Balancing Questions

[Baserati]

Couple of novice questions regarding balancing

1) Im balancing 20 cells in parallel. Surely the voltage at the furthest terminal from my power supply will end up with less voltage due to resistance on my busbars. How do i solve for this?

2) Related to this, how tight do I need to make my busbar connections for balancing. I want to minimise the stress on my aluminium threads?

Any clever ways people have done the set up for balancing would be appreciated.

 

[RCinFLA]

Voltage difference only when significant current during initial charging. As cells get close to full charge their current drops off and voltage difference will diminish.

If you connect charger Pos and Neg to opposite ends of stack it will minimize initial difference.

Biggest mistake is not setting voltage limit on charger with no load on power supply, or cranking up voltage while power supply in current limiting.
Wire gauge to power supply matters. Try to keep voltage drop from cells to power supply less then 50 mV. Too much wiring voltage drop increases top balancing time. Cranking up power supply voltage to make up for wiring voltage drop is a good way to overcharge and end up with bloated cells. Throw away the power supply leads supplied with typical low cost 30v 5 amp or 10 amp supplies. They are junk.

Paralleling cells will not fully balance without charging supply. If cells greater then about 30-50 mV difference some current will flow between cells towards balance but once the difference is less then 30-50 mV they will not get closer without external charging source. Paralleling cells should be within 3.24v to 3.33v. A fully discharged cell paralleled with fully charged cell can produce upwards of several hundred amps for 280 AH cells depending on resistance of bus bars used.

As a precaution against short-cycled manufacturer charge forming, it is good not to push new cells greater then 0.2C current for first six to ten cycles. This allows additional forming of SEI layer if incomplete job done by manufacturer. Insufficient SEI layer growth protection with high cell current can damage electrolyte which will coat anode and cathode layer surfaces restricting ion flow, yielding higher cell resistance..

Higher initial capacity and lower initial cell resistance then cell spec is not necessarily a good thing. Both are also a sign of incomplete SEI layer growth. Manufacturing SEI growth process takes several days to over a week to perform correctly. If manufacturer is being pushed to supply cells where do you think they are going to cut corners.

 

[Baserati]

Makes sense, thank you

 

[time2roll]

I would just snug up the terminals. Maybe 10 to 20 inch-pounds for the balance. Check the surfaces are clean. Monitor the cells are all rising in voltage together. Attache the power supply to each end of the stack.

 

[Baserati]

Im using a Xantrax XHR 20-50. Max Amps is 50. Batteries are 200AH with 1C. Am I right in that .2c would be 40 Amps multiplied by 20 batteries = 800Amps?

 

[toms]

As a precaution against short-cycled manufacturer charge forming, it is good not to push new cells greater then 0.2C current for first six to ten cycles. This allows additional forming of SEI layer if incomplete job done by manufacturer. Insufficient SEI layer growth protection with high cell current can damage electrolyte which will coat anode and cathode layer surfaces restricting ion flow, yielding higher cell resistance..

That goes against most of what i know about SEI formation, where did this information come from?

 

[time2roll]

Im using a Xantrax XHR 20-50. Max Amps is 50. Batteries are 200AH with 1C. Am I right in that .2c would be 40 Amps multiplied by 20 batteries = 800Amps?

Your are at 20 x 200 = 4,000 AH / 50 amps would take up to 80 hours. Although probably half that if half charged.

 

[RCinFLA]

That goes against most of what i know about SEI formation, where did this information come from

SEI formation comes from reaction primarily between lithum ions carried by electrolyte, free electrons from graphite anode, and other solvent chemicals in electrolyte, at anode-electrolyte interface. It's primary function is to prevent electrons back washing into electrolyte which will cause it to decompose. The reason why anode-electrolyte interface is a problem is there is little layer potential difference at this interface making it easier for electrons to migrate back into electrolyte. The SEI layer provides a field barrier restricting the backwash of electrons into electrolytes. It has some, but not too excessive restriction to lithium-ion flow to anode graphite..

To prevent too much damage to electrolyte, the initial charging on a newly manufactured cell must start out at low voltage, then gradually build to higher level as the SEI layer grows. This takes a lot of time and factory resources in charge forming equipment stations. Its initial growth consumes about 5% of cells total lithium content, reducing capacity by about same amount. It also increases cell impedance, which relative rise is used by some manufacturers to determine progression of growth process. You have to know the initial pre-charge impedance of cell for the change to be meaningful. Another way to evaluate SEI growth on new cells is leakage current at elevated temps. Too much leakage on a new cell means insufficient SEI growth. Some manufacturers don't like this leakage method of evaluating SEI growth because it requires more equipment and time.

Assuming a cell is not subjected to other detrimental use by end user, like overcharging or excesssive current, the gradual, continuous growth of SEI layer due to normal cycling use will eventually eat up significant percentage of available lithium causing capacity loss and cause increasing impedance of cell. High current, keeping cell fully charged put more stress on graphite anode and SEI layer. It is normal for small fractures to develop in thin SEI layer that will be rebuilt / repaired on subsequent charge cycles but consume some of cell's available lithium, and increasing cell impedance over time.

A Google search will yield many techical papers on topic. Attached is a simple Youtube videos on topic. Jump to 7:55 in video to get to explanation.

 

[toms]

SEI formation comes from reaction primarily between lithum ions carried by electrolyte, free electrons, and other solvent chemicals in electrolyte, at anode-electrolyte interface. It's primary function is to prevent electrons back washing into electrolyte which will cause it to decompose.

Not sure where you are getting your info from - even in the videos you linked it mentions the SEI keeps electrons in the electrolyte.

The last video speculated about Teslas formation methods without going into detail.

I’d be curious to see any manufacturers protocol for SEI formation, it would have to be the most closely guarded information in the field.

It is worth investigating, and is one of the reasons i’m opposed to parallel top balancing at low currents.

 

[RCinFLA]

Not sure where you are getting your info from - even in the videos you linked it mentions the SEI keeps electrons in the electrolyte.

The last video speculated about Teslas formation methods without going into detail.

I’d be curious to see any manufacturers protocol for SEI formation, it would have to be the most closely guarded information in the field.

It is worth investigating, and is one of the reasons i’m opposed to parallel top balancing at low currents.

You need to rewatch video, that is not what they say.

Jump to timestamp 7:55. The verbage of 'electrons in graphite is a problem' should be more appropriately worded as 'electrons in graphite create a potential problem'. Having electrons in graphite is normal cell operation, the result of external current flow. The problem they create is possibility they migrate into electrolyte which the video does explain.

Most of the cell layer potential difference is at the LiFePO4 to electrolyte interface. The electrolyte to graphite layer is low potential difference making it easier for electrons to jump the layer interface and break into electrolyte where they activate chemical reactions detrimental to electrolyte. SEI creates an additonal electrostatic barrier that helps to repell electrons back to graphite anode, while still allowing Li Ions through with minimal blockage Without the SEI layer the cycle life of a cell will be significantly reduced.

Most commonly used electrolyte is hexafluorophosphate, ‘salt’ (LiPF6) and Ethylene Carbonate (C3H4O3) ‘solvent’. LiPF6 salt dissolves into solvent to create Li+ and PF6- free floating ions. Electrolyte is the transfer ‘conveyor belt’ for Li+ cat-ions movement between cathode (LiFePO4) and anode (graphite) of cell. The solvent is a hydrocarbon (petroleum) and is somewhat flammable although it has a very high flash point, similar to fuel oil or diesel.

As an aside, the electrolyte stable range is cell voltage between 0.2v and about 4.3v. This range has some temp dependancy. LiPO cells use similar electrolyte but the different cathode material gives the cells a slightly higher voltage. The 4.2v max charge voltage for LiPO cells is very close to the limit for electrolyte making max charging voltage very critical. LFP cell's lower voltage gives them more margin to the electrolyte limit. The LiFePO4 cathode is also more robust against overcharging. 4.3v is not a hard limit, it is just where the decomposition of electrolyte exponentially accelerates. If you bloated a cell during charging you likely took it above 4.3v.

The charge forming requirement is openly known with a lot of research papers on topic. A given manufacturer's process is a closely guarded secret partially for propriety reasons, partially to avoid industry scrutiny challenging of an accelerated process that could negatively impact long term quality of cells. Most common trick is electrolyte additives and elevated process temp to enhance formation speed. Understanding possible long term negative side effects of additives is the tough part.

 

[toms]

I notice you removed the first video from your reply that more accurately reflects the purpose of the SEI. The video you left in your link is misleading. Why did you remove the other videos?

Electrons will only be trying to move from Anode to Cathode through the SEI under a discharge condition, and there will be no redox reaction at the Anode in this situation.

It is very clear that the SEI is formed during a charge cycle, and in this instance once the SEI is formed correctly electrons are prevented from further reaction with the anode.

 

[toms]

The charge forming requirement is openly known with a lot of research papers on topic. A given manufacturer's process is a closely guarded secret partially for propriety reasons, partially to avoid industry scrutiny challenging of an accelerated process that could negatively impact long term quality of cells. Most common trick is electrolyte additives and elevated process temp to enhance formation speed. Understanding possible long term negative side effects of additives is the tough part.

As you have seen, blindly accepting what is presented on the internet without a good fundamental understanding of the chemistry involved will allow you to be easily mislead.

I’ve yet to see an online article on SEI formation that comes close to what the quality cell manufacturers are currently doing.

I understand this a beginner level forum, but it surprises me that there isn’t more interest in this topic as it goes hand in glove with the issue of low current parallel top balancing.

 

[Baserati]

So what is the best way to top balance based on this understanding?

 

[toms]

Individually charge cells to top knee voltage (typically 3.65V) until current drops to 0.05C.

Fit a cell monitor to the cell that will disconnect the charge source if the cell voltage goes above target.

Repeat for all cells.

 

[John Frum]

Individually charge cells to top knee voltage (typically 3.65V) until current drops to 0.05C.

Fit a cell monitor to the cell that will disconnect the charge source if the cell voltage goes above target.

Repeat for all cells.

Assumes a charge current > 0.05C
For the 280 amp hour cells that are popular here that is > 14 amps.

 

[time2roll]

Assumes a charge current > 0.05C
For the 280 amp hour cells that are popular here that is > 14 amps.

So the typical 10 amp supply is done immediately at 3.65

 

[toms]

That is correct.

 

[toms]

Assumes a charge current > 0.05C
For the 280 amp hour cells that are popular here that is > 14 amps.

There are plenty of suitable chargers available that go well over 10A.

The parallel top balance method at low current is a short cut method that gets the job done - it just has associated risks that are unacceptable to me.

 

[Gazoo]

I’ve yet to see an online article on SEI formation that comes close to what the quality cell manufacturers are currently doing.

And that's the problem...lack of data. So no one knows if parallel top balancing using a typical 10 amp power supply is damaging the cells. In the end I think we might get more data from users of this forum than we can get from the manufacturers.

 

[toms]

And that's the problem...lack of data. So no one knows if parallel top balancing using a typical 10 amp power supply is damaging the cells. In the end I think we might get more data from users of this forum than we can get from the manufacturers.

Eventually we will. There are many that have been working with this chemistry for well over a decade. The information is available directly from my manufacturer, just not published. It’s a shame to ignore that information.

As i said, not my money so if people want to take their chances then good luck to them.

The main question is, with all the different ways of getting your cells balanced - why choose the way that damages your cells at all?