Battery drain faster than other

[stekicar]

Hi all,
I have a 48V system with 12.5kW solar and 3kw + 12kW inverters. My battery pack was made of 20 AGM batteries (Universal UB121000-45978 100Ah), currently 16, and counting down.
I have built a battery monitoring system that measure each battery voltage and temp using Raspberry Pi and Ina219 with voltage dividers. I also have Thornwave Powermon to monitor total voltage and current going in and out of batteries. What I have found out is that batteries closer to positive side drain faster than the rest of the batteries with exception of one row. I tried different configuration from serial/parallel to parallel/serial connection. All cables are 4AWG, same length and as short as possible. No matter what I do, batteries closer to positive side main terminal drain much faster than the rest of the pack. At first everything worked as calculated, but now batteries are giving me big problem that I do not know how to solve. Well, there is a solution, but it would be expensive one ...
Does anyone knows what can cause this drain? Personally, I think that these batteries are dud, and should not be used for solar system.

[Mike90250]

Attempting to parallel AGM batteries (or any battery) is going to fail because of internal battery impedance differences.

However, there is no explanation I know of that would explain the batteries close to the + cable, other than thermal differences, hotter near the + and causing the batteries to drain. You didn't "tap off" of the 12V anywhere did you. That will unbalance a bank quickly.

I also expect that 4ga interconnects are too small, and you should be using heavier wire for such a large bank and array

[Tecnodave]

I know what is happening there.....this explains it well......... http://www.smartgauge.co.uk/batt_con.html

[littleharbor]

5 strings of AGM batteries will be nearly impossible to maintain equal current paths. You really need to switch to larger batteries if your power needs require that many batteries. Look into 2 volt cells. If you don't really need a 500 Ah. 48 volt battery bank then possibly 6 volt L-16 batteries will work.

[stekicar]

Attempting to parallel AGM batteries (or any battery) is going to fail because of internal battery impedance differences.

However, there is no explanation I know of that would explain the batteries close to the + cable, other than thermal differences, hotter near the + and causing the batteries to drain. You didn't "tap off" of the 12V anywhere did you. That will unbalance a bank quickly.

I also expect that 4ga interconnects are too small, and you should be using heavier wire for such a large bank and array

There is no "tapping off" present with exception for voltage divider. But resistors are very high resistance so no big drain there. As for 4AWG, cable length is 4". I use 4/0AWG for main wire to inverters.
I have measured internal resistance using 25W resistor and they were all over the place. But, nowhere near 5 Ohms manufacturer states. They were at 50+Ohms. I tried to connect them in series with close resistance to each other, but still nothing changed.

[stekicar]

I know what is happening there.....this explains it well......... http://www.smartgauge.co.uk/batt_con.html

Nope. That's not it. My batteries are already connected per picture #4 per web site you provided. That is why I reduced number od batteries to 16. I really hoped that this will fix this weird issue but it did not.

[stekicar]

5 strings of AGM batteries will be nearly impossible to maintain equal current paths. You really need to switch to larger batteries if your power needs require that many batteries. Look into 2 volt cells. If you don't really need a 500 Ah. 48 volt battery bank then possibly 6 volt L-16 batteries will work.

My calculation, based on power consumption, is that I need 250ah from dusk to dawn which is approx. 10 hours to 12-13 hours in the winter. So, since, these are 100ah batteries, if I draw 5amp for 10 hours times 5 strings, it should last 10 hours max for half DOD and 25ah continuously. At first, everything was as calculated, but then this issue started. This is the reason why I built monitoring system, to check what is going on. Also, I think these batteries are not for high amp draw. Although manufacturer specifies drain at 3C, I do not think that these batteries are capable to provide even 1C.

[stekicar]

48 volt battery bank then possibly 6 volt L-16 batteries will work.

I do not know if I want to do this. I am not willing to gamble again with AGM batteries. I am looking into 3.2V LiFePo prismatic batteries due to high amp capability. They look promising, but I have not found anyone with 300ah capacity installed and running. Even if I get 3x16 of LiFEPo I will have to parallel them and what if issue starts again?

This is what I have also tried. Made two of each strings running through two Gigavac GX14 contactors and run them intermittently. I was monitoring voltage for running battery pack and if voltage dropped below 48V I was switching to other battery pack. And same thing happened. First batteries to main positive were draining fastest.

Just for the test I used one battery that leaked at terminal and connected it closest to main positive but that one did not drain fastest, second one did!
So, I think that internal resistance is the biggest culprit which I cannot get rid off. I also started thinking making some kind of floating resistors attached to batteries ... do not want to even think about it.

[stekicar]

My theory is, since battery closest to main positive is at highest voltage, electrons travel faster from negative to positive terminal within battery, while other electrons coming from secoind to fourth battery are slower due to accumulative internal resistance. This is causing first battery to drain fastest.
But, I do not know how to explain why leaking battery did not drain fastest, but second one did. Maybe, leaking battery has higher internal resistance?

[Tecnodave]

AGM batteries do not work in parallel wery well at all, as you are discovering.....but LiFePO4 batteries do, but parallel them at a cell level so that each two in parallel are 1 battery, there is a very good post on this forum with lots of advise on paralling batteries, with LiFePO4 batteries you need a BMS, battery monitoring system that will balance the batteries. There is also bottom balancing and top balancing to deal with. Lithium is not a drop in conversion from lead acid batteries. There is a learning curve to lithium, if you get it wrong, things get expensive rather quickly. Lithium is not as forgiving as lead acid. There is much reading and learning to do. Read what Sunking has to say about parallel batteries, he really has a handle on the issue.

[stekicar]

Thank you for the advice. I am mostly interested in Nissan battery modules and LiFePo prismatic batteries. And that is what I am currently doing - trying to get, as much as I can, a valid info on these two types. So far, (maybe we are getting off the subject in this post though) what I have understood is that parallel batteries level out equally so only one BMS line is needed per parallel connected batteries. As for balancing, I have found some active balancers 1A and 2A on AliExpress so I am looking into those ones as well because they do not waste energy but move energy from high voltage to low voltage cells. At present, I will continue to use existing batteries and by the time I am confident that LiFePo batteries are good for my use and price fall down enough, I will make a move. Luckily, my solar system works perfect (knock on a wood), so I can run my system full capacity. I can even charge my EV with it when weather permits. All of my calculations turned out to be correct with exception of batteries. It's a bummer, I know ...

[Tecnodave]

I searched for that post on parallel batteries but did not find it, maybe someone will post the link to it, the post gets into the deep on paralling issues.

Yes that is what I'm talking about, active balancing, and with cell level paralling only one BMS is required, I'm no expert on lithium but I do have two sets of Prisimatic LiFePO4 cells and many 18650 LILO packs for my repowered power tools that were NiCad or NiMh earlier in life.

my main banks are large format FLA, not paralled unless huge loads are expected that day, I have twin solar systems, 100% redundancy...

[stekicar]

I have found one research from some scientists explaining why parallel and then serial is preferred way of connecting lithium cells. Unfortunately, I cannot find the bookmark for this website. I tried this method for my AGM batteries and it was a total disaster. At least I know why - current was flowing through batteries with least resistance causing them to drain faster. But parallel pack closest to main positive drained fastest again.

[stekicar]

AGM batteries do not work in parallel wery well at all, as you are discovering.....

Speaking of this. I think if I did not have an issue with first battery draining faster then others I could develop a system with 5 contactors per stream and run them intermittently. This way, I would run all batteries in series only. I sent an e-mail to manufacturer about this issue, but they are useless. They did not even respond.

[Mike90250]

Before you consider Li battery packs, you really need to solve the issue with the simple LA batteries.
How old are they ?
Were they commissioned properly ?
Had any been overcharged and vented gas ?

Understanding what is happening, and solving it, needs to be done before doing the same to a very expensive Li bank.