Top Balancing "How to"

snoobler

Solar Honey Badger
For current best practices on top balancing cells, please watch the video and download the PDF at the following page:

Top Balancing Cells Using a Low Cost Benchtop Power Supply

If you are interested in a more general tutorial of what balancing is about and *Why* to balance, check this out:

Explanation for Beginners of Top and Bottom Balance

If you're asking yourself if you should top balance your cells, the answer is always: "YES!" If you don't, it's almost certain that even a small imbalance in the cell SoC will cause "runners" at near full charge and will be VERY frustrating.
 
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BarkingSpider

Carbon Lifeform
Thanks for posting the great info.
For a great bench top power supply like Will uses, i have this unit, best part is they are offering a $45 Amazon gift card making out almost free. Not sure when it runs it.
 

Dzl

Perpetual Newbie & Unofficial Forum Librarian
Staff member
Moderator
I encourage everyone to read through @FilterGuy's resources linked above (and all the other great beginner friendly resources he has been producing on a variety of topics). I very much respect his commitment to helping beginners and creating resources to answer many of the questions beginners struggle with most.

And more broadly, as a general point, make use of the resources section! There is a ton of useful info there. Everything from explainers, to cell datasheets, to product manuals, to full on E-Books, to section of the electrical code.

I also encourage anyone who wants to go further to read the Nordkyn Design series and the Marinehowto article. Generally speaking, a good amount of our collective knowledge on the forum comes from these two resources. They are long reads, but both are great comprehensive in depth introductions to the lifepo4 chemistry in marine systems (which has a ton of overlap and applicability to mobile and stationary off-grid systems as well). Both have good clear explanations of balancing as well
 

DJSmiley

Solar Addict
Cheapest way to get a CC / CV powersupply for topbalancing is getting an old server power supply ($10-$20 or so on Ebay), and a buck converter like https://www.aliexpress.com/item/4001231282413.html ($22 for a 20A version)

A lot of (eg HP Proliant) supplies provide 12 V / 40 up to 100A and can be easily turned on with a simple wire, making it a great and very cheap power supply. The buck converter will handle the CC for topbalancing to 3.65V or whatever you set it to.

If you series connect 2 (this requires some more modification since you have to disconnect the GND from the case) you can also use it to charge a 12V (4S) bank. This can speed up the balancing afterwards, if you serie-charge it with 20A till 1 cell hits its HVD the others are also almost full. Reconnect them in parallel and do the final top balance.

You can get a single one for <$5: (Providing 38A output @ 12V)
 

PeterBC

Solar Enthusiast
I've read many resources on Top Balancing, including Nordkyn Design series and the Marinehowto article.
(mind you my mind is exploding with the LiFePO4 learning curve I'm on at present !). I'm just about to order 52 x 3.2V 272AH Lishen cells from Basen for 2 sets of 24 cells (3 x 8 Cells each with BMS) for 2 boats. My questions seem to multiply as I digest more information !

If we assume 280AH cells, 50% charged initially

WARNING: Top each cell up, to a similar SoC level, prior to wiring them in parallel.

Q1. What voltage difference is it safe between cells to connect them parallel ? What surge currents can be expected ? for how long ?

(eg in extreme, if one plugged a 90% SOC cell in parallel with a 10% cell (about a 300mv difference ? ) then approx 224AH would flow over tens of hours, but what would flow in the first seconds ? minutes ?

Q2. Whats the best way to safely connect such batteries together. Separately charge the lower till within X mv diff ?

Similarly if one had 3 strings of 8 cells usually connected in parallel but one taken off line for repair and wanted to connect back to the others.
Q3 how close should their voltages be and what current flows might be expected/tolerated initially. If one has BMS systems on each string, what risk of blowing one with initial surge current ? Perhaps disconnect the BMS' initially till the 3 are settled ?

Specific questions re the Parallel Step-Method Top Balance:
assume 8x 280AH cells, 50% SOC initially, 20A 30V DC supply..

1- Wire the cells in parallel
2- Set the power supply to 3.400V and 80% or less of the rated amperage (80% to not burn it out)
3- Turn on power supply and charge cells to 3.400V

Q4 if initially at 50% SOC, expect to take 120AH?/16amps * 8 Cells = 60hrs ?
4- When current has dropped to 0.0A at 3.400V turn off the power supply & set it to 3.500V
Q5 how close to 0A ? How long to take 8 x 280AH from 3.4 to 3.5V ?
5- Turn on power supply and charge cells to 3.500V
6- When current has dropped to 0.0A at 3.500V turn off the power supply & set to 3.600V
7- Allow current to drop to 0.0A (or very close) at 3.60V

Q6 How long to take 8 x 280AH from 3.5 to 3.6V ?
8- Done, pack is balanced.
Q7. Is 3.5 as a reasonable more conservative value to top balance too ?
 

snoobler

Solar Honey Badger
Q1. Many don't care. I personally like 0.2V

Q2. Let them sit overnight, or just connect them.

Q3. See Q2.

Q4. 140Ah*8/16 = 70 hours.

Q5. Very close is fine.

Q6. Varies, but it doesn't typically take very long.

Q7. No. 3.5 is not the top. Close, but not.

There will be a notable voltage difference between what the power supply reads and what you read directly off the cells. Direct cell voltage is what matters.
 

PeterBC

Solar Enthusiast
Q1. Many don't care. I personally like 0.2V

Q2. Let them sit overnight, or just connect them.

Q3. See Q2.

Q4. 140Ah*8/16 = 70 hours.

Q5. Very close is fine.

Q6. Varies, but it doesn't typically take very long.

Q7. No. 3.5 is not the top. Close, but not.

There will be a notable voltage difference between what the power supply reads and what you read directly off the cells. Direct cell voltage is what matters.
Thanks for your concise replies..
but "very close" and "not very long" are the 2 numbers I'm trying to quantify a bit.
Is Q5 1 , 10 , 100 ma ? other ?
Re step from 3.4 to 3.5 and then 3.6 .. are times similar ? Is it 1 min, 10mins, 100mins, 1000 mins ?

Sorry if I'm being pedantic , just trying to learn from the experience of those who've done this the numbers a bit better ..
 

snoobler

Solar Honey Badger
There's no empirical data. Everybody's setup is different with different response. It will take how long it takes.

The reality is that these cells are full @ 3.65V and 0.05C. At 2240Ah, the cells are full at 3.65V and 112A, so once they hit true cell 3.X and start to taper from a measly 16A, you can move on whenever you like.
 

FilterGuy

What, me worry?
There will be a notable voltage difference between what the power supply reads and what you read directly off the cells. Direct cell voltage is what matters.
This is true, but as the current starts to drop, so will the voltage difference.

The temptation is to turn up the voltage at the power supply to get better voltage at the cells. Don't do it. Before you hook up to the cells, set the power supply to 3.65 (or whatever your target is) and leave it there. When you first hook up to the parallel cells, the supply might become current limited and you will see the voltage go well below your target..... Don't adjust it.

Once the supply is no longer current limited, you will see the voltage at the power supply climb up to your target voltage. If it drifts above the target voltage, turn it back down to the target voltage. You will still see a voltage drop across the leads so the cells won't be at target yet, but don't try to adjust for it. Just be patient.

At the very end, when the current gets low (less than an amp) the voltage drop across the leads will be getting small as well. At that point, watch it closely. Things can start changing quickly. If the power supply drifts off of the target voltage, you may have to adjust it, but use your volt meter to measure it at the supply. When the current gets to zero, the voltage drop will also be zero and the voltage will be the same at both the supply and the cells.

The reason I say never let the voltage at the supply get higher than the target voltage is that if you forget and leave it there.... it will eventually drive the cells to the higher voltage...and that could be bad.

This is one reason why balancing them to a lower voltage first may be a good thing. If your target is 6.5... you can first charge to 5.5 or even 6V and if the supply drifts to 6.2 or 6.3 or even 6.4... it is no big deal so you don't have to watch as closely. When you then charge to 6.5, it will be relatively quick so it is not a long time to babysit it.
 

UncleTodo

New Member
Thanks Snoobler, I'd ended up going with one of those batteries I'd linked in my other thread.
I found something that said cells should be top balanced before connecting the BMS and in searching I came across Will's video on it a couple days ago then this thread.
I have an RC/hobby charger I bought a few years ago that charges LiPo & LiFe, it's been cranking along for about 20 hours now at max charge rate of 5 amps. I'm fairly certain it wasn't meant to charge anywhere near this length of time at max amperage and it was getting rather warm after a few hours so I got a USB powered 120MM fan to put behind it and now stays nice and cool even after 12 hour charge shifts.
Cells started at 2@ 3.27 & 2@ 3.28 and now all 3.31.
I'm waiting for my BMS to arrive, and will probably leave the cells where they be being I'm not ready to install them quite yet.
I'm thinking too I'd like some better battery bars than what was included...
IMG_20210101_114013596.jpg
 

snoobler

Solar Honey Badger
Thanks Snoobler, I'd ended up going with one of those batteries I'd linked in my other thread.
I found something that said cells should be top balanced before connecting the BMS and in searching I came across Will's video on it a couple days ago then this thread.
I have an RC/hobby charger I bought a few years ago that charges LiPo & LiFe, it's been cranking along for about 20 hours now at max charge rate of 5 amps. I'm fairly certain it wasn't meant to charge anywhere near this length of time at max amperage and it was getting rather warm after a few hours so I got a USB powered 120MM fan to put behind it and now stays nice and cool even after 12 hour charge shifts.
Cells started at 2@ 3.27 & 2@ 3.28 and now all 3.31.
I'm waiting for my BMS to arrive, and will probably leave the cells where they be being I'm not ready to install them quite yet.
I'm thinking too I'd like some better battery bars than what was included...
View attachment 31892

Would have been better to exploit its balance function and charge it as a 4S battery with balance leads attached. Would have taken about 25% the time.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Just to toss a monkey wrench into the works.

I've just received 4x EVE 280's to build a 12V pack. Before doing so, I figured on top charging each cell and doing a capacity test on them, curiosity got the better of me. I'm only using one of those 180W / 20A load testers, so it's 14 hours run per cell... talk about tedious. Charging though is why I'm popping in.

I've been charging these 280's with a TekPower 1540E power supply and doing one cell at a time. Starting with 3.60V/30A CC which eventually flips to CV mode and slowly the amps decrease as the impedance changes. The DOCS say 0.05 of the C-Rate which is 14A is fine but as I am doing per cell, cutting off at 1.5A taken. That sets the cells to 3.650. and they settle out to 3.62-3.63 within an hour. At one point I allowed the cells to sit a couple of days after that charge level and they independently settled between 3.51-3.56 "Without Paralleling". The Capacity tests are showing between 276 & 278AH but I am cutting off at 2.65V and starting from 3.60V after being charged & rested for an hour. A Side benefit of doing this, is being able to charge each cell from 2.65V with the same CC-CV charge, helps get things balanced up.

An observation: While it may be a PITA to do a single cell charge (not so bad with 4 but) it is quite a bit faster per cell. It's also easier to top that single cell down to 1-2A taken. Once I've completed my current charge run per cell, I'll tie them together in parallel, let sit a few hours (overnight) and then apply a topping charge to 3.60 to the parallel set until the Amps drop down to 4 or so. Then it's reconfigure & BMS Time.

Remember, the cells can take 0.5C Charge rate, the more amps you can put behind the charge the faster the cells will top off.
 

UncleTodo

New Member
Would have been better to exploit its balance function and charge it as a 4S battery with balance leads attached. Would have taken about 25% the time.

Thanks, I was wondering about that. I do have balance tap adapters, I'll solder up some wires for 4S and finish off with a balance charge.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
i use a active cell balancer.i build the packs,connect bms/balancer/charger and let er rip.works great.
Done it both ways.
Doing a full top charge & top balancing does make a significant difference, especially as you watch the batteries go through their cycles.
I'm using Chargery BMS8T's & QNBBM-8S on my 24V packs. The 12V get a JDB-4S/100A with PC-UART and no active balancer.
The 24's that were not Topped deviate much more above 3.390V while the topped ones don't get hyper till after 3.450.

Passive balancing does little on cells larger than 100AH capacity and especially if they are not matched & batched properly.
 

nwillitts

Solar Addict
Done it both ways.
Doing a full top charge & top balancing does make a significant difference, especially as you watch the batteries go through their cycles.
I'm using Chargery BMS8T's & QNBBM-8S on my 24V packs. The 12V get a JDB-4S/100A with PC-UART and no active balancer.
The 24's that were not Topped deviate much more above 3.390V while the topped ones don't get hyper till after 3.450.

Passive balancing does little on cells larger than 100AH capacity and especially if they are not matched & batched properly.
yea,these are overkill fortune 100ah lfp.matched well.i have built several of these in 12,24 and 48v configs.these batteries are great.
 

snoobler

Solar Honey Badger
Have 9 cells from Amy arriving Sunday (Ordered 11/14). I can charge/discharge all cells simultaneously at 20A. Next week I'll be:

Measure as-received Voltage, IR and capacity
Charge to full at 20A

I likely won't discharge them as the charge and discharge are nearly identical, and I'll be charging from empty. I'll be terminating at 3.65/4A.

I will attempt to log all data. My IoT connections aren't that reliable, so I may have a few hiccups on one or more channels with 21+ hours of testing.

I'm very curious to see how they settle to compare to @Steve_S's results. My CALBs consistently plummet after charging, yet they deliver full capacity from 3.35V after a full charge to 3.65V. If I see the same slow settle as @Steve_S, I'll eagerly quit busting his chops... :)
 
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