pack. At the same time, an active BMS is trying to add resistance to the higher capacity groups to get them in-line with the lower capacity groups. All of this adds up to your BMS possibly cutting off power due to imbalance across the pack, or cells heating up as power is trying to surge back/forth through the pack. The 2600mA cells with also go through more "abusive" cycling as they try to keep up with the higher capacity, likely resulting in much shorter battery pack life.

      A second option that sounds like you could "get away with" is building with slightly mixed batteries distributed across all parallel groups. With each parallel group having the same mix of batteries, they should discharge at equal rates. With heavy load, the smaller battery would again lose voltage more quickly, but the big reverse flows between packs, and across the BMS are avoided. Also, because the smaller cells are all in parallel with the larger cells, as voltage balances within each pack, you reduce the load moving across higher resistance parallel groups, which should mean more efficient balancing as you use the whole pack, and less heat. In use, the 2600mA cells should drain more quickly when under heavy load, but should recover actively/quickly when load reduces.

      I might be willing to experiment with smaller battery packs, but upwards of 50V, that is a whole lot of power to take risks on. Given that these "free" cells I am gathering are all limited to 5A, I am thinking I have two choices. Keep collecting and hope I end up with 96 of each capacity, or build a couple of lower voltage packs with more parallel groups. I am just 6 batteries away from being able to make a 12S5P pack of the 3000mA cells (15Ah), and three packs away from making another one with the 2600mA cells (13Ah). That would be good for 1200W (48V at 25A). I'll need to watch my power consumption at different settings to see if that would be good for putting around town. I suppose I could just set my controller to 50% current/phase and that would show me what that 25A would feel like.

      Jose

      Comment


        #4
        Originally posted by DaHose View Post
        Ok,

        A second option that sounds like you could "get away with" is building with slightly mixed batteries distributed across all parallel groups. With each parallel group having the same mix of batteries, they should discharge at equal rates.
        Jose
        This is what you want. You DO NOT want all 3000s in some of the 5ps and all 2600s in others. Each of the 5ps should be balanced and there are calculators over at Second Life to help with this. What I don't see is that you have cycled the cells and tested their capacities to be sure of their capacities.

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          #5
          Hey Diggs UT. Good to know I'm on the right track about mixing cells. You are correct. I am continuing to amass my bounty of used cells, so I only made sure all batteries would charge. I left them all at 3.8V for storage.

          My small 4-slot testing charger that can fully cycle test batteries, but if I test all batteries now, I will be storing them fully charged until I get all my batteries collected, which is not desirable. Once I have at least enough cells for one 5P pack, I'll do full capacity tests. It will take quite some time, but I'll have accurate numbers for each cell. If I have enough for two small packs, I think that would be lighter/better for when I'm riding trails, and wanting PA levels of power. My big battery will be for putting around town.

          Jose

          Comment


            #6
            I added an imax B6 charger to my toy box. It has a storage function that drains the cells to 3.65v for storage. After I cycle them for capacity I drop them in the imax with storage function turned on while the next batch of cells is cycling for capacity. They B6s are inexpensive and a nice addition for anyone that works with cells (of all types) and packs.

            Comment


              #7
              This the tool you looking for?

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