How to Parallel Balancing. (YEP 99% of us is doing it wrong)(PART#1)

ghostwriter66

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So my boss who has a PhD in Chemical Engineering went to China on business about a month ago and visited one of the largest LiFePO4 battery manufacturers in the world that we use. They gave him a very good tour and today we reciprocated their hospitality by hosting their Lead Design Engineer who is in the States for an upcoming Solar conference. It was a VERY interesting day with him with lots of hands on "Show and Tell". So I thought I would pass what I can on to those interested.

MOST OF THIS INFORMATION IS TAKEN/COPIED FROM THE SLIDE DECKS FROM HIS CLASS

How to Parallel Balancing. (YEP 99% of us is doing it wrong)

So we have all heard the instruction that by just wiring the cells in parallel they will somehow all get themselves magically in balance. This is NOT entirely true unless you have several days-weeks to waste. Now let me caveat something first. 95% of all the LiFePO4 (LFP) cells coming out of China now are made in batches and shipped in batches. This means that if you order four 100aH batteries that the odds are they are going to all be charged at 3.2v at the factory and by the time you get it ALL the cells are probably within a 0.1V variance. SO simply paralleling those four batteries for the next 24 hours will probably do the trick.

BUT if you get batteries that are 0.25v or more out of whack - or you don't want to wait 24 hours - here's how the Manufacturing Design engineers recommend. Remember - Balancing requires a voltage differential to move current between or from/to the cells. That’s why just putting them together in parallel and leaving them does NOT do much.

So to QUICKLY achieve a PERFECT balance.

1. If possible - top each of the cells up to the voltage of the highest cell prior to wiring them in parallel.
2. Wire the cells in parallel
3. Set the power supply to 3.40V and the amperage to about 80% of the max it can do. (Setting it at 80% is only to prolong the life of the Power Supply)
4. Turn on power supply and charge to 3.40V
5. When the current has dropped to 0.0A at 3.40V turn off the power supply & set it to 3.50V
6. Turn on power supply and charge cells to 3.50V
7. When current has dropped to 0.0A at 3.50V turn off the power supply & set to 3.60V
8. Allow current to drop to 0.0A at 3.60V
9. Disconnect Power.
10. The Battery pack is now perfectly balanced.

NOTE: Due to LFP characteristics, It will take about 70% of the time to get from 3.2 to 3.4V. Then about 25% of the time to get 3.4 to 3.5V... then 3.5 to 3.6 will be very short ...

NOTE: Measure the input voltage right at the cells - do not trust the voltmeter on the power supply.

NOTE: You have to do each step as described – you CAN'T simply set it at 3.6V and walk away.
 
Last edited:

Craig

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Thanks. This makes total sense to me. The way you recommend should allow the batteries to absorb the power more evenly. I basically do this for testing my batteries however I didn't step the voltage up I charged to X volts let settle then charge again. Seems like the step up approach would save some time.
 

Solarfun4jim

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So my boss who has a PhD in Chemical Engineering went to China on business about a month ago and visited one of the largest LiFePO4 battery manufacturers in the world that we use. They gave him a very good tour and today we reciprocated their hospitality by hosting their Lead Design Engineer who is in the States for an upcoming Solar conference. It was a VERY interesting day with him with lots of hands on "Show and Tell". So I thought I would pass what I can on to those interested.

I am going to divide this up between a few posts as to not overwhelm.

How to Parallel Balancing. (YEP 99% of us is doing it wrong)

So we have all heard the instruction that by just wiring the cells in parallel they will somehow all get themselves magically in balance. This is NOT entirely true unless you have several days-weeks to waste. Now let me caveat something first. 95% of all the LiFePO4 (LFP) cells coming out of China now are made in batches and shipped in batches. This means that if you order four 100aH batteries that the odds are they are going to all be charged at 3.2v at the factory and by the time you get it ALL the cells are probably within a 0.1V variance. SO simply paralleling those four batteries for the next 24 hours will probably do the trick.

BUT if you get batteries that are 0.25v or more out of whack - or you don't want to wait 24 hours - here's how the Manufacturing Design engineers recommend. Remember - Balancing requires a voltage differential to move current between or from/to the cells. That’s why just putting them together in parallel and leaving them does NOT do much.

So to QUICKLY achieve a PERFECT balance.

1. If possible - top each of the cells up to the voltage of the highest cell prior to wiring them in parallel.
2. Wire the cells in parallel
3. Set the power supply to 3.40V and the amperage to about 80% of the max it can do. (Setting it at 80% is only to prolong the life of the Power Supply)
4. Turn on power supply and charge to 3.40V
5. When the current has dropped to 0.0A at 3.40V turn off the power supply & set it to 3.50V
6. Turn on power supply and charge cells to 3.50V
7. When current has dropped to 0.0A at 3.50V turn off the power supply & set to 3.60V
8. Allow current to drop to 0.0A at 3.60V
9. Disconnect Power.
10. The Battery pack is now perfectly balanced.

NOTE: Due to LFP characteristics, It will take about 70% of the time to get from 3.2 to 3.4V. Then about 25% of the time to get 3.4 to 3.5V... then 3.5 to 3.6 will be very short ...

NOTE: Measure the input voltage right at the cells - do not trust the voltmeter on the power supply.

NOTE: You have to do each step as described – you can simply set it at 3.6V and walk away.
Looking forward to the next parts. (y)
 
Last edited:

FilterGuy

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Thanks,
Great post. I can see where active charging them while hooked up in parallel would get them balanced much quicker. If they are 'just' hooked together, as they get closer to the same voltage the current will asymptotically approach zero....and therefore they would only asymptotically approach balanced.

Question: If we get them close, hook them in parallel and then start using them, won't it achieve the same thing as they are charged and discharged?
 

Solarfun4jim

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So my boss who has a PhD in Chemical Engineering went to China on business about a month ago and visited one of the largest LiFePO4 battery manufacturers in the world that we use. They gave him a very good tour and today we reciprocated their hospitality by hosting their Lead Design Engineer who is in the States for an upcoming Solar conference. It was a VERY interesting day with him with lots of hands on "Show and Tell". So I thought I would pass what I can on to those interested.

I am going to divide this up between a few posts as to not overwhelm.

How to Parallel Balancing. (YEP 99% of us is doing it wrong)

So we have all heard the instruction that by just wiring the cells in parallel they will somehow all get themselves magically in balance. This is NOT entirely true unless you have several days-weeks to waste. Now let me caveat something first. 95% of all the LiFePO4 (LFP) cells coming out of China now are made in batches and shipped in batches. This means that if you order four 100aH batteries that the odds are they are going to all be charged at 3.2v at the factory and by the time you get it ALL the cells are probably within a 0.1V variance. SO simply paralleling those four batteries for the next 24 hours will probably do the trick.

BUT if you get batteries that are 0.25v or more out of whack - or you don't want to wait 24 hours - here's how the Manufacturing Design engineers recommend. Remember - Balancing requires a voltage differential to move current between or from/to the cells. That’s why just putting them together in parallel and leaving them does NOT do much.

So to QUICKLY achieve a PERFECT balance.

1. If possible - top each of the cells up to the voltage of the highest cell prior to wiring them in parallel.
2. Wire the cells in parallel
3. Set the power supply to 3.40V and the amperage to about 80% of the max it can do. (Setting it at 80% is only to prolong the life of the Power Supply)
4. Turn on power supply and charge to 3.40V
5. When the current has dropped to 0.0A at 3.40V turn off the power supply & set it to 3.50V
6. Turn on power supply and charge cells to 3.50V
7. When current has dropped to 0.0A at 3.50V turn off the power supply & set to 3.60V
8. Allow current to drop to 0.0A at 3.60V
9. Disconnect Power.
10. The Battery pack is now perfectly balanced.

NOTE: Due to LFP characteristics, It will take about 70% of the time to get from 3.2 to 3.4V. Then about 25% of the time to get 3.4 to 3.5V... then 3.5 to 3.6 will be very short ...

NOTE: Measure the input voltage right at the cells - do not trust the voltmeter on the power supply.

NOTE: You have to do each step as described – you can simply set it at 3.6V and walk away.
Only thing i would query, is that i thought the voltage had to be above 3.4v just the get the ions moving, so how does this manage to actually charge to this voltage. I read somewhere, that if you dropped the 'float' voltage to 3.4v after charging to say 3.5v, then no further amps would flow?
I'm perplexed. :)
 

Supervstech

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So my boss who has a PhD in Chemical Engineering went to China on business about a month ago and visited one of the largest LiFePO4 battery manufacturers in the world that we use. They gave him a very good tour and today we reciprocated their hospitality by hosting their Lead Design Engineer who is in the States for an upcoming Solar conference. It was a VERY interesting day with him with lots of hands on "Show and Tell". So I thought I would pass what I can on to those interested.

I am going to divide this up between a few posts as to not overwhelm.

How to Parallel Balancing. (YEP 99% of us is doing it wrong)

So we have all heard the instruction that by just wiring the cells in parallel they will somehow all get themselves magically in balance. This is NOT entirely true unless you have several days-weeks to waste. Now let me caveat something first. 95% of all the LiFePO4 (LFP) cells coming out of China now are made in batches and shipped in batches. This means that if you order four 100aH batteries that the odds are they are going to all be charged at 3.2v at the factory and by the time you get it ALL the cells are probably within a 0.1V variance. SO simply paralleling those four batteries for the next 24 hours will probably do the trick.

BUT if you get batteries that are 0.25v or more out of whack - or you don't want to wait 24 hours - here's how the Manufacturing Design engineers recommend. Remember - Balancing requires a voltage differential to move current between or from/to the cells. That’s why just putting them together in parallel and leaving them does NOT do much.

So to QUICKLY achieve a PERFECT balance.

1. If possible - top each of the cells up to the voltage of the highest cell prior to wiring them in parallel.
2. Wire the cells in parallel
3. Set the power supply to 3.40V and the amperage to about 80% of the max it can do. (Setting it at 80% is only to prolong the life of the Power Supply)
4. Turn on power supply and charge to 3.40V
5. When the current has dropped to 0.0A at 3.40V turn off the power supply & set it to 3.50V
6. Turn on power supply and charge cells to 3.50V
7. When current has dropped to 0.0A at 3.50V turn off the power supply & set to 3.60V
8. Allow current to drop to 0.0A at 3.60V
9. Disconnect Power.
10. The Battery pack is now perfectly balanced.

NOTE: Due to LFP characteristics, It will take about 70% of the time to get from 3.2 to 3.4V. Then about 25% of the time to get 3.4 to 3.5V... then 3.5 to 3.6 will be very short ...

NOTE: Measure the input voltage right at the cells - do not trust the voltmeter on the power supply.

NOTE: You have to do each step as described – you can simply set it at 3.6V and walk away.

I LOVE learning more about cell balancing, THANK YOU for sharing with us!

Please edit your last sentence in this instruction.
It is contridicting
 

Supervstech

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Only thing i would query, is that i thought the voltage had to be above 3.4v just the get the ions moving, so how does this manage to actually charge to this voltage. I read somewhere, that if you dropped the 'float' voltage to 3.4v after charging to say 3.5v, then no further amps would flow?
I'm perplexed. :)
True, it charged to 3.5v a float voltage of 3.4 would not do anything.
 

Solarfun4jim

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Well I'm assuming they are starting at less tgan 3.4 volts. Or am I not understanding your guys whole question.
The first step say, set the power supply to 3.4V
ref this as regards the minimum voltage needed to charge

It states the following..

"Charge Voltage Needed
So what Voltage is enough to get those ions moving? A little experimenting shows that 13.6 Volt (3.4V per cell) is the cut-off point; below that very little happens, while above that the battery will get at least 95% full given enough time. At 14.0 Volt (3.5V per cell) the battery easily charges up to 95+ percent with a few hours absorb time and for all intents and purposes there is little difference in charging between 14.0 or higher Voltages, things just happen a little faster at 14.2 Volt and above."

Ok, on re reading it, i see it states that 'BELOW' 3.4v is the cut off point for charging. Something else i had read stated 3.42v, but no worries...i get it, it is just within the limit.
 

SolarRat

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That's the way I've been doing it, because that is how it is written here: https://marinehowto.com/lifepo4-batteries-on-boats/

That link is a must read for anyone here, and is probably where this info came from (or vs versa) because it's almost word for word.


"Parallel Step-Method Top Balance:

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
4- When current has dropped to 0.0A at 3.400V turn off the power supply & set it to 3.500V
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
8- Done, pack is balanced."
 

ghostwriter66

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That's the way I've been doing it, because that is how it is written here: https://marinehowto.com/lifepo4-batteries-on-boats/

That link is a must read for anyone here, and is probably where this info came from (or vs versa) because it's almost word for word.


"Parallel Step-Method Top Balance:

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
4- When current has dropped to 0.0A at 3.400V turn off the power supply & set it to 3.500V
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
8- Done, pack is balanced."

Totally bizarre -- so I just copied the info that he had given us on a handout from his slide deck ... you have to wonder how many times this has been posted in the last 20 years ..LOL .. or maybe since this method obviously works there is no need to reinvent the wheel ...
 

SolarRat

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...Seems like the step up approach would save some time...

According to my link above, one main advantage of the step method is to limit the time the cells spend in the upper knee:

"Through testing and experimenting with numerous balancing processes I’ve found the “parallel step-method top balance” (PSMTB) has proven to be the absolute fastest method that also keeps the cells in the upper-knee the shortest. This means less upper-knee time for the cells. "
 

SolarRat

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Totally bizarre -- so I just copied the info that he had given us on a handout from his slide deck ... you have to wonder how many times this has been posted in the last 20 years ..LOL .. or maybe since this method obviously works there is no need to reinvent the wheel ...

Yup, the info is out there...just most people don't look for it or read it all.
 

ghostwriter66

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Yup, the info is out there...just most people don't look for it or read it all.

At the same time there is allot of information out there that's incorrect that people assume that "if its on the Internet it must be true" and just simply follow ... The battery manufacturer today said that literally 50% of these companies data sheets that posted on their sites (unless they ARE the manufacturer themselves) - especially the C-rate (Charge rate) is wrong and thats dangerous ...

He also said that he personally would never purchase a LFP battery with a built in BMS because almost everyone that they have tested has been wrong - mostly in it couldn't remotely handle the amperage that the battery said it could .... he said that most of these companies just assume if you are getting a 100aH "drop in" battery that you probably only have a 20 or 30aH charger - or getting 30aH maximum charge from a SCC ...
 

electric

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NOTE: You have to do each step as described – you CAN'T simply set it at 3.6V and walk away.
This is nonsense. You certainly can and should just set your supply at 3.6V and walk away. There is no reason to split up in steps, unless your power supply is not current limited and would shutdown. If you have a good CC/CV supply you can certainly do the balance in one step.
 

SolarRat

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This is nonsense. You certainly can and should just set your supply at 3.6V and walk away. There is no reason to split up in steps, unless your power supply is not current limited and would shutdown. If you have a good CC/CV supply you can certainly do the balance in one step.
According to my link above, one main advantage of the step method is to limit the time the cells spend in the upper knee:

"Through testing and experimenting with numerous balancing processes I’ve found the “parallel step-method top balance” (PSMTB) has proven to be the absolute fastest method that also keeps the cells in the upper-knee the shortest. This means less upper-knee time for the cells. "

The 3.6v is in the upper knee. Stepping up prevents any cells from staying in the knee too long with a slooow balance charge.

2018-09-19_8-52-12.png
 

Will Prowse

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I agree that we need to balance at high SOC, but I don't understand the purpose of doing it in steps. They can handle the charge rate just fine. Setting power supply to 3.65v and waiting for current to drop will top balance all of the cells.
 
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