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

[FilterGuy]

As for what SOLAR RAT is saying about different cells getting to different levels of charge faster then others I see his logic ... NO equalization between cells is going to occur as FAST as the charge is occurring ... If you take four cells at different voltage .. 3.2, 3.3, 3.4, 3.5 and throw a 13.6 volt charge on them in series - or parallel them and hit them with 3.6 ... the 3.5 is going to get to 3.6 exponentially faster then the rest and stay up there in the "Upper Knee" until the 3.2 catches up ... unless i am missing something ...

I'm totally open for thoughts?

This is a re-post of my earlier comment about parallel voltages:

I could believe current flowing, but a voltage difference?

Unless there is resistance between the cells and current flowing, I don't see how there would be a voltage difference between the cells.
If there is a measurable difference either the resistance or the current (Or both) would have to be pretty dang significant.

If there is current flowing between the cells, the cells are not yet balanced, but the voltage will be the same at the terminals.

Now let me contradict myself (kinda)
If there is a current flowing between the cells, it means there is voltage differences internal to the cells, (Otherwise current would not be flowing)

 

[electric]

This is a re-post of my earlier comment about parallel voltages:

I could believe current flowing, but a voltage difference?

Unless there is resistance between the cells and current flowing, I don't see how there would be a voltage difference between the cells.
If there is a measurable difference either the resistance or the current (Or both) would have to be pretty dang significant.

If there is current flowing between the cells, the cells are not yet balanced, but the voltage will be the same at the terminals.

Now let me contradict myself (kinda)
If there is a current flowing between the cells, it means there is voltage differences internal to the cells, (Otherwise current would not be flowing)

The moment you connect cells in parallel, their previous OCV ( Open Circuit Voltage ) immediately becomes moot. Even reference to any voltage at time zero becomes moot, unless you specify exactly 2 points where you measure it. Resistance of conductors, internal resistance of cells and initial voltage difference would define amount and direction of initial currents flowing between cells. If initial dV was high and conductors resistance is low enough, initial current could be 10s or even 100+A. This is why no one in their right mind should connect fully discharged and fully charged cells or batteries in parallel, unless you count on resistance of conductor to limit the peak current.
However, if SOC delta is not too high and dV is low enough, then parallel connection is safe and should not be a concern.
Time to equalize would depend on conductors resistance and internal resistance. This is why charging to above 3.4V helps, it reduces the time to balance and gives you a well known starting SOC point. It's just to make things practical.

 

[Solarfun4jim]

Yes but because the discharge/charge curve is relatively flat with LiFePO4, active balancers usually don't kick on till high or low SOC. The cells can be out of balance at 50% SOC, and the voltage difference between the cells can be so minimal that balancer won't be triggered. Also, some active balancers need a higher SOC voltage to push current from one cell to another. Depends on the circuit design. Some work horribly slow at low soc vs high soc for this reason. Also, cells like lto with lower nominal voltage will pass very little current to the cell next to it with most balancers. It again, depends on design of balancer. Some will boost voltage with tiny converter circuit and that helps a lot.

Thanks Will, much appreciated.

 

[Solarfun4jim]

Ok, excuse my ignorance, but is this not a case of cells being paralleled for balancing purposes, but will be used in series configurations in actual use? Thus the balancing takes on different meaning???

From the point of view of someone like myself, that wishes to do a belt and braces system, such that everything is done correct at the initial commissioning stage on an RV, but will never be looked at again over the next 15 years use, would a simple BMS suffice without active balancing, using say CATL equivalent cells? Thinking about up to 32 cells, gives a bit of room for slight mismatch. If running at low C rates and charging to 3.4v/discharging to 3.25v....is this likely to remain balanced over the 15 years?
This to me, is the difference between electrically minded guys and the typical DIY'er, who wishes to find a solution to a problem, build it and then promptly forget about it. They do not want to be scratching their head again in 4 years, trying to re balance their RV battery bank.
Apologies if this seems at a tangent, but electric guys wont mind checking banks every 6 months or so, cause that what they like doing. I built a solar thermal system(flatplate-nothing complicated) back in 2010 and it simply runs. An El sid pump hooked up to a small solar panel, with a solar controller activating diverter valve relay's. I dont have to rethink it...it just runs. I expect to get the same from a vell constructed battery bank. Folks talk about very little drift of cells over months or a few years....what about 10 years when cells are aging(i'm not talking about being worked hard either, simply calendar aging)?

 

[electric]

Ok, excuse my ignorance, but is this not a case of cells being paralleled for balancing purposes, but will be used in series configurations in actual use? Thus the balancing takes on different meaning???

Thinking about up to 32 cells, gives a bit of room for slight mismatch. If running at low C rates and charging to 3.4v/discharging to 3.25v....is this likely to remain balanced over the 15 years?

Parallel for balancing is a one time thing you do when you buy new cells, you never do that again with those cells. Initial balance and good quality cells will stay balanced forever, unless another factor kicks in, like adding a stupid active balancer, which could wreak havoc and give zero benefit to an otherwise healthy battery.

Also, you don't think about 32 cells unless you are making a 100V battery. Assuming you make a 12V battery you only have 4 cells, or 8 in 24V or 16 in 48V. Whatever cells you put in parallel blocks to increase AH become one cell forever. They are not separate cells anymore. Less to worry about.

You don't charge to 3.4V, it is not practical, you'd be wasting most of your battery investment. Same for 3.25V discharge, it is just silly. Whatever you read to come to such conclusions is exactly the FUD I was ranting about. Find better source to follow. I won't repeat what I already stated a few times in previous posts, you can find it on the forum. Many reputable sources here, but a lot of FUD also. Spend time reading and thinking and question everything.

 

[FilterGuy]

Ok, excuse my ignorance, but is this not a case of cells being paralleled for balancing purposes, but will be used in series configurations in actual use? Thus the balancing takes on different meaning???

From the point of view of someone like myself, that wishes to do a belt and braces system, such that everything is done correct at the initial commisioning stage on an RV, but will never be looked at again over the next 15 years use, would a simple BMS suffice without active balancing, using say CATL equivalent cells? Thinking about up to 32 cells, gives a bit of room for slight mismatch. If running at low C rates and charging to 3.4v/discharging to 3.25v....is this likely to remain balanced over the 15 years?
This to me, is the difference between electrically minded guys and the typical DIY'er, who wishes to find a solution to a problem and then promptly forget about it. They do not want to be scratching their head again in 4 years, trying to re balance their RV battery bank.
Apologies if this seems at a tangent, but electric guys wont mind checking banks every 6 months or so, cause that what they like doing. I built a solar thermal system(flatplate-nothing complicated) back in 2010 and it simply runs. An El sid pump hooked up to a small solar panel, with a solar controller activating diverter valve relay's. I dont have to rethink it...it just runs. I expect to get the same from a vell constructed battery bank. Folks talk about very little drift of cells over months or a few years....what about 10 years when cells are aging(i'm not talking about being worked hard either, simply calendar aging)?

I am one of those 'electrical guys' by training and career.... but I want my systems to be as 'set-and-forget' as they can be.. (You would hope the civil engineer doesn't have to come back to the bridge every 3 months to check if it was working ) In my career I have also built an appreciation of the beauty of simplicity. I have fun exploring complex ideas with people on the forum, but I am always looking for the simple solution. A clever but complex solution may create a bigger reliability problem than the problem it was trying to avoid.

The more I can understand the internal details of how something works, the more I can be confident what will work or not. Unfortunately, I just don't have the many years experience with LiFePO4 that would tell me what the drift would be, and you can find almost any answer you want on the wild wild web..... That is frustrating to me. My *guess* is that if they were well matched to begin with they would drift in similar ways and the differences would be small... but I certainly don't know.

 

[Solarfun4jim]

Thanks electric. Looking at a 48V bank, of 400Ah(19.2kW) i had thought i'd read it somewhere that you did not want to go above 200Ah cells in a RV due to structural integrity within the cell, but Ghostwriter kinda dispelled that myth earlier in another conversation.

 

[Solarfun4jim]

I am one of those 'electrical guys' by training and career.... but I want my systems to be as 'set-and-forget' as they can be.. (You would hope the civil engineer doesn't have to come back to the bridge every 3 months to check if it was working ) In my career I have also built an appreciation of the beauty of simplicity. I have fun exploring complex ideas with people on the forum, but I am always looking for the simple solution. A clever but complex solution may create a bigger reliability problem than the problem it was trying to avoid.

The more I can understand the internal details of how something works, the more I can be confident what will work or not. Unfortunately, I just don't have the many years experience with LiFePO4 that would tell me what the drift would be, and you can find almost any answer you want on the wild wild web..... That is frustrating to me. My *guess* is that if they were well matched to begin with they would drift in similar ways and the differences would be small... but I certainly don't know.

Difference is filterguy, in 4-5 years time, you are still going to understand the basic principles...i, on the other hand, would need to re-study

 

[electric]

Thanks electric. Looking at a 48V bank, of 400Ah(19.2kW) i had thought i'd read it somewhere that you did not want to go above 200Ah cells in a RV due to structural integrity within the cell, but Ghostwriter kinda dispelled that myth earlier in another conversation.

Yea, another piece of FUD. Get the largest cells you can get your hands on, less connections to worry about.

 

[electric]

My *guess* is that if they were well matched to begin with they would drift in similar ways and the differences would be small... but I certainly don't know.

Your guess is correct, probably because your training and experience tells you that things should be really simple, otherwise there would not be a huge industry growing around this technology. Lithium and LFP are not new at all, been around for decades. Everything is already developed and understood. It is really simple and reliable, but you need good cells. If the price seems too good to be true, it probably is. Good cells and a simple reliable BMS rated for the job, that is all you need. No active balancers.

 

[Solarfun4jim]

Your guess is correct, probably because your training and experience tells you that things should be really simple, otherwise there would not be a huge industry growing around this technology. Lithium and LFP are not new at all, been around for decades. Everything is already developed and understood. It is really simple and reliable, but you need good cells. If the price seems too good to be true, it probably is. Good cells and a simple reliable BMS rated for the job, that is all you need. No active balancers.

How do you actually know if the cells are as good as you would like? Some of the CATL ones from china are a very good price and there seems mixed reports....but how does a novice make that judgement? You can pick up four times the capacity of battleborns at similar price, but that doesnt necessarily mean they are duff cells.

 

[electric]

A clever but complex solution may create a bigger reliability problem than the problem it was trying to avoid.

These words should be written on the tombstone over the grave where active balancers are buried

 

[GVSolar]

I love this forum. It's like hanging out with Robert Heinlein in a disco bar.

 

[electric]

How do you actually know if the cells are as good as you would like? Some of the CATL ones from china are a very good price and there seems mixed reports....but how does a novice make that judgement? You can pick up four times the capacity of battleborns at similar price, but that doesnt necessarily mean they are duff cells.

All cells are made in China, the best ones and the worst ones. You cannot buy from OEM, you can only buy from VARs. So, the question becomes which VAR is reputable, been around long enough, cares about their name and reputation, has good business model setup for shipping and warehousing, etc.
Battleborn doesn't offer cells, so it's apples to oranges. If you decided to make your own battery, then you forget about Battleborn. If you don't think you have skills to make a battery, then cell prices are irrelevant to you, you pay Battleborn for their experience in making a solid battery product.

 

[Solarfun4jim]

Yea, another piece of FUD. Get the largest cells you can get your hands on, less connections to worry about.

Tapping your experience electric, which configuration would you say was most 'reliable'....16S, 2P8S or 4P4S?

 

[electric]

Tapping your experience electric, which configuration would you say was most 'reliable'....16S, 2P8S or 4P4S?

This question is nonsense. What does reliability have to do with system voltage? You pick system voltage based on various application factors, then you build a battery to match system voltage. All configurations are reliable if you engineered everything correctly.

 

[ghostwriter66]

All cells are made in China, the best ones and the worst ones. You cannot buy from OEM, you can only buy from VARs. So, the question becomes which VAR is reputable, been around long enough, cares about their name and reputation, has good business model setup for shipping and warehousing, etc.
Battleborn doesn't offer cells, so it's apples to oranges. If you decided to make your own battery, then you forget about Battleborn. If you don't think you have skills to make a battery, then cell prices are irrelevant to you, you pay Battleborn for their experience in making a solid battery product.

I agree with all the above -- The only correction I would say is that Battleborn does NOT make a solid battery product -- THEY package a solid battery product. Like everyone else - they do not make anything here -- their cells and BMS comes from China - they just package it in the states and slap a sticker on it ...

and there are some excellent OEM companies like DELIGREEN that not only manufacturer but also sell ...

 

[Solarfun4jim]

This question is nonsense. What does reliability have to do with system voltage? You pick system voltage based on various application factors, then you build a battery to match system voltage. All configurations are reliable if you engineered everything correctly.

Many thanks for your reply Electric.

 

[FilterGuy]

Difference is filterguy, in 4-5 years time, you are still going to understand the basic principles...i, on the other hand, would need to re-study

You are assuming I have not fried my other brain cell by then

 

[electric]

I agree with all the above -- The only correction I would say is that Battleborn does NOT make a solid battery product -- THEY package a solid battery product. Like everyone else - they do not make anything here -- their cells and BMS comes from China - they just package it in the states and slap a sticker on it ...

I don't like Battleborn any more than you do, but I refrain from negative comments because the devil is in details and your statement is just too simplistic. Make/package are loose terms and much depends on how well the business is setup and financed and how honest management is.
I work for a competitor, but don't reveal which one, because it's not my decision and I don't lower myself to anonymous slander. I can only say that things are often not as they seem and will leave it at that. Time will sort out winners from losers.