Active Cell Balancers are Usually Pointless for DIY Solar Batteries


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36 replies
  1. IMHO
    IMHO says:

    The big use of these active balancers are in commercial applications where each cell group is monitored and the data is transmitted to the company responsible for maintaining them.

    For example, if your city buys some electric street sweepers, with a 4 parallel cells per group, if one cell gets a fault and disconnects, the whole battery is only at 75% capacity without active balancing. If used in this condition for long, the other 3 cells will go to cell shutoff voltage each discharge cycle, accelerating degradation on those cells.

    With the active balancer, the street sweeper will keep working with 99% capacity until the company can get out to it and replace the bad cell or module.

    The active balancer only needs to have a fraction current capacity of what is pulled from the battery when there are a lot of cells in parallel and the load is intermittent.

    As active cell balancing gets cheaper I think it can be incorporated in any BMS and it will allow a weak cell group to stay healthy longer. Think of all those laptop batteries that have one bad cell group because it got weak and then was sent to high and low voltage extremes till it degraded to a useless value. An active balancer would have kept many of these alive for another year or so.

  2. KlausenArve
    KlausenArve says:

    Hi Will, love your videos,….Have you ever looked at these 2 BMS's? 123BMS from GWL i think and SBMS0 from Elecrodacus??? Would love to see you test them for a 4s setup…thanks

  3. Jeff Winter
    Jeff Winter says:

    Hey Will really enjoy your videos. Need to figure out a system to keep my batteries for my solar system warm in the winter in Northern Ontario. Some sort of insulated box with temp controlled heat that can be powered by 2 x 100w 12v, in parallel panels and the 2 x 27 series Deep Cycle batteries. Any ideas? Video on how to do it?😁

  4. Michael Eric Menk
    Michael Eric Menk says:

    My CAR do not have active balancing!!!

    And in winter while fast charging, the temperature difference in the pack is between 20 and 30 °C… (Air cooled battery)..

    When a car can be without active balancing, your pack can be as well….

  5. jotatsu
    jotatsu says:

    I already have my first thermal runaway with a 24v pack, all new cells. One of the cells internally shorted and self discharged, the other batteries in the pack begin to dump the charge (it was like 80%) to that one cell. The only thing that saved me was the bms, a fuse and some 3m intumescent foam i have between the cells.
    Until you see a thermal runaway, recommending less safety features seems fine. But is not.

  6. Benny Banger
    Benny Banger says:

    Thanks – that was good info. However, I haven't had problems with that side of the equation. I've just finished building a solar/battery/inverter for my car. Really the biggest learning is getting the current to flow unimpeded and connectors, terminations and cable are usually the culprits producing voltage drop and heat. Just going by current ratings alone are not enough. I see connectors rated at 10A drop 2V at less than their maximum current, similar problems with cable. I think that this subject would be a good one to tackle Will. Perhaps even some demonstrations?

  7. Dave Ulmer
    Dave Ulmer says:

    Will, I sense that you still don't completely understand how LifePO4 cells work when charging. The cells are like glasses of water, as you pour liquid in they fill up just fine. Now when a glass gets full water it flows over the top and spills out No matter how hard you try the level of water can go no higher. Now if you had glasses of water in series such that the first glass that got full would cut off the flow to the other glasses then you could never get the other glasses full. LifePO4 cells work like this. All cells can never be perfectly matched no matter what you do one cell will charge to full first and cut off the charging current to the other cells and possibly go into a damaging high voltage state. This is why a balancer MUST provide a current shunt around the first cell that gets full so that the others can catch up and get full too. The larger the current that the shunt can bypass the faster all cells can get filled.

  8. zawilious
    zawilious says:

    could you please make a video about the benefits of not charging lifepo4 above 80% state of charge and not discharging below 20% , and how to relate charging voltages to a specific state of charge

  9. Yuzi
    Yuzi says:

    HI Will, tilll today i still keep wondering how does the BMS handle the high current application such as using the Lifepo4 in auto start application which will burst 500-700 amp in a few seconds? Some more i can see that most of the BMS are rated at 200-300amps. Above that the price does not make sense anymore.

  10. Honu Moorea
    Honu Moorea says:

    Just explain that if you need a balancer all the time… It's because there is a probleme with your batteries, the blancer will kind hide it a little… But in the end you'll have to deal with the real problem.
    Your cells are unbalanced once cause.. whatever the cause, charge them independently… Then reconnect the pack. If the problem occur a lot, fix the bad pack.
    I got an electric car from 2001…yea.. It got 70km of range (it's fine for what we do with this vehicule) it use NiCd cells… And once a year, cells are discharged and charged fully, independently… This is how the balancing was done in 2001, there is not bms… 😊
    Believe it or not, we only swapped 4 battery modules.. and it still got the original range, well hard to say if it's exactly the same cause temp and weight got an influence but we still from time to time go past 70km…so..

  11. magicmanspaz
    magicmanspaz says:

    Please can you let us know if the daly and bluetooth bms have a minimum current needed before they will allow charging to commence? Some bms i have seen and used in the past required a few amps minimum before it would turn on and charge the battery.

  12. Ted McFadden
    Ted McFadden says:

    Looking forward to the active balancer use case videos! Will, could you do [or have you done] a video on wind power? It seems like everybody focuses on the particulars of the turbines, but I figure there's back-end considerations (dump load? turbine stop?) that could be addressed, even if you're not in a windy area.

  13. Michael Rath
    Michael Rath says:

    Will, could you please pull apart and review a Greenlife, 100AH battery? thanks love your channel. I would also point out to you that your audience is not just home and camper solar enthusiasts. I use your knowledge and learning to educate myself for using lithium batteries on my boats I am making and the company I am starting for next generation boating. You offer a lot in more areas than you know and I would recommend you expand based on that. Thank you.

  14. Johnny Lightning
    Johnny Lightning says:

    Will, Will, Will, what you're saying can't be true I saw it on the Internet so it has to be true! Oh, wait you're on the Internet too. I'm so confused I guess I'll have to go drink a beer and sort it out. Beer done, the Internet is wrong and you're right, as always. Keep up the good work buddy.

  15. landon Ferguson
    landon Ferguson says:

    Very true. I build battery packs with used cells, so I always put these in any battery pack I sell. Even when using used cells they don't balance often unless something's wrong with the battery pack.

  16. Doctorbass
    Doctorbass says:

    Great video man. One more thing about cell unbalance is about current leak. It is one of the major responsible for the need of cell balancing. DIY battery with various cells with different internal resistance and capacitance can be well matched in both total capacity and internal resistance for each groups, however if there is some bad apples cells in these groups the entire cell matching will end out to be ruined anyway!. So this is SUPER IMPORTANT to make the BOUNCEBACK TESTING!! It consist of draining all lithium cells to 3.00 or 2.50V depending on the specs and then to wait for few days. Then you re-measure their voltage. If some have significant voltage drop then these ARE BAD APPLE. Just discard these cells to recycling and keep all other cells. what happen during this test is that you bring cells to very low SOC which allow you to see current leak effect at a much more delta V then when these are recharged. In fact what it does is that it bring the cells to their knee point where the voltage slope get nearly vertical so every uAh or nAh loss are easyer to perceive!… remember: capacity test, then internal resistacne test then bounce back testing.. and THEN you know everything you need about each cells you are assembling so that your pack will be a succes… but yes it is more work as well !! I did that back in 2008 on my ebike with 432x 18650 from makita salvaged pack and the bike ran 12000km with bery minimal balancing… it had NO BMS ! Doctorbass

  17. David
    David says:

    Although its true that new batteries are unlikely to need balancing, "unlikely" is not "never".
    Its like saying you don't need to buy a backup parachute until the first one fails.

  18. Jim Maxwell
    Jim Maxwell says:

    I was one of the folks under the misconception that i needed an active balancer. This was because it is the drift over 15 years i was concerned about, not 6 months. Once built, i have no intentions of continually checking the battery balance….it simply needs to work flawlessly from day one, for the next 15 years.
    Thanks for the input

  19. Steve Clunn
    Steve Clunn says:

    Here is a scenario I don't see many people thinking about. You have a set of batteries that are all the same capacitance but with time their internal resistance changes. Now those cells that have higher internal resistance will have a higher end voltage when charging and possibly have some of that bled off by the BMS. So over time these batteries that all held exactly the same amount get thrown out of balance by the BMS which bleeds the weeker,higher internal resistance cells . I'm not a big fan of bottom balancing, draining everyone down to the same point but I can see where it has some Merit. As you say most of the time batteries stay in sync very well.

  20. MZR0682
    MZR0682 says:

    Bottom balance… Let's beat this dead horse a bit more. Why not top balance instead, so that the bms does not have to spend days top balancing at 100mA ? After that, balancing will be minimal.


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