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  1. #1
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    Default Balancing 2x Lithiumís in series

    A mate bought one of these for his system, Kodak 5kva 48v with 4x AGM batteries. https://shop.thesunpays.co.za/collec...-battery-banks

    Reason was because the batteries werenít in sync, sometimes reading up to .5v difference between the highest and lowest.

    My 2x BlueNova 108ís with builtin BMS does the same, but havenít thought as much about it. When testing thereís a 0.4/0.5v difference between them as well, but always thought the BMS in each battery will sort itself out and protect the battery. The BMS in the BN108 is on itís own island, no way of communicating to the Axpert King I have.

    Would it be advisable and beneficial to get something similar, if it can be done on Lithium with builtin BMS?

  2. #2
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    Default Re: Balancing 2x Lithiumís in series

    Before any tech intervention, first check if you have any resistance/ voltage drop on the connection between the 2 batteries. Do this when there is significant charge or discharge current present. Check on the actual batt terminals, not on the stuff bolted on....

    Which Blue Nova type is installed here?
    + are they of the same age?
    Last edited by Dungbeetle; 2021/05/01 at 04:47 PM.
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  3. #3
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    Default Re: Balancing 2x Lithiumís in series

    Iíll run the inverter on batteries and do as suggested, thanks.

    Itís these, bought at the same time: https://sunstore.co.za/products/blue...SABEgJvZfD_BwE

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    Default Re: Balancing 2x Lithiumís in series

    My 2 cents worth.

    If your batteries are in parralel, I think they would be fine and self balance over time.

    If in series, based on the description from your friends setup, and assuming the BMS inside would shut down and protect, you could try it. This is with out any knowledge where the bms on each pack would cut out. Worst case I think your cells would be protected from under and over voltage. But, the two packs would still grow an indifference. One would cut out, and the whole set would be off line. Thing is, you never know what internal BMS is doing, and if it is working.

    You could also, on a weekly basis, disconnect them when they are full, pair them up in parralell. This should equalize them. I would definitely start here if it where my packs. Actually, I would take one or both packs, discharge them, and see where the BMS cuts out. Not sure if it would activate automatically on charge, or voltage rise with no load would enable them again. Worth checking with manufacturer.

    You could also get a single active balancer, which is cheap. Various pros and cons to them, but you still have internal BMSes taking care of the risk, so I think it could work.

    There might also be other risks associated, so you could also check international forums for putting 2 packs with their own BMSes in series.

    Biggest risk factor here, I think, is knowing the internal BMS is working, and when.

    Edit. Uuuhh, title says series.
    Last edited by CeeZee; 2021/05/04 at 07:38 PM.

  5. #5
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    Default Re: Balancing 2x Lithiumís in series

    Can takes weeks to balance the cells.

    Quote Originally Posted by CeeZee View Post
    You could also, on a weekly basis, disconnect them when they are full, pair them up in parralell. This should equalize them.
    Cells cannot balance themselves. Their has to be a current passing into them to do that.
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  7. #6
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by the_terrible_triplett View Post
    Can takes weeks to balance the cells.



    Cells cannot balance themselves. Their has to be a current passing into them to do that.

    Well, if I where a casual reader, I would have asked: Which is it now, can it be done but takes weeks, or, can't it be done? But, I'm not a casual reader, in this case, but I might be a dilettante!

    So, I ran some tests today for some pictures. After all, pictures tells a story, don't they? (Damn, need more camping pictures. Winter around the corner.)

    Test 1.

    4 Old 18650's.
    3.92V , 3.99V, 4.04V, 4.07V. Hooked them up after 7am, by 12 am, they where balanced to .01V. Zero external input. Now, my Batrium balancer, not a mickey mouse peace of gear, would consider this balanced.

    Test 2.

    Took 2 LifePo4 cells, with a 0.02 V difference. Now, again, my Batrium, would not have considered these cells out of balance. But, for my own learning, I hooked them up, with a amp meter. So, initial amp draw was 1amp, at 12pm. By 5pm, the amp draw or shift by amp meter where still .25A. Balanced both to 3.29V (with minor fluctuations.)

    To put the amps in perspective, this was about lets take a round figure, 2AH for the 4 hours. (Give or take.) These are on cells already 'balanced' if I had to put them on Mr. Batrium. So, on my 1000AH bank ( uuuhhhh, this is F-huge) my Batrium balances about, 1-2AH per day, or for the time it is in balance mode/range. Maybe not even every day of the week. These numbers on a already close to balanced cells for a 120AH cell, are quite significant. No external power input.

    Now, for some more interesting reading, if you are not a 'casual' reader. Pay attention to the time it takes. Weeks? (This article, for my own understanding, I will keep and come back to it, I think)

    http://liionbms.com/php/wp_parallel_balance.php

    On my 1000ah bank, or my other packs, I never had do 'burn off' with drastic measures or implement other drastic measures to get them in balance, before I hooked them up. Simple self balancing, worked for me.

    My advice, given available tech to the original poster, time and budget, would be the same, but, I would, stress the following point as highlighted in the white paper as well!

    Be careful when you self balance with 100% charged packs. This is a change to my original comment, especially if they are not close to each other in voltage or SOC.

    On more thing: I would check as per DungBeetles post. Also, swop the batteries around. If the problem remains on one side, it might tell you something, or not at all.

    Bees, as per article, I would balance them when they are 50% initially, then more at a higher rate. My understanding of Lifepo4, and it's flat curve, is that 0.5V/4=1.25, depends on where you are on the curve, could start to become significant, depends on SOC and capacity. So, the answer to your initial question of whether you should do something? If it where my setup, I would definitely not ignore the warning lights!


    Enjoy!

    Edit.... 0.125v. Tx.
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    Last edited by CeeZee; 2021/05/06 at 06:42 AM.

  8. #7
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by CeeZee View Post
    Well, if I where a casual reader, I would have asked: Which is it now, can it be done but takes weeks, or, can't it be done? But, I'm not a casual reader, in this case, but I might be a dilettante!
    Oooo, you have now SO opened a can of worms, you dilettante you!

    What you just posted, I'm SO going back to those two guys who told me, with authority, that when I connected cells in parallel to let them self balance, I wasted my time.
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by the_terrible_triplett View Post
    Oooo, you have now SO opened a can of worms, you dilettante you!

    What you just posted, I'm SO going back to those two guys who told me, with authority, that when I connected cells in parallel to let them self balance, I wasted my time.
    Please provide feedback here as to their sentiments when you tell them - what the fudge dudes?
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  10. #9
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by Die SwartKat View Post
    Please provide feedback here as to their sentiments when you tell them - what the fudge dudes?
    Ps. Swartkat, you know whom the source is/was ... the other was a engineer who posted same. No need for names.

    Slight change in "tune". I was told in no uncertain terms and without any debate that connecting cells in parallel, leaving them stand, makes no difference.
    Fact that it took me <4 days of manual intervention to Top and Bottom balance a very unbalance bank, made no impact on the debate.
    Yes not all the cells are equal Top and bottom, so either balance Top OR Bottom, the chance that both would be near identical is heavily dependent on the age and manufacturing quality of the cells.

    The reply:
    Not sure why anyone would think that does not work. If you connect them in parallel, then those with a higher voltage will dump current into those that have a lower voltage, until they have the same voltage.

    But that does not mean they have the same amount ďchargeĒ (aka coulombs). Perhaps that is why some say it does not work. Perhaps they mean it is somewhat meaningless.

    As a thought experiment, consider what happens if you connect a 7Ah SLA battery to a 80Ah car battery. Well, they are probably going to settle somewhere around 12.5V, and until that happens some current will flow from the one to the other.

    But that does not mean they are now equal. The same applies, although on a much smaller scale, between almost-identical LFP cells.

    Edit: But, because the cells are ALMOST identical, putting them at the same voltage does help in the sense that they will then be ALMOST charged the same. It eliminates some of your problems. You probably donít need more than a few hours for this to work.
    Last edited by the_terrible_triplett; 2021/05/06 at 10:29 AM.
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by the_terrible_triplett View Post
    Oooo, you have now SO opened a can of worms, you dilettante you!

    What you just posted, I'm SO going back to those two guys who told me, with authority, that when I connected cells in parallel to let them self balance, I wasted my time.
    Just a thought on this. All laptops are running with them in parallel and their BMS manage the 3 BIG cells that are then in series on most laptops.
    This seems to work fine.....

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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by ekkekan View Post
    Just a thought on this. All laptops are running with them in parallel and their BMS manage the 3 BIG cells that are then in series on most laptops.
    This seems to work fine.....
    We are talking about connecting cells in parallel with no BMS nor any source of power, to help balance the cells faster.
    Political Correctness
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  13. #12
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by the_terrible_triplett View Post
    We are talking about connecting cells in parallel with no BMS nor any source of power, to help balance the cells faster.
    Sorry. I misunderstood it.
    To me the OP was about more than 1 Bluenova being used together with each one having it's own BMS.

  14. #13
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    Default Re: Balancing 2x Lithiumís in series

    Quote Originally Posted by ekkekan View Post
    Sorry. I misunderstood it.
    To me the OP was about more than 1 Bluenova being used together with each one having it's own BMS.
    It derailed ... if the BMS's are designed to be connected to each other in parallel, perfect.
    Otherwise it can become interesting, as was discussed on other threads.
    Political Correctness
    Political Correctness is a doctrine, fostered by a delusional illogical minority, rabidly promoted by a unscrupulous mainstream media which holds forth the proposition that it is entirely possible to pick up a turd ... by the clean end!

    Saw this in a pub - no idea why the wife does not see it so:
    I'm not opinionated ... I'm just always right!

  15. #14
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    Default Re: Balancing 2x Lithiumís in series

    Now HERE is a very GOOD video from Will Prowse.

    Political Correctness
    Political Correctness is a doctrine, fostered by a delusional illogical minority, rabidly promoted by a unscrupulous mainstream media which holds forth the proposition that it is entirely possible to pick up a turd ... by the clean end!

    Saw this in a pub - no idea why the wife does not see it so:
    I'm not opinionated ... I'm just always right!

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