Update (~2.5yrs) - 1100ah Xuba - Sailboat

[CaptNordy]

Hello everyone,

I wanted to post an update plus some questions. I purchased 1100ah from Xuba on July 20, 2020 after finding this forum and have been living with these batteries full time on our live aboard sailboat. Overall I have had ZERO issues with these batteries, have done ZERO maintenance and I'm very pleased considering their price point.

Brief overview of system:
BMS = REC-BMS
Charger = Victron MultiPlus being externally controlled by the REC-BMS (100% run from generator since we haven't stopped in a marina)
Alternator = Balmar 140amp with their programmable regulator
Solar = ~1000w with 5 MPPT Controllers

Brief usage pattern:
We need to run our watermaker every 5 days, so initial system was designed so that we could live off of solar until the 5th day, then run generator to make water and top off batteries. This has worked in the dry/summer months, but in the winter and rainy seasons we can only go about 3-4 days before needing to charge. I typically charge when we get in the 35-45% remaining range, but I have gone down as low as 25% on occasion.

Observations/Questions:

1. One thing I've noticed is w/ the 140amp alternator, the REC-BMS cannot keep up with balancing, so the first pack always hits 3.6v while the others are in the 3.5v or 3.4v range.
2. When the packs were new, I could get to 14.2 at 100%. After year one, I was at ~13.8 and 96%, today I'm at ~13.6 and 92%. These voltages are hit by the Victron Multiplus charger (externally controlled by the REC-BMS), when it start to hit these voltages, it goes from putting in its normal amperage down to ~10ams/hr. Basically it seems to drop in to float (or is it absorbtion). When I shut down the generator, the solar panels will continue charging and can bring the final percent up a couple more (so from 92% to 94%).
3. Any guidance on what maintenance I can do to get these cells back up to 14.2 and 100%
   1. Should I pull them all and do a top-balance?
   2. Should I rebuild the packs to change up the cells since pack 1 has seen more 3.6v than the other cells?

Thanks for any help and let me know if anyone has any questions about my setup or would like more info!

 

[MisterSandals]

1. Should I pull them all and do a top-balance?
2. Should I rebuild the packs to change up the cells since pack 1 has seen more 3.6v than the other cells?

When your pack hits 3.6V, are all the cells equal or do you have a couple runners? How about the other batteries, are the cells pretty much the same within each battery?

How are your batteries wired? Is one battery getting more charge current and load draw than the others? Happen to have a pic or diagram of how the batteries are wired?

Answers to these questions should narrow down the problem to the cell level or the battery level. Any solutions or courses of action would depend on this too.

 

[CaptNordy]

MisterSandals,

Sorry for the delay, I wanted to charge things up so I could get voltage readings. It is hard to 'freeze' the voltages for this test since I didn't want to disconnect the REC-BMS (so balancing was actively occurring), but you can see cells (packs) 1 and 4 hit the 3.6 in a couple cells before packs 2, 3). I think the main issue is the REC-BMS balancer cannot keep up with the 100+ amps going in, so things start to get out of balance. You can see if I let them sit for a few hours, they balance nicely.



Other thing I noticed was the REC-BMS app says I'm at capacity (only used 25ah), but it shows only 90% charge which doesn't make sense. the 13.7 voltage and 90% seem to make more sense, but I cannot get above the 90% since cells are going above 3.6v. (the 3.718 readings shown below are because I turned alternator back on for a couple minutes to show max again since they balanced while I took the readings and all dropped).



Let me know if you need any more data from my side and thanks for any help!

 

[MisterSandals]

The battery diagram shows your problem. The wiring is very unbalanced.
The current path of least resistance is straight across the bottom cells between the positive and negative wires.

Moving the main battery connection point up between 2nd and 3rd rows should help a bunch but still not perfect.

Look at this thread for a explanation of the concept. After a few posts just looking at pictures is probably enough to see good, bad and best.

Calculation of parallel string battery currents

Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current...

diysolarforum.com

 

[MisterSandals]

Also, with a 4P4S config, it’s important to have the 4P pack capacities as equal as possible (combined capacities of each pack close to 1100Ah/4.)

If your 2 weakest cells happened to be in 2nd column, that pack would be weakest and charge fastest and discharge fastest. The weakest link.

Did you do anything to mix and match cells or was it random?

 

[MisterSandals]

I think a better place to move the wires is like this. Move positive wire up slightly between 3 and 4. Move negative up between 1 and 2.
This is very good balance.

 

[CaptNordy]

Mister Sandals,
Thanks so much for this. When I first purchased the cells I top-balanced and all where matched well after I let them settle. I matched the packs with cells that were closest in voltage.

I can move the positive and negative to see if that helps any, thanks for that recommendation. I'm assuming you mean move the positive up a position, but still within what I have labeled as Cell 4, and negative up a bit, but still within what I have labeled Cell 1. So something like this:

I read through the SolarRod post and it seems that using the terminals like this doesn't change much. He recommends drilling holes in the bus bars and making a new attachment point in-between the bus bars...

What do you make of the whole 90% full at 13.7 vs REC-BMS saying I've only used 25ah? Also, why does it say SOH 98%. If max I can charge to is 13.7 am I not lower than 98% SOH? I'm guessing REC-BMS is just using some crude algorithms to measure, so nothing that I can rely on.

 

[MisterSandals]

I matched the packs with cells that were closest in voltage.

So you "may" have all the strongest together and the weakest together. I dunno though. I tried to make correlations between how quickly or how much cells settle vs cell capacity and did not come up with anything conclusive (i am in the middle of actual capacity testing to get real numbers.)

What do you make of the whole 90% full at 13.7 vs REC-BMS saying I've only used 25ah?

BMSs aren't made to be good SoC meters so its no surprise that they are not good SoC meters.



But, I consider 13.7V to be 100% full. I only charge to 13.8V (3.45Vpc). Mine quickly settle to 13.4V (3.35Vpc) so THAT is what i really consider 100% full without a surface charge. Your cells may settle elsewhere.

Is there a reason you are concerned with SoC and the voltages around 13.7V? If you look at the attached chart, anything over 13.5V is over 99% full. Does 99.0% mean much compared to 99.5% in terms of capacity? (i'd say virtually no difference in capacity, but the voltage may be 13.5V vs 13.7V).

 

[CaptNordy]

Reason I'm concerned is that the SoC has reduced over the years. The BMS was recording a steady 99% when I would charge, now I only get to 92%, but like we are both saying, I think I'm just over-focused on that number.

I'll play around with re-wiring to help w/ the balancing issue when I haul out in February. Thanks MisterSandals for the help!

 

[MisterSandals]

I'll play around with re-wiring to help w/ the balancing issue when I haul out in February.

Label your cells (A, B, C...) and take good notes so you have "some" idea which ones are strong and which ones are weak when you deal with this in February. Pairs weaker and stronger in the 4P groups will help you make similar 4P packs.

In your battery diagram, the cell in the upper right looks like it might be lower capacity if it is that high and out of the main current path. Just little notes on things like this will help.

So something like this:

I missed this pic earlier. "Best" would be like this (midpoint on the bus bar if possible). Otherwise how you have it connected (+ to second from bottom, negative to second to top ) will leave the top row and bottom row under charged and under loaded (but MUCH better than you have it wired now).

 

[MisterSandals]

What do you make of the whole 90% full at 13.7 vs REC-BMS saying I've only used 25ah? Also, why does it say SOH 98%. If max I can charge to is 13.7 am I not lower than 98% SOH? I'm guessing REC-BMS is just using some crude algorithms to measure, so nothing that I can rely on.

I know nothing about REC-BMS.
If i understand what you are asking, why does it show 90% Soc when you've used 25Ah/1100Ah = 2.2%?

Probably because the algorithm is not calibrated to your cells and weighs the big voltage swing when burning off surface charge to high in relation to the actual capacity used. Looks like you used to charge to 14.2V and told you that was 100%. Then indicated 13.8V was 96%. I think the reality is that 14.2V vs 13.8V is far less than 1% Soc.

And, this is a big and, you had such an imbalanced pack, you were not saturating all of your cells so who knows what your actual Soc was. It really only had voltage to go by and that voltage was from an imbalanced charge.
I wouldn't read too much into how it used to be because of the balance problems.

Solar = ~1000w with 5 MPPT Controllers

What MPPT controllers do you have and what are the charge parameters? You really charge to 14.2V without any cells hitting HVD?

 

[John Frum]

@CaptNordy how much balance current can your rec BMS bring to bear on those 1120 amp hour logical cells?
I think you have done pretty well with commodity cells.
From your description I estimate you have racked up ~250 cycles.

 

[John Frum]

You could get 4x the balance current by making a 4s4p configuration which would require 3 more BMSs.

 

[CaptNordy]

John,
I agree I've done well considering the price point!

The REC-BMS states, "active cell balancing up to 2.5 A DC per cell". For my setup, a 'cell' is really a pack consisting of 4 cells. I get 'pack-level' visibility from the REC-BMS instead of true cell-level since the REC-BMS has the ability to monitor 4 things, not the 16 that would be required for me to get true 'cell-level' monitoring.

But again, for the price it has done well. What are people using now for BMS as this was the hot thing a couple years ago when I was on here and spec'ing out this system. Seems I could do with a more active balancer, capable of monitoring all 16 cells, but I would need it to be compatible with Victron, like the REC-BMS, as all my solar controllers and chargers are Victron, but controlled by the REC-BMS over CAN-BUS.

Thanks!

 

[John Frum]

John,
I agree I've done well considering the price point!

The REC-BMS states, "active cell balancing up to 2.5 A DC per cell". For my setup, a 'cell' is really a pack consisting of 4 cells. I get 'pack-level' visibility from the REC-BMS instead of true cell-level since the REC-BMS has the ability to monitor 4 things, not the 16 that would be required for me to get true 'cell-level' monitoring.

But again, for the price it has done well. What are people using now for BMS as this was the hot thing a couple years ago when I was on here and spec'ing out this system. Seems I could do with a more active balancer, capable of monitoring all 16 cells, but I would need it to be compatible with Victron, like the REC-BMS, as all my solar controllers and chargers are Victron, but controlled by the REC-BMS over CAN-BUS.

Thanks!

Your current configuration is 4p4s.
You could also make 2 batteries of 2p4s each with their own rec bms which would effectively double your balancing capcity.
You could also make 4 batteries of 4s1p each with their own rec bms which would effectively quadruple your balancing capacity.

 

[MisterSandals]

You could also make 4 batteries of 4s1p each with their own rec bms

You could also make 2 batteries of 2p4s each with their own rec bms

This is all true and makes for solid, proven battery solutions.

Personally, I really like the idea of a single BMS (i have 2P4S with 206Ah cells) that works great for me. Managing multiple parallel BMSs is not appealing to me, especially if i can get away with just 1. I've taken extraordinary steps to get my pairs equal which has paid dividends in simplicity and performance.

Given what the OP has now, and how well he's done, for years!, with horrifically unbalanced cell wiring, I think he has a very good chance of success by just fixing the cell wiring to balanced. Its certainly the easiest thing to try (first).

Failing that, the solutions that you have outlined are very solid, but more expensive and involve rework.

 

[John Frum]

Given what the OP has now, and how well he's done, for years!, with horrifically unbalanced cell wiring, I think he has a very good chance of success by just fixing the cell wiring to balanced. Its certainly the easiest thing to try (first).

Failing that, the solutions that you have outlined are very solid, but more expensive and involve rework.

Agreed.

 

[John Frum]

On the other hand this is a sailboat where redundancy and fault tolerance are particularly important.
@CaptNordy you might like this channel https://www.youtube.com/@TheDigitalMermaid/videos

 

[CaptNordy]

@John Frum - Thanks so much for the new YouTube channel, I'll subscribe now!

Regarding a move to 2 battery banks and 2 BMS systems, I don't think there is a way to get the 2 BMS systems to talk to the Victron Cerbo that manages all the charge sources (i.e. stops the solar, etc when voltage gets high).

 

[John Frum]

@John Frum - Thanks so much for the new YouTube channel, I'll subscribe now!

Regarding a move to 2 battery banks and 2 BMS systems, I don't think there is a way to get the 2 BMS systems to talk to the Victron Cerbo that manages all the charge sources (i.e. stops the solar, etc when voltage gets high).

That could be a problem.
I have 0 experience with the rec bms so I can't help there.