TL;DR: Putting some of the latest "smart" batteries in parallel is showing an odd behavior. The batteries are not discharging equally. Sort of like they take turns discharging. This is not a desired result. WattCycle and Li-Time are two batteries that are showing this behavior.

Here is a thread on the DIYSolarForum by Will Prowse (owner of the forum) that explains the issue. Full disclosure: I'm a moderator on that forum..
https://diysolarforum.com/threads/bu...g-issue.111783

For those of us that have parallel batteries in our battery bank this is a big deal. We want the batteries to remain balanced. Often we install parallel batteries to increase the number of amps we can pull at one time for large loads such as a microwave or air conditioner. If one battery has been carrying all the normal loads and it goes down to a low state of charge while the other battery remains at 100%, when the large load kicks in the combined amps of both batteries may not be available.

 

My Renogy batteries do not have this behaviour, but they are linked with an RJ45 data cable so they can coordinate their charge/discharge and remain balanced.

 

That’s crazy behavior. I have 3 of the “dumb” LiTime 200ah and so far they have been good. I’m on that site a lot trying to absorb information. Thanks for sharing the video too.

This might be a dumb question, but I haven’t had any with WiFi or Bluetooth tech. Can the BMS be updated to possibly correct this through their app? Or would the manufacturer have to fix the software issue and then you would need to buy a whole new battery?

 

This is a recent issue. Older batteries (bought more than 6 months ago) won't have this problem. The problem is likely limited to vendors that are using the same BMS. At the time I created this thread, the brand/model of the BMS wasn't known. There is an expectation that the vendor(s) either will make a fix soon or will have already done so. There is a lot of visibility for this issue.

you can remove the BMS from an off-the-shelf battery and replace it with a different one. Usually.

 

I built my 4 batteries using 304ah EVE batteries and they all have a separate 200A JBD BMS. None of them drain equally. None of the charge equally. All my batteries are balanced individually. Just because they all read slightly different, does not mean they are imbalanced. At least that is how I look at it.

My first step in my build was having 2 304ah batteries. One with a Daly BMS and the other one a JBD BMS, they discharged/charged differently. I got rid of the Daly because I thought that was the cause of the different charge/discharge rate. Sadly, it did not help.

Is this a 'real' problem? With my little experience my answer is no. Our expectations don't match what we see. That is why we think it's a problem. If you never saw the indifference's, you would never know you have a 'problem'.

I have learned that just because I think it's wrong, does not make it wrong. My system does what I want it to do and that is all I care about. Since I don't believe I have a problem, I will focus my attention on other things I can't fix.

I don't expect anyone to agree with my opinion. The only experience I have is what I have done. I spent months researching what I elected to do and I am very pleased with the outcome. I am not an expert whatsoever. Just another clown on the internet with an opinion.

 

I believe the OP was saying that his was not just a few mV difference - one was discharged completely while the other was at 100%. Or was he just extrapolating from a few mV difference? You are correct that a slight diff isn't something to worry about. I have that too.

 

It is a very large discrepancy in SOC between the one battery that is initially discharging and the remaining battery(s) in the bank. Below is the link to the Will Prowse video that details the issue. He is talking about WattCycle batteries but LiTime batteries have the exact same issue. I have had the one battery that is discharging actually hit low voltage disconnect on the BMS before another battery in the bank kicks in. The amount of charge that flows at that point between the deeply discharged battery and the one that just woke up can be huge.

Link to video explaining the issue in detail ->

 

DIY batteries are not at issue within this thread. The problem is with recent "Smart" batteries.

DIY batteries that don't discharge equally can be due to a few issues:
1. Different Internal Resistance (IR) of the cells between batteries
2. Different size cables
3. Different length cables.
4. Poor connections

 

Sounds to me that smart batteries aren't all that smart
Denny

 

Yeah! And the programmers that programmed them aren't in tune with how the batteries are used in the real world.

 

I charged my Li-Time 100ah Mini batteries individually with the same converter and then like they recommended tied them together to balance with no load and started using them and they work great and the only thing I monitor is the voltage when they are in use dry camping. When hooked to shore power after they are charge I disconnect the batteries and run the trailer off the converter. I used 3 Mini batteries because they fit into my battery box.
Denny

 

If that is the case, then I would agree there are issues.

I guess what I don't understand is the term smart. Maybe someone can define that definition to an idiot like me.

 

Or that people are using the batteries in a way they were not designed for.

When I was young mechanic, I used to bad mouth the engineers and said they were stupid. In simple terms, my way was much better than theirs. As I got older, I worked with many of those engineers and once I understood why they did what they did, it was clear, they had good reasons why it was that way. Now, I tend to look at situations like this differently. I tend not to blame engineers/programmers on what I think should be done.

 

We've been using LiFePO4 batteries in parallel for years without the BMS causing problems like this. The DIY batteries in my prior camper discharged almost exactly the same.

 

"Smart" can mean whatever they want, but in the LiTime product line it simply means that it has bluetooth monitoring.

Not sure how or if that means anything with regard to the unequal discharge issue that people are seeing, other than the fact that they can look at their phone app for each battery and see it happen. Maybe they all do it and only the smart batteries with built-in BT make it easy to see? Probably not true since that would not fool a guy like Will Prowse.

I have the LiTime 230ah smart battery in our fiver. I went that way instead of trying to parallel multiple smaller batteries. Problems with multiple BMS connected in parallel or series really isn't anything new. So I just felt I could avoid all that with a single higher capacity battery. All of that is really above my pay grade, but I will say that I totally love the LiTime bluetooth method of monitoring the battery SOC. No fooling around with shunts and dedicated monitors.