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Discussion Starter · #1 ·
The patient: a home built 280AH LiFePo4 battery, using 4 Liitokala prismatic cells. It is less than one year old. Each of the 4 cells were "grade A, unused" and they are all in good physical condition. no scratches, no dents, no bloating. The cells were top-balanced and I confirmed that they were within .001v.

Last season this battery performed amazingly, I was very happy with it. FWIW my boat is a light electrical user so I am not ever pushing the house bank very hard.

I originally built the battery with an el-cheapo BMS. But I just replaced it the very nice Overkill Solar BMS with bluetooth. I can monitor this BMS with an app on my phone. With no load the cells are in good balance with a variation of less than .001v. This was reported by the app and confirmed with my multi-meter.

Under load of ~5A I see that one cell drops significantly. Here is the app, and you can see that cell #2 is ~.3v lower (note I have no idea what the color coding of the cells mean ... maybe grey is lowest, red is highest?)
Sky Light Azure Operating system Font

When I turn off the load, the cells equalize again and that #2 cell rises to the same voltage as its friends. Note that last season I had no way to monitor the cells, so #2 might have always been behaving like this.

So:
- is this a sign that cell #2 is seriously compromised? or just a little bit "off"?
- How much is this affecting the capacity of the bank?

And what to do?
- replace that one cell?
- replace all cells?
- do nothing?

If #2 was performing normally the bank voltage would be 13.2, but with #2 as it is bank voltage is 12.9. So clearly the BMS will disconnect much sooner as the bank voltage drops.

I confirmed that there is no physical damage to any of the cells, no bloating.
#2 looks perfectly normal.
 

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Liitokala has a pretty terrible rep.

Dodgy sellers say Grade A and new, when anything but.

Lots of counterfeits.

Winston, CALB, GBS, Sinopoly, CATL and A123 (now Lithium Werks B.V. / LithiumWerks/ Valence (Saphion) / Super B)

are the good quality makers

but the seller issues above still need eliminating unless you find a way to buy direct from the factory.
 

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A full fledge capacity test, using a suitable Constant Current dummy load and data logger is required to know SoH% for sure and with accuracy.

Should be run as a benchmark on each cell when first commissioned after breaking in.

At a low C-rate, if actual capacity is well below nameplate, or cells are widely divergent, grounds for returning under warranty.

Then that benchmark used as a comparison periodically to detect wear over time, if performance issues are perceived, or say every 500 cycles, or after an unexpected "event".
 

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Have you confirmed these readings with a voltmeter directly on the battery terminals?

Have you checked all connections on the battery?

Does your BMS have cell balancing functionality? If so, when does it start balancing the cells?

When you charge, does cell #2 come to full charge at the same time as the other cells?

Mark
 

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Thanks Bristol and John.
Learning tons 😁
I use a solar panel and controller I got from parts at Creatronics to charge a 20Amp. 12v For the Dinghy.
The panel is an 18watt cheapie probably glacé out in 5 years. The battery so far is fine.
 

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Discussion Starter · #8 ·
Have you confirmed these readings with a voltmeter directly on the battery terminals?
yes, voltmeter at the cells confirm the readings reported by the BMS.

Have you checked all connections on the battery?
yes, all connections confirmed good, tight. I'm using bus-bars to connect cells.

Does your BMS have cell balancing functionality? If so, when does it start balancing the cells?
Yes. here are the parameters I can adjust (and the values)

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When you charge, does cell #2 come to full charge at the same time as the other cells?
I am not sure. From memory (so all caveats apply!) it seems to come up to the same voltage pretty quickly but I will check and confirm.
 

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yes, voltmeter at the cells confirm the readings reported by the BMS.



yes, all connections confirmed good, tight. I'm using bus-bars to connect cells.



Yes. here are the parameters I can adjust (and the values)

View attachment 142768



I am not sure. From memory (so all caveats apply!) it seems to come up to the same voltage pretty quickly but I will check and confirm.
Perhaps you need to break apart the cells, capacity test and top balance?

It's possible that starting balancing that early gets the cells chasing their tails and one is drawn down out of balance because it shows an early higher voltage and becomes the source for others.

Maybe turn off the balancing and watch the cell voltages as they go to full charge. See what each are doing.

Mark
 

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Discussion Starter · #10 ·
thanks all!

just to follow up, I cycled the bank a few times charging and the discharging to about 50%. The overkill solar BMS balances during charging and it seems that it did it's job. the cells are in perfect balance now and are holding constant as it discharges.

just a quick reminder: this bank was running with an "el cheapo bms" for about a year.I noticed the imbalance after upgrading to the the Overkill Solar BMS. That bms includes Bluetooth to monitor the bank and individual cells. And that is when I saw the initial imbalance. however running the new bms thorough a few charge/discharge cycles allowed the BMS to do it's thing.

my take away: the overkill solar BMS is excellent and at $120, not all that more expensive than an el cheapo BMS.

Mark: I believe that your suggestion to capacity test and top balance was right on. I would have done that next but decided to give he BMS an opportunity to do its thing.
 

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Pretty hard to get EV grade cells as a DIY. The term A grade is nothing more than Chinese marketing..There are only a couple of companies in the US that are actually selling true EV grade cells.

You cannot tell how well the cells are balanced or matched by voltage alone. Liitokala is a known supplier of B grade and lower cells..

Whenever building any lithium battery pack. You are much better off to do a manual top balance. While the overkill BMS(JBD/JIABAIDA ) is certainly better than a Daly,they still have very minimal balance Capability and can only do so much if your cells are not well matched...
 

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Discussion Starter · #13 ·
For the record, my budget was 100s not 1000s. My goal was to experiment and learn about the technology. And Fwiw I'm happy with the system that I put together. It was built on a budget so yes, not top shelf components. But the system has far more capacity than any lead bank I could have installed. it provides plenty of power for my needs. And I learned a lot.

Maybe my 280ah cells would capacity test at something less... 260? 250? im still way ahead than a lead bank that would fit. Maybe these cells will provide a shorter lifetime? fewer cycles? Im still ok with it. And the longer I run it, the more I'll learn.

In the end I've got a pretty great house bank and had fun building it.
 
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