A faster & better way to top balance?

nate_syd

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In theory: What if you connect 4 cells (or whatever BMS you have supports), and when the BMS shuts off charging, remove that one high cell and replace it with a fresh one. Repeat the process each time a cell trips the BMS. Eventually you'll have them all around the same level so you can go back to putting them all in parallel to finish a low voltage equalization.
yeah that would make sense - very similar to mine
2 differences: you do it with all the safety of a BMS, you're using 4 cells - instead of 8, 7, 6... so less voltage difference & slightly slower.
by the end of the process there could be 3 low ones, but they would have been there charging since the start - so should be close to full by the end.
 

Just John

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This has probably been done before, and maybe there's a better way - but as a n00b i read the usual sticky post on balancing & in parallel it was going to take AAAAAGES for a 24v 300AH pack at 3.6v
So i thought about it & have a really simple way to do it much faster.

The issue is that you need the biggest voltage difference (charger vs battery pack) to push current when charging, when you put all cells in parallel - this is the lowest voltage drop you could possibly have. A full pack in series is the highest voltage drop.

So here's the process...
  • Get yourself a hobby battery charger that can do LiFe chemistry, they're soooo cheap!!! & can use a 12v computer PSU, and have awesome (1.5A) balancing, and big current capacity. Drones, remote control whatever... that type of hobby charger.
  • Connect up the battery bank in series, connect all the balancing & start the charge, lets say its a 24v 8s pack
  • 1 cell will hit high voltage, remove that cell from the pack & re-assemble the pack - its now a 21v 7s pack
  • Set the hobby charger to 7s cells, connect all balancing & charge
  • the next cell will hit high voltage, remove it from the back & re-assemble - a 18v 6s pack
  • .... repeat
All cells will be removed 1 by one when they hit peak voltage, and you've kept the highest charge current at all times, you've had balancing & safety on - so its all "safe". There is NO BMS!!! so not completely safe, you need a good charger that stops when it hits full voltage, which is easy enough for a hobby battery charger.

Feel free to point out any errors or problems, or link to a better way/information if there is any. The biggest downside is the removal of the BMS, so watch like a hawk & make sure you KNOW that the charger disconnects & stops when it should on the voltage.

Hope that helps any other n00bs & speeds up everyone's balancing.

If you have a BMS properly set up, charge in series and then parallel to finish, speeds things up considerably. Explained in detail.

 

Just John

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Let's try this another way.
The big LFP cells we deal with are "tedious" to Top Balance because they hold a LOT of AH.
Take a 200AH LFP cell. It can discharge 200A (1.0C) and take 100A to charge (.5C)
From 0%SOC (2.50v) to 100% 3.65v would take 2 hours at 100A, 4 hours at 50A and so on.
To TOP BALANCE, all cells are placed in PARALLEL ! NOT Series.
They are then charged to 3.650V with as many amps as possible but NOT exceeding the cell Limits. This continues until the amps taken falls below 1.5A (this is considered as saturated). Technical spec says cutoff can occur at 0.05C, for a 200AH cell that is 10A.
Once all the cells have reached 3.65 & saturated, then they should be left connected in parallel while they settle for the next couple of hours. Typically the cells will drop to around 2.550 within 1 hour and 3.500+ within two hours. This is NORMAL for LFP.
Once settled, then configure that battery pack into Series Config with BMS and fuse and your ready to go (casing etc of course being done too).

Shortcuts & rushing usually do NOT end well, Patience Testing happens in this process. BE PATIENT ! Very patient !! smoke'm if ya gottem LOL.

One "shortcut" that is safe:
You can Top Charge 4 cells - This is the same as Top Balancing but with 4 cells at a time it will be faster to get the cells up to 3.65. Even after they settle, getting them back to 3.65 will be Much Quicker. Do the cells in "lots" of 4 cells, once you have 8 cells Top Charged, then do a Top Balance with all 8 cells in parallel, (it will be quicker) and then assemble the battery packs per standard.

I have built several packs and suffered many of the hard lessons and costs that go with that... some pretty $$ One of the reasons I wrote the Luyuan Tech LifePo4 Assembly guide (link in my signature). I gave up playing & bought a TekPower TP1540E bench Power Supply that can output up to 15V @ 40A and charging the cells at 3.65V/40A goes fairly well. The CC ( Constant Current) get's the charge to 90% and CV (Constant Voltage) takes it the rest of the way as the amps decrease as the cells saturate.

BMS' with Passive Balancing do not do much for Large Capacity cells, that is NOT their target, they can only handle small capacity cells as it only burns off Hi Voltage from cells and is very slow. At this time, very few BMS' have Active Balancing which is suitable for Large Capacity LFP cells. Active Balancers do work and QNBBM, Heltec are Known Good devices that work as designed but they are not that cheap either.

Bulk Commodity ESS grade cells CAN deviate up to 1,v per AH of capacity when reaching above 90% SOC. That's 200mv for a 200AH cell. Top Charging & Balancing can resolve "some" of that and smooth things out.

Matched, Batched & Binned cells (cost more) will behave identically to each other through their entire voltage & load cycles (just like in EV's) and they will not generally deviate more than 30mv and will often settle in at below 10mv difference.

Example of Bulk versus "properly" Matched EVE-280 Bulk = $92, Matched = $125 (and THAT is a deal, typically it is 50% above Bulk Cost) due to the 3 full cycles of Charge/Discharge at various C-Rates with measurements logged every 10 minutes.

FYI: Many Chinese Vendors rely on the fact that the Translator Software is partly borked and inaccurate... (the excuse)
Many say they sell "Matched" cells BUT what they mean to say, is they are Voltage & IR (Internal Resistance) matched so they are identical when shipped. THIS IS NOT MATCHED ! This is Voltage & IR Matched at the storage voltage NOT through operational Test Cycles which is what Proper Matching IS.

Several links in my signature will help you, so grab the downloads etc.
Hope it helps, Good Luck.
Steve
FYI, thanks for the Tekpower supply recommendation, mine has performed flawlessly. I have ordered a Wanptek 30v at 40A supply, so once it arrives and I have tested it, I will sell my Tekpower. I am pretty sure Wanptek was the manufacturer of the Tekpower, but the 30v range of the new supply makes it good for 24v battery packs as well. Of course it costs more as well, but my new one has arrived in the USA per tracking, so I am still waiting. Just wanted to thank you again for the recommendation, great time-saver and of course good for 12v packs as well.
 

nate_syd

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If you have a BMS properly set up, charge in series and then parallel to finish, speeds things up considerably. Explained in detail.

The first line in my post stated that i read this material.
The link you referred to said "be patient it can take days"... my post is about doing it much, much faster & how its achieved.

I read the manual, i know why its the manual, but there's alternatives to achieving the same outcome.
 

Just John

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The first line in my post stated that i read this material.
The link you referred to said "be patient it can take days"... my post is about doing it much, much faster & how its achieved.

I read the manual, i know why its the manual, but there's alternatives to achieving the same outcome.
I have noticed a lot of people are impatient. You can't charge any faster than in series with a BMS. That is explained in the doc. How long it takes depends on cell size and how many amps your charger or power supply puts out. Pretty simple math. However to perform the math, you must supply details (like cell size, charger output). We don't know unless you tell us, and the last person who "read the manual", didn’t actually, he watched a video that left out a lot.

I am sure you will "watch it like a hawk" and have no problems not using a BMS. If a cell is seriously out of balance compared to the others, then it will take a while.

On an Eve 280AH cell, using 40 amps like I do, it takes approximately 7 minutes to go from 3.4v to 3.65v on a single cell. Always use a BMS, because to get to 3.4v will take some time, and most people get bored watching paint dry.
 
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nate_syd

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You can't charge any faster than in series with a BMS. That is explained in the doc.
Thats exactly what my method is doing, charging in series MORE than that published method. That method says put all in series, then move to parallel. Mine says to put all in series, then n-1 in series, n-2 in series until you're done. I do a lot more in series - so do a lot more power in.

Agree with you 100% - on the watching, with my method i literally watch the paint dry - but because its quicker its quite bearable.
Nice info on the 7minutes too - gives great perspective
 

2Big2B

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Can I stack multiple power supplies in parallel to boost amperage?

I have a 8S 29.2v 100A battery pack I just built with an Overkill 8S 29.2v 100A BMS.
I bought a matching (AliExpress) 8S 29.2v 10A Wheelchair battery charger for it.

charger (small).png


I notice someone is selling some used "28.8V Battery Charger LiFePo" chargers, cheap. If I were to use my BMS and say, wire five of their outputs in parallel, would I effectively end up with a 28.8V Battery Charger to rapid charge the pack?

used charger.png

That might be an appealing way to build a rapid charger very cheaply.

Does it work that way? - Or is that a noob question?
 

Just John

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Can I stack multiple power supplies in parallel to boost amperage?

I have a 8S 29.2v 100A battery pack I just built with an Overkill 8S 29.2v 100A BMS.
I bought a matching (AliExpress) 8S 29.2v 10A Wheelchair battery charger for it.

View attachment 55012


I notice someone is selling some used "28.8V Battery Charger LiFePo" chargers, cheap. If I were to use my BMS and say, wire five of their outputs in parallel, would I effectively end up with a 28.8V Battery Charger to rapid charge the pack?

View attachment 55011

That might be an appealing way to build a rapid charger very cheaply.

Does it work that way? - Or is that a noob question?

Yes and no. With lithium chargers, you have no clue what their settings are, what the charge voltages are, how long they charge for, etc.
They could wind up fighting each other (one wanting to float, one wanting to bulk, etc.). Probably better if all were the same brand and model.

That said, the cheap 10 amp power supplies commonly used for top balancing would not be a problem, just set however many to your voltage and 10 amps, and they indeed work well. Depending on price, you might be better off buying a higher amperage supply, or battery charger.

I just bought the 30v 40 amp model here (so far works well, but at 3.65v at 40 amp, it does generate a lot of heat compared to my Tekpower 15v 40 amp supply):

 

2Big2B

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That 30v 40 amp model costs $239.86.

At about $20 each, assuming they are all the same make/model, if I wired 4 of them together and it worked I would end up with roughly the same thing for $80 ! (maybe even cheaper if seller accepts a lower offer for all 5...)

That is what I am investigating.

I already sent the seller this question, looking to see what the seller might know about it. I should follow up and get more details about if they are all the same and their make/model to further investigate.

Meanwhile I was just offered a 28v 72A (airplane 24v SLA type) supply for $35. The catch is, it is 208v, 3 phase AC. I guess I would have to unplug my electric clothes dryer in my apartment to plug it in ...

I had made a $35 offer (plus $35 shipping) but then realized it wasn't 110v - so immediately retracted the bid and sent the seller an email to verify that. He responded that it was 208v AC 3 phase. So I decided it probably wasn't the trouble. Then he accepted, obliging me to complete the purchase. I have sent him back a protest about my retraction, asking him to cancel the sale. I haven't heard back yet. Now i shall investigate how much trouble it would be to get to that 3 phase outlet. It still might be do-able ...
Power One 28 Volt DC, 72 Amp Power Supply.png
 
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Hedges

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Some power supplies can be paralleled. Go by the manual for the supply.
208V 3-phase - is it really 3 phase input, not just 208V? If a rectifier front end, will probably work on single phase, just lower power output. Some may care about he difference between 208V and 240V - some of my equipment has distinct switch settings for those.

How big is your battery bank? If 280 Ah and completely discharged, a 10A supply would take approximately 28 hours to charge in series. Big deal, just wait for it. Whatever parallel balancing might still be needed will be a fraction the Ah.
 

time2roll

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Parallel stacking should be fine with fixed voltage power supplies. Multistage charges may drop voltage at different times so they may drop out. Still no harm. As long as you understand the limits it will not be an issue.
 

Just John

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That 30v 40 amp model costs $239.86.

At about $20 each, assuming they are all the same make/model, if I wired 4 of them together and it worked I would end up with roughly the same thing for $80 ! (maybe even cheaper if seller accepts a lower offer for all 5...)

That is what I am investigating.

I already sent the seller this question, looking to see what the seller might know about it. I should follow up and get more details about if they are all the same and their make/model to further investigate.

Meanwhile I was just offered a 28v 75A (airplane 24v SLA type) supply for $35. The catch is, it is 208v, 3 phase AC. I guess I would have to unplug my electric clothes dryer in my apartment to plug it in ...

Yes, I know how much it costs, I just bought one.

The 28v, 75 amp charger is over 2000 watts, it is over what can be supported by a normal 120v outlet in the USA.

Try ordering 2 and see. It probably will work well, just no guarantee. Just remember that 50 amps at 24v is 1200W assuming 100% efficiency. You probably wouldn't want anything else on that breaker while charging. I wouldn't personally try to run more than 4 on a single breaker. I would say it is about 90% certain that 4 in parallel would work.
 

Hedges

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Some power supplies will crowbar a battery and suck all the current it can supply. Test with a fuse or light bulb.
 

2Big2B

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I have a 8S 29.2v 100A battery pack I just built with an Overkill 8S 29.2v 100A BMS.
I bought a matching (AliExpress) 8S 29.2v 10A Wheelchair battery charger for it.

I am also building a second pack just like it using 8 3.2v 100ah cells, ergo 8S 24v 100Ah. I will use a Daly 8S 24v 100Ah for that one. I presently have that on parallel hooked up to my 20v/10a variable shop power supply that I used to top balance my first pack. The difference is that my first pack was top balanced to 3.5v as a courtesy from the seller, so the process to top balance it up to 3.65 was not so bad. But these came in all at just a hair below 3.0v and it has been on that power supply set to 4v/10A in parallel for a few days and I am only upto about 3.24v, with almost no movement. I began to think there was something wrong - but thinking about the SoC flat curve profile on these LF batteries I can understand why - and looking up this thread just kinda' confirms it for me.

My application is for running my power wheelchair- designed for a pair of 105a Gel batteries (huge Group 27 truck type!). I bought the second set sort of by accident and will probably use it in my other power wheelchair. I might not need to have a fast charger, but it might be nice to have if I go out exploring all day - discharging down to say 30% if I really go wild. Then I might find myself limited in using the power wheelchair while it recovers using a 10A charger..
 

chrisblessing

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Let's try this another way.

Shortcuts & rushing usually do NOT end well, Patience Testing happens in this process. BE PATIENT ! Very patient !! smoke'm if ya gottem LOL.
Thanks Steve. I'm facing something of a charging dilemma and would love a shortcut, but your advice regarding patience is sound, not just with batteries, but all things in life.
 

Just John

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Now i shall investigate how much trouble it would be to get to that 3 phase outlet. It still might be do-able ...
Installing a 3 phase outlet in a normal house will be pricey.
That's why he accepted the bid, nobody wants to buy it. Good luck.

Yes, we hijacked this thread, might want to start a new one.
 

2Big2B

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I am thinking I might be able to just plug it into the 3 pole socket for the clothes dryer. I can't find a picture or any docs about its plug though.

Apologies for hijackings the thread... my thought was to bounce this off of Steve in context to the subject in hand ...
 

time2roll

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Clothes dryer in North America is either NEMA 10-30 or NEMA 14-30 single phase 240v power 30 amps available.
 

Just John

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I am thinking I might be able to just plug it into the 3 pole socket for the clothes dryer. I can't find a picture or any docs about its plug though.

Apologies for hijackings the thread... my thought was to bounce this off of Steve in context to the subject in hand ...
You might want to spend a bit more and get a PowerMax, they have a good reputation.

Cheap Chinese chargers have a tendency to die suddenly with not always good results.

 
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