Still struggling to fully understand top balancing with bench top power supply

[snoobler]

Charging to just 3.4V is going to get you well under 97.5%. You should likely go to 3.5 or 3.55. With the cells parallel balanced, you should see no wonkiness.

The BMS exists to save the day. .01V between the two means you're more influenced by meter accuracy. You should use your equipment to establish your SoC operating range.

BMS LVD not above 2.9V.

 

[Solarfun4jim]

Charging to just 3.4V is going to get you well under 97.5%. You should likely go to 3.5 or 3.55. With the cells parallel balanced, you should see no wonkiness.

The BMS exists to save the day. .01V between the two means you're more influenced by meter accuracy. You should use your equipment to establish your SoC operating range.

BMS LVD not above 2.9V.

Snoobler, that was what i expected after the first parallel charge, but unfortunately i saw wonkiness right after 3.42V
This was taken from averaging the total voltage shown on the chargery and dividing by 8
I watched the Wh rise all the way up, calculating as a percentage SOC against an expected 3.25v x8cells x 280Ah(7280Wh). So for each 10% threshold(initially) on SOC i calculated the average voltage. At 3.3638v = 70%, 3.3688v = 80%, 3.3725v = 90% of expected. At 3.38v exact, that equated to 95% SOC.
3.3862 = 96%, 3.3925 = 97%, 3.4175=98% and it terminated me off(one cell hit 3.65v) at 7190wh or 98.76% of expected. I have similar figures at the bottom end as well.
When the wonkiness started, this was the resulting figures....3.4238v = 7142wh, 3.4262v = 7144wh, 3.4275v = 7146wh, 3.4300v = 7148wh, and 3.4362= 7150wh (but these were averages during very expanding mV differences.)
Hope my ramblings make sense.

Edit....now here's the rub, i worked out the WH as per the chargery spec sheet suggestions, but this rated the cells at 3.25 as matter of fact. Looking at the eve spec sheet, these cells are 3.2v nominal not 3.25v
So, if we apply this info the total Wh to be expected would be 3.2v x 8cells x 280Ah = 7168Wh
In other words, at 3.42v average, i would have achieved 7190wh/7168wh = 100.30%

 

[ArthurEld]

Oh man, I was hoping to get to 3.5 with no wonkiness

 

[Deleted member 9967]

Oh man, I was hoping to get to 3.5 with no wonkiness

Most should be able to after top balancing them.

 

[FilterGuy]

Late to the conversation.

1) When top balancing with a small power supply (less than 20A) voltage drop across the leads does not matter by the time you are done. Why: As the cells charge and the current drops, the voltage drop across the leads will also drop. At the end of the top balance, the current will be zero or near zero and the voltage drop accros the leads will also be zero or near zero (Ohms LAW: V=IR). The cells that see the higher voltage will charge faster and get to the target voltage faster, but then they will just stop taking current..... As the current drops, the cells that see the lower voltage initially will eventually get to the higher voltage and charge till they stop taking current.

Note: This only applies to top balancing. In normal use, the voltage drop is bad and must be minimized.

2. I document my process for top balancing here:
https://diysolarforum.com/resources...ls-using-a-low-cost-benchtop-power-supply.65/

 

[DeniseN]

LFP can be charged to 4.2V. There is essentially no benefit as you might only gain 1-2% of capacity between 3.65 and 4.20, and above 4.20, damage can occur; however, the occasional run up to that point is likely not going to harm anything, and keeping it at something lower, like 3.9V reduces the risk further.

Hello. How can you know if a LFP is damaged when accidently charged to 4.2v (no bloat)?

 

[snoobler]

Hello. How can you know if a LFP is damaged when accidently charged to 4.2v (no bloat)?

No bloating.
No change in capacity.
No change in IR.
No change in charge retention characteristics.

In short, it still meets the data sheet to the same degree it did before.

If you don't have pre-4.2V exposure performance data, you just kinda have to hope. Bottom line is it really shouldn't matter to a healthy grade A cell if it's briefly exposed to 4.2V.

 

[DeniseN]

If you don't have pre-4.2V exposure performance data, you just kinda have to hope. Bottom line is it really shouldn't matter to a healthy grade A cell if it's briefly exposed to 4.2V.

That is encouraging. I will have to take two of them out of the heap and observe! Thank you.

 

[Sanwizard]

@Will Prowse @FilterGuy @snoobler
Looking at the operation of these bench top power supplys, you can either use as a regulator source or a steady flow source. In the regulator mode, you adjust the current knob clockwise to maximum, adjust voltage to the value required then connect the load. In this case the CV led is illuminated.
In steady flow mode, you adjust the voltage knob to the required voltage, adjust the current to minimum, connect the load, then adjust to the current value you wish. In this case the CC led is lit.
Now, in Will's video(at around 3:20), he appears to do the first, by cranking up the amps to full, before connecting the load/battery bank.
When i done this in my top balance, the current/wattage continually fell all the way through the charge, whilst the voltage climbed up to the set point.
I had been expecting the current to remain static up until the set point was reached and then see the expected fall off in current, but that didn't happen. However, i felt i was following the instructions given....so this had to be correct.
Looking at charging charts, the first phase is the CC phase where the current remains constant.....so, this begs the question, did i set up my bench top supply wrongly? Should it be set up for steady flow, so that the current remains the same till the voltage hits the 3.65v, then expect it to drop? Unfortunately, having zero experience in all this, i made the assumption, that the falling current/wattage input all the way through was a function of the bench top power supply providing less than the needed 0.05C end point charging current, not that i had screwed up following the instructions provided in wills video.
Hoping this makes sense and that someone can illuminate. When top balancing, should the CV led or the CC led be lit up until the voltage reaches 3.65v?

This may be a stupid question for all tou experts, but let me ask if this makes sense.
When my cells arrive, and the first thing I do is hook them up in 16s for 48v 208ah, then attach my Heltec 200A BMS with 2a active balancer, setup the Lifepo4 parameters, and then hook that up to my MPP LV 6548, and plug that into a 120V outlet with utility charge mode and all the parameters setup for 3.65v cuttoff, and then wait a week, would that exclude the need to do a top balance? Wouldn't the active balancer simply handle it? Would a week be long enough?

What am I missing?

 

[snoobler]

This may be a stupid question for all tou experts, but let me ask if this makes sense.
When my cells arrive, and the first thing I do is hook them up in 16s for 48v 208ah, then attach my Heltec 200A BMS with 2a active balancer, setup the Lifepo4 parameters, and then hook that up to my MPP LV 6548, and plug that into a 120V outlet with utility charge mode and all the parameters setup for 3.65v cuttoff, and then wait a week (1), would that exclude the need to do a top balance(2)? Wouldn't the active balancer simply handle it(3)? Would a week be long enough?

What am I missing?

Your unit has a 120A charge ability. If your cells, are only rated for 0.5C charge, 104A, then you should reduce it.

(1) You never want to hold LFP at high voltages for extended periods if you mean to hold the bank at 58.4V for a week waiting for the 2A active balancers. 54.4V is the peak recommended float at 3.4V/cell. With long charge times, you can get the battery to 95% SoC @ 3.4V/cell. I would take that route before I considered holding at higher voltages.
(2) If the active balancers can get you there, yes.
(3) See #1.

Active balancers can help and hurt. If they can be set to only balance during charge, that's preferred. If they balance during discharge, they will undo any top balance.

 

[John Frum]

When my cells arrive, and the first thing I do is hook them up in 16s for 48v 208ah, then attach my Heltec 200A BMS with 2a active balancer, setup the Lifepo4 parameters, and then hook that up to my MPP LV 6548, and plug that into a 120V outlet with utility charge mode and all the parameters setup for 3.65v cuttoff, and then wait a week, would that exclude the need to do a top balance? Wouldn't the active balancer simply handle it? Would a week be long enough?

What am I missing?

Not an expert but will take a stab.
Lets assume the charge voltage is 58.4 volts which is 3.65 per cell and the charge current is 41.6 amps which is .2c.
Ideally balancing is not restricted to the charge cycle.
Cells usually come ~50 percent full.
Its reasonable to assume one of the cells will trigger the high cell cutoff in ~2.5 hours.
Balancing will continue even though charging is disabled by the bms.
Charging will resume when the cell voltage gets to the release trigger.
Its crude like using a skid steer as a golf cart but it should work.
Holding the cells at charge voltage is stressful but 1 week in the life of a lifepo4 pack is probably not that horrible.
After the cells are in sync at the top, disable balancing except while charging.

 

[Sanwizard]

Not an expert but will take a stab.
Lets assume the charge voltage is 58.4 volts which is 3.65 per cell and the charge current is 41.6 amps which is .2c.
Ideally balancing is not restricted to the charge cycle.
Cells usually come ~50 percent full.
Its reasonable to assume one of the cells will trigger the high cell cutoff in ~5 hours.
Balancing will continue even though charging is disabled by the bms.
Charging will resume when the cell voltage gets to the release trigger.
Its crude like using a skid steer as a golf cart but it should work.
Holding the cells at charge voltage is stressful but 1 week in the life of a lifepo4 pack is probably not that horrible.

Thanks for the fast response! I figure if all the parameters are set as they should be during normal operation, the system should find cell balance one it hits full SOC, and the active balance will kick in at the BMS level, thus eliminating the need to go through the entire top balance before hookup procedure. I also just bought one of the Heltec 5A active balancers. Would it be a good idea to have the BMS active balancer in parallel with the 5A active balancer? So 7 amps of balancing current available at all times?

 

[John Frum]

Thanks for the fast response! I figure if all the parameters are set as they should be during normal operation, the system should find cell balance one it hits full SOC, and the active balance will kick in at the BMS level, thus eliminating the need to go through the entire top balance before hookup procedure. I also just bought one of the Heltec 5A active balancers. Would it be a good idea to have the BMS active balancer in parallel with the 5A active balancer? So 7 amps of balancing current available at all times?

Only let one balancer work at a time they may fight each other.
Remember to limit balancing to the charge cycle and only above float voltage after the initial balance.

 

[ArthurEld]

This may be a stupid question for all tou experts, but let me ask if this makes sense.
When my cells arrive, and the first thing I do is hook them up in 16s for 48v 208ah, then attach my Heltec 200A BMS with 2a active balancer, setup the Lifepo4 parameters, and then hook that up to my MPP LV 6548, and plug that into a 120V outlet with utility charge mode and all the parameters setup for 3.65v cuttoff, and then wait a week, would that exclude the need to do a top balance? Wouldn't the active balancer simply handle it? Would a week be long enough?

What am I missing?

You are missing the fact that your BMS will keep hitting the over voltage limit because the balancer can't keep up.
You have to gradually raise the voltage. Start at 3.4V until it is balanced. If you hit the over voltage limit reduce the amps until you can get it balanced at 3.4V. The thing about 3.4v is that you can stay charged at 3.4V for a long time. Just check it now and then until it's balanced down to .003V delta.
When you gradually move up from 3.4V the 3.65 it won't take so long and you can keep an eye on it.
You'll get a feel for it. Adjust CV and float to the same value and move them both up small amounts until you get the amps up but without hitting the over voltage limit. You get used to it
You should be able to get 3.6V balanced down to .003V delta without hitting the 3.65V limit.
That should be balanced good enough.

Adjust your CV and float back below 3.4V

I'm not an expert but that's how I do it

 

[John Frum]

@snoobler how hard on the cell's do you think my method would be?

 

[John Frum]

To clarify lets say the hi cell cutoff is 3.65 volts and the release is at 3.55.
I figure the bms will spend the vast majority of its time balancing with charging disabled while the cells settle.

 

[John Frum]

I suspect it will be harder on your bms than on your cells.
What kind of charge termination logic does your charger have?

 

[ArthurEld]

Thanks for the fast response! I figure if all the parameters are set as they should be during normal operation, the system should find cell balance one it hits full SOC, and the active balance will kick in at the BMS level, thus eliminating the need to go through the entire top balance before hookup procedure. I also just bought one of the Heltec 5A active balancers. Would it be a good idea to have the BMS active balancer in parallel with the 5A active balancer? So 7 amps of balancing current available at all times?

The problem is that the states of charge can be way out of balance.
Some cells can be 70% charged but still check to 3.3V. Other ones can be 30% charged but check to 3.3V. Top balancing fixes that.

It is possible to do what you are saying if you started using a small percentage of your capacity and gradually open the limits and let the balancer pull the states of charge in line.
That is more work than it sounds and just doing the top balance is probably easier.
The reason why it doesn't work well is because in normal usage there's no stopping near the knees.
You need to stay near the knee long enough for the balancer to get the balancing done. Otherwise it will probably take months to get through the whole process until you can use full capacity.

I'll give it a try for the heck of it.

I don't know about using 2 balancers at the same time. Everything I have heard warns or recommends against doing that.

 

[ArthurEld]

This may be a stupid question for all tou experts, but let me ask if this makes sense.
When my cells arrive, and the first thing I do is hook them up in 16s for 48v 208ah, then attach my Heltec 200A BMS with 2a active balancer, setup the Lifepo4 parameters, and then hook that up to my MPP LV 6548, and plug that into a 120V outlet with utility charge mode and all the parameters setup for 3.65v cuttoff, and then wait a week, would that exclude the need to do a top balance? Wouldn't the active balancer simply handle it? Would a week be long enough?

What am I missing?

You could just float at 54.4V (3.4V per cell) until the balancer gets delta down to .003V.

That alone should balance your cells well enough for you to have full usable capacity.

 

[snoobler]

@snoobler how hard on the cell's do you think my method would be?

Dunno. Just don't like.

Don't know how the BMS or the charger will react to repeated cut offs.

With a little more thought... charge to 3.4V @ 100A until current drops below 5A. Lower charge current to 2A and charge to 3.65V. Balancers should nearly keep up with charge current, and even at 2A, the last 20% of the charge can be accomplished in less than 24 hours.