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diy solar

Runner cell "trigger" voltage

OK, I made some better leads. With ring terminals on both ends. I gained a little. Not much, all it can do is 4.5A - and decreasing.
Cells are now 3.55ish. Supply is set to to 3.65.
I'll be patient. Thanks all for the advice.
-
 
(3.65 - 3.55)V / 4.5 A = 0.022 ohms
It would take 14' of 12 awg to make that much resistance.

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Check voltage at power supply terminals with DMM. If close to 3.55, the drop is inside supply, and just be patient.
Otherwise, check voltage drop from each supply terminal to cell terminal.
Check from ring terminal to cell terminal, etc.
When you reassemble battery, low resistance will matter at high current. Now is a good time to observe any high contact resistance, and plan to eliminate.
 
I'm a bit confused. I finished top-balancing yesterday. Today I re-assembled.
Now, I have two cells that go up in voltage. #2 and #7.
Screenshot_0726_145051.png

Balancing alternates every few seconds, and after a few hours, it doesn't seem to get much better. For example, now both are at 3.648.
Granted, #7 used to go much higher, over 3.7, and trigger protection. Now they stay just below that.
Oh well. I guess I'll give it a day or two and see what they do...
-
 
Did you balance to (near) zero amps? What was each cell's voltage as tested with a DMM individually at the time?
Let them rest for an hour, where did they settle?

I ran my first top balance a while ago as a trial run and I found I had some not-so-stellar bus bar connections (didn't surprise me) and some cells just didn't charge up compared to the others, so they didn't really top balance after all. I need to totally redo the top balance after properly cleaning the terminals fully. I noticed a thin film of red loctite on top of one terminal from when I put the grub screw in so that one is more obvious. Gotta figure out how to clean that off with the grub in place.
 
Don B. Cilly said:
About replacements
Click to expand...
Have you determined the runner has less capacity than the others? Any good cell could be a runner if it is put in a pack with a higher SOC than the others. Parallel top balancing can be your friend. I had good luck with Amy. One of the first questions she asked when I thought I had a problem was had I done a capacity test?
 
Uh. I didn't check the individual cells. It was late.
The bus bars are new, looking good and clean, torqued at 10.0 nm.

If I have to re-do, I'll check the voltages :·/

I have not done a capacity test, nor do I plan to if I can avoid it.
-
 
Don B. Cilly said:
I have not done a capacity test, nor do I plan to if I can avoid it.
Click to expand...
As mentioned by others, if you have not bothered to do a capacity test you have not bothered to answer Amy’s initial question and I do not blame her for not responding. She has not gone AWOL, you have leaped to the conclusion that you have a bad cell without bothering to provide evidence. Set your voltage lower and given enough time your pack will probably balance itself
 
I mean, obviously, I don't have enough bars to connect 8 cells in parallel, so some of them I connected with (reasonably thick) wires with (reasonably big) alligator clips. I have enough of those.

But then, once the charger goes to near-zero amps, any connection should do, there's only a few mA going from one cell to the other.
After which they laid at rest for more than six hours...

Amy did eventually reply, asked what I set my BMS to, I replied, she said,
Screenshot_0726_160347.png
I didn't bother her further as I wanted to see how the top-balancing went.

Yes, I'll give it a few days and see how it goes.
As long as it doesn't trigger protection and stop charging, I'm not particularly worried.
-
 
Don B. Cilly said:
I mean, obviously, I don't have enough bars to connect 8 cells in parallel, so some of them I connected with (reasonably thick) wires with (reasonably big) alligator clips. I have enough of those.
-
Click to expand...

Alligator clips? Bad contact resistance. On studs? Poor conductivity, can clips can't bite in for better contact.

All connections ought to be identical. The objective is balancing.

Some people have balanced a set of cells, found one cell didn't reach same voltage as others, charged it again and things were OK.


Don B. Cilly said:
The bus bars are new, looking good and clean, torqued at 10.0 nm.
Click to expand...

Aluminum, copper, or tinned copper?
Aluminum has native oxide, should be scrubbed off. Instantly regrows, but thin at first. Oxide inhibitor could help.
The terminals will have this, regardless of busbars. Bare copper direct on aluminum may be worse than tinned.

Sometimes an assembled pack has one cell voltage deviating from others and busbar hot under load. Re-cleaned and torqued fixe the issue.

The metallurgy of these things is all wrong. Electrical industry never uses bolted connections to bare aluminum except for some ground connections. Either tinned aluminum or crimp connections.
 
This is getting quite interesting.
I now have three cells in the 3.6s (2,5 and 7, one going slowly up (8) and four lagging a bit.
Screenshot_0727_162605.png
Setting the charge voltage just right (27.7, doesn't trigger protection), and even though the BMS is trying to balance them down, I'm hoping that in a few days they can balance up.
We'll see.
-
 
Should have been able to get all cells above 3.4, maybe above 3.5, without any running for over-voltage. That would let battery charge to a target voltage.

Since you used alligator clips for some busbars I'm going to assume cells didn't get balanced perfectly.
Maybe if you had held pack at 3.65V longer it would have been closer. You reported cells below supply voltage - what did cells reach? Lowest cell?
If you can hold a pack voltage that doesn't cause disconnect for cell over-voltage (your 27.7V), maybe BMS bleeding off high cells will bring them into balance.
 
sunshine_eggo said:
You indicated you got them on 7/15, but you got them "balanced" by the time you posted the next day after cells were at 3.282V? I'm not saying you're lying, but if you're not, you currently hold the fastest record for someone top balancing their cells.

Guide, Page 1, Item 1, Note 3.
Click to expand...
Newbie question what is the guide???
 
HRTKD said:
See the download button in the upper right corner.

diysolarforum.com

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
diysolarforum.com diysolarforum.com
Click to expand...
Sorry no I don't. Not the sharpest tool in the shed when it comes to computers! :(
 
Don B. Cilly said:
27.2 still triggers protection.
27.1 does not, but the 7th cell still runs quite high.
27.0 looks fine.

About replacements, Amy Wan has gone AWOL. We'll see next week.
Qishou say they can only ship 4 cells. When I asked why they offer "samples" on their site (1 piece) they did not reply. They seem rude in general.
Exilporc advertise EU stock. They did give me a quote for a 1-piece sample. Shipped form China.
eBay EU vendors (expensive), one (German) says "Sorry we can't ship one", the other (Dutch) did not reply.
Let's see what Amy says (if and) when she comes back to (business) life.
-
Click to expand...
Since several people have asked, I'll ask too. Did you top balance?

diysolarforum.com

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
diysolarforum.com diysolarforum.com

Why you should top balance.

diysolarforum.com

Explanation for Beginners of Top and Bottom Balance

This is a beginners explanation of top and bottom balancing cells that should help you understand what it is all about. To get the document, click on the orange button at the top of the page. NOTE: This is not intended to be a tutorial of how...
diysolarforum.com diysolarforum.com
 
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