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Old 09-12-2021, 04:18 PM   #21
Skoolie
 
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Hey all!

Quick question:

Everything I've seen indicates that performing a top balance, I should expect to see amperage up to and including the max on my power supply, which in this case is 10A. I have it set for an output of 3.65v and the amps are cranked to max, however I'm only seeing ~.5A at the most...also, the cells have been sllllooooowwwwwwlllly cahrging up from a start volatage of 3.25ish and are now at a whopping 3.3 after 48+ hours. This happens across both packs I have tried.

Should I be worried I about having bad cells? Is there something else I am missing here? Thanks!

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Old 09-12-2021, 04:28 PM   #22
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Each cell needs to be cap tested individually.

Resistance as well if you like but that is a lot trickier. Only bother with that step after doing your capacity go/no-go sorting, obviously only on those that pass.

Personally only worth going to all this trouble if your hourly labor is worth very very little.

Best to just start with known-good Grade A brand new cells, bought from a trusted vendor.
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Old 09-12-2021, 05:20 PM   #23
Skoolie
 
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Quote:
Originally Posted by john61ct View Post
Each cell needs to be cap tested individually.

Resistance as well if you like but that is a lot trickier. Only bother with that step after doing your capacity go/no-go sorting, obviously only on those that pass.

Personally only worth going to all this trouble if your hourly labor is worth very very little.

Best to just start with known-good Grade A brand new cells, bought from a trusted vendor.
These were known good grade a brand new cells when I bought them. They got out of balance accidentally, and I never top balanced to begin with because I (mistakingly) believed that new cells that were within .01v of each other did not need to be. Since I bought new BMSs with bluetooth, I figured I might as well top balance the packs, replace the BMS and reinstall just to make sure everything is good to go.
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Old 09-12-2021, 06:13 PM   #24
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Originally Posted by jjaj823 View Post
They got out of balance accidentally, and I never top balanced to begin with because I (mistakingly) believed that new cells that were within .01v of each other did not need to be.
That is indeed balanced, at least at whatever point in the SoC/voltage curve that was the case.

If you then start top balancing, that becomes that point.

And at the bottom is where imbalances if any would be revealed.

But if you keep your stop-discharge / LVC high enough, as you should for longevity, you won't ever notice.

Keep your start-balance lower than your CV setpoint charging

Edit: say 4.00V vs 4.10V only applies to li-ion or LiPo, not LFP

A target delta of 50-70mV is tight enough

Don't let the cells sit at Full for long.
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Old 09-12-2021, 06:33 PM   #25
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Quote:
Originally Posted by john61ct View Post

But if you keep your stop-discharge / LVC high enough, as you should for longevity, you won't ever notice.
I was planning on keeping the low voltage disconnect around 21.2...is that too low? If so, how low would you go?

In regards to the charge differential (i.e. 4.10 -> 4.0) I am understanding that to mean if I want 3.65, then I should set the voltage to something a bit higher, like 3.75?

Thanks,

Jeremy
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Old 09-12-2021, 06:49 PM   #26
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Originally Posted by jjaj823 View Post
I was planning on keeping the low voltage disconnect around 21.2...is that too low? If so, how low would you go?
Best for discussion to quote voltages at per cell level.

21.2 8 = 2.65Vpc

In theory that might not immediately "damage" the cells, but it may well shorten cycle lifespan by 90% compared to what I advised - go back and read that.
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Old 09-12-2021, 06:54 PM   #27
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Are these LFP chemistry cells?

If so ignore the stop-charge voltages I quoted above.
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Old 09-12-2021, 06:58 PM   #28
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Sounds like you're charging the cells in parallel and they are charging just fine. They take forever to charge up; moreover, you will see a drop in current as you approach full charge (no more than 3.65v per cell), and the process essentially attenuates to near-zero amps making the last bit of charging a real snoozer. Days.

You can't control how many amps the cells take. They pull the amps based on charge state and voltage. The higher the voltage, the more current however don't go over that 3.65v mark to avoid damaging the chemistry.

Check out the Off Grid Garage; that guy discusses these topics in a clear way. He's done testing and shows you don't get a lot of amps out of the tippy top end (3.5 or higher) so there may not be a lot of advantage cranking the charge voltage all the way up.

Also, most folks recommend keeping the cells at about 50% charge if you are not using them regularly. And avoid the extremes (+90% or <10% charge) 80% of 280 is still a very large number.
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Old 09-13-2021, 02:20 PM   #29
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Quote:
Originally Posted by Rucker View Post
Sounds like you're charging the cells in parallel and they are charging just fine...
Good to know. Thanks!

Quote:
Originally Posted by Rucker View Post
Also, most folks recommend keeping the cells at about 50% charge if you are not using them regularly. And avoid the extremes (+90% or <10% charge) 80% of 280 is still a very large number.
If I could get them working right, I would definitely be using them. Basically, as soon as I get these darn things working right, me and the fam will be in the bus full-time, at least while we move east.

Quote:
Originally Posted by john61ct View Post
Are these LFP chemistry cells?
Yes, they are.

Rucker - based on what you mentioned about 90% - 10%, that would give me an effective capacity of 100AH per cell. Using cell values, if the max charge per cell is 3.65 and the minimum cell charge (per manufacturer) is 2.5, that gives a difference of 1.15v from 100% - 0% SOC.

Therefore, for best battery life, I should set my high and low cutouts to be:
- approximately 3.285v (26.28v for an 8s pack) on the high side
- approximately 2.75v (22v for an 8s pack) on the low

EDIT: Assuming the above, does it even make sense to *balance my batteries above the 3.3v/cell they are at now?

Thanks to everyone for your continued input and patience!

Jeremy

EDIT: changed 'charge' to 'balance'
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Old 09-13-2021, 02:52 PM   #30
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Quote:
Originally Posted by jjaj823 View Post
Rucker - based on what you mentioned about 90% - 10%, that would give me an effective capacity of 100AH per cell. Using cell values, if the max charge per cell is 3.65 and the minimum cell charge (per manufacturer) is 2.5, that gives a difference of 1.15v from 100% - 0% SOC.

Therefore, for best battery life, I should set my high and low cutouts to be:
- approximately 3.285v (26.28v for an 8s pack) on the high side
- approximately 2.75v (22v for an 8s pack) on the low

EDIT: Assuming the above, does it even make sense to *balance my batteries above the 3.3v/cell they are at now?

Thanks to everyone for your continued input and patience!

Jeremy

EDIT: changed 'charge' to 'balance'
You are asking exactly the right questions.

LiFePo voltage is not directly correlated to state of charge, so the better way to track battery state is with BOTH a charge controller (for controlling how much charge and discharge you get) and a BMS that you can configure for max and min voltage (among other things) to avoid battery damage.

I recommend both Will Prowse and the Offgrid Garage as two valuable youtube resources for details on this topic.

The process for commissioning batteries is to one time charge them all the way up to 3.65V per cell, combine them into the battery; install the BMS, smartshunt and top balancer, then top balance them, then discharge them all the way down some low voltage-2.7 or so (but never ever lower than 2.4).

The reason it kind of doesn't matter whether you use 2.8 or 2.5 as the low voltage point is because there's only a couple of amp hours difference in storage capacity at those voltages. The charge curve is non-linear, meaning it goes down quickly from 3.65v, to about 3.4 to 3.2v, then sits there forever as the cell is discharged to about 20% capacity, and then it starts to creep down from there fairly linearly. This is easier to see with a visual, so go to the youtube videos for a better understanding.

Now with the battery built and the smartshunt added you can monitor the amps you pump into the battery while you charge it back up. Charge the darned thing all the way back up to something near 3.6v per cell. That will give you the total capacity of the battery.

Again, at the top end, there aren't a lot of amp hours stored from 3.5 to 3.65 volts, so you aren't going to lose capacity if you keep the top voltage below its theoretical max.

You can now see from the smartshut what the battery capacity is between those voltages. Take ten percent off the top and set the upper limit on the charge controller. For example, if your 280 AH battery actually has a capacity of 290 based on the above, you can set the upper limit of charge to 261 (290-29) and let your charge controller manage charging up to that limit. Same on the bottom end-set the charge controller to 29 amp hours in this example for the low end cutoff. As long as you've also set the low and high voltage cutouts with the BMS and balanced the cells at the top end, your system will do the rest and give you optimal performance and life.

Other folks may have additional points to raise or comments from their practical experience; this is just from my experience (and YouTube).
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Old 09-13-2021, 03:16 PM   #31
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Quote:
Originally Posted by jjaj823 View Post
EDIT: Assuming the above, does it even make sense to *balance my batteries above the 3.3v/cell they are at now?
Balancing those LiFePo cells at anything less than full capacity does nothing. The charge curve is almost totally flat at 3.3, so it is only when you are at the limit, right there at 3.6v or slightly above, that the voltage correlates to state of charge. If you balance the cells now, they will be off by the time they are fully charged. In that case, if you're using a BMS, as soon as the first cell hits the upper limit, all charging ceases.

Again, the Off Grid Garage guy did a bunch of experimentation to verify this.

Moreover, if you buy one of those cheap active balancers (basically a capacitor bank on a small printed circuit board) it will just waste energy if it is left on throughout the charge cycle. Cells vary in voltage naturally throughout the charge cycle, and that's perfectly fine. Balancing only adds value at battery commissioning, and then perhaps once a quarter check thereafter until the batteries approach their end of life.
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Old 09-13-2021, 03:55 PM   #32
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Originally Posted by jjaj823 View Post
Yes, they are.
OK, so 3.45Vpc is best charge-termination for longevity. Hardly any capacity loss.

Absolutely no point going over 3.55V, just stressing the cells reducing lifespan for no purpose at all.

3.285v stop charge will lose quite a lot of capacity. Just how much depends on the specific model cells and current-related aspects of the charge profile
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Old 09-13-2021, 04:01 PM   #33
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You should stop discharging at 3.1Vpc, especially if current is low, say 0.2C

At higher current rates, calibrate your LVC by checking after at rest an hour and voltage should have bounced back above 3.1V.

Going any lower will reduce cell lifespan by a very large percentage, with very little added capacity utilization.
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Old 09-25-2021, 04:31 PM   #34
Skoolie
 
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Thanks again everybody for your input!

After some trial and error, I figured out that the wires I was using to connect the cells to the power supplies were not connecting efficiently, leading to a drop in current. I made a couple sets of wires and now they are humming along at a full 10a!

Now, to figure out why the new BMSs show only 22v after the balance leads, but the balance lead harness shows 28...

- Jeremy
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