Top balancing, recommendations (fastest or best practices)

[snoobler]

Okay.

But what's your actual current?

 

[Alfalfameister]

At 16A, puny cables (I suspect 16 AWG or even only 18AWG) that came with PS were getting rather warm (you could hold them long, but we're definitely warm).

Replaced them with fatter cables (6ish AWG).

Current according to PS is 18.32A (I just raised it).

What should I be looking for/observing?

Thanks!

 

[snoobler]

That's just pretty impressive. Most see a 0.3V drop and their 10A PSU will rarely put out much more than 4A.

That supply must be a pretty good one with low internal resistance. Are you using ring terminals?

 

[Dzl]

One advantage I see with step balancing (other than the peace of mind of making sure you are around and paying attention in the 3.6-3.65v territory, and the benefits of being there to witness/monitor performance in the knee) particularly with these large capacity cells/banks and low current chargers is that you can minimize the time your batteries are being held at nearly full charge/max voltage.

I've heard here and there that even at 3.65v, some damage can occur if held there too long during balancing. I'm not 100% convinced of this if we are talking hours to a few days, but I'm also quite positive I'm not informed enough to say one way or the other, and at the very least I know that holding at a high state of charge does negatively affect lifespan.

EVE defines end of charge as when current drops to 0.05C @ 3.65V. With 4 x 280 Ah cells, the max a 10A benchtop supply can supply is 0.009C and 16 cells in parallel the max rate for a 10A benchtop supply is 0.002C, Even with just a single cell, the max a 10A supply could deliver is 0.036C. Point being, the common chargers everyone is buying can't even supply the current that EVE considers 'end of charge' cutoff point (orders of magnitude less if parallel balancing). So at a minimum, cells will be held near 3.65v for longer than they should, while the power supply trickle charges the large cells. Stepped balancing, is one way to minimize the time spent near 3.65v, if you make sure to be present and attentive during that last step. Does this make sense?

 

[Alfalfameister]

To Dzl - i think i read somewhere in this forum that holding them at high SoC (3.6 to 3.65) for a long time is not good, but "long" here means months. Several days or few weeks doesn't really count (but, yeah, why stay there longer than you have to?)

To snoobler

Couple of things, before it got wonky:

1. I disconnected the battery first;
2. Measured at the end (alligator clips) of the ~1m 6-ish AWG cable: 3.561v. At the output side of the PS, 3.581v (0.03v loss by the time of the alligator clips). Cable attached using ring terminals (ring terminal lug one end, alligator clip at other end).

Then it got wonky:

Re-connected alligator clips to battery. Current showed zero. Tried adjusting knobs. No joy. Turned off PS. Disconnected battery. Did all sorts of combinations (turned on PS with battery disconnected, with battery connected, adjusted knobs, etc).

Finally, with battery disconnected, raised voltage and current knobs a little, and quickly shorted them (quick tap of + and - of alligator clips).

Got my voltage reading back, then reconnected to battery, and adjusted voltage and current to "safe" (sub 3.6v) levels.

I do remember something about cheap PSs that need to be shorted quickly to "reset" them. I don't like that.

 

[Alfalfameister]

That's just pretty impressive. Most see a 0.3V drop and their 10A PSU will rarely put out much more than 4A.

That supply must be a pretty good one with low internal resistance. Are you using ring terminals?

$125ish (locally to me) PS. I use the output at the back.

 

[Dzl]

To Dzl - i think i read somewhere in this forum that holding them at high SoC (3.6 to 3.65) for a long time is not good, but "long" here means months. Several days or few weeks doesn't really count (but, yeah, why stay there longer than you have to?)

This was my impression too, but I have also heard, that even in the course of a balance some damage can occur, I'm not yet convinced this is true, but why risk it if the stepped method is virtually the same amount of effort and time, and is recommended by at least three reputable sources (1, 2, 3). One other source (don't have the link), advocates balancing cells individually, not in parallel (which I don't fully understand), the purpose is the same, minimizing time spent 'in the knees.'

All that said, since a parallel balance is at most a once a year deal, and for most people a one time thing, I can't imagine too much damage could come from pushing charge into a near-full or full pack for a few hours or days. But what do I know (hint: not much )

edit: I will try to dig up where I heard this, and the link.

 

[Alfalfameister]

I think some damage I have seen is the bloating of cells. A slight compression should fix that.

I've seen those reputable sources, but I find Will Prowse and a guy named "electric" here also reputable sources.

At such low c-rates, and for a few days, i think overly babying the cells is not required.

I hope everything turns out okay, though.

 

[Gazoo]

I'm not yet convinced this is true, but why risk it if the stepped method is virtually the same amount of effort and time, and is recommended by at least three reputable sources (1, 2, 3).

Do any of those sources explain why step balancing is a good idea? Do any of those sources suggest a rest time between charging? I do recall reading about the step suggestions but I don't remember.

Anyways I am most likely going to top off my cells individually. While rare it makes sense to me a cell could be damaged parallel top balancing. If one or more cells is at a high SOC compared to the others I think it could be damaged. Of course that's just my thoughts, and what do I know? I read the document upnorthandpersonal posted.

https://files.ev-power.eu/inc/_auto88/_info/Doc/GWL-Power-Cell-Damage-OverCharge.pdf

 

[Dzl]

I've seen those reputable sources, but I find Will Prowse and a guy named "electric" here also reputable sources.

This is part of the reason I feel the parallel stepped balancing makes sense. Given differing perspectives from semi-knowledgeable sources, I think its reasonable to lean towards the more cautious approach (all other things being equal), particularly when it doesn't cost you anything.

I have a lot of respect for Will he is pretty knowledgeable and experienced, and good at explaining things simply and clearly, but my impression is he is towards the less conservative/cautious end of the spectrum, this is not a positive or a negative, just a subjective observation.

At such low c-rates, and for a few days, i think overly babying the cells is not required.

I don't have a full command of the facts, but in this particular case, I think low C-rate might potentially cause the problem, not mitigate it. In this case, I think one of the problems the stepped method is trying to address is minimizing the time the cells are being charged at or near 3.65v. The lower the C-rate, the longer the time in the knees.

Regardless of whether the stepped method is better or not, I suspect with a healthy moderate sized pack, a one time (or occasional) top balance will not lead to any immediate or substantial damage whichever method you choose. But its easy enough to do it this way, and it will either be a little better and safer, or the same as non-stepped parallel balancing, so I see no downside and a potential upside, and can see why some consider it a best practice to balance this way.

 

[Dzl]

https://files.ev-power.eu/inc/_auto88/_info/Doc/GWL-Power-Cell-Damage-OverCharge.pdf

That is the last link I was trying to find, thanks!
I plan to reread it soon, I recall having some questions that needed addressing the first read through.

Do any of those sources explain why step balancing is a good idea? Do any of those sources suggest a rest time between charging? I do recall reading about the step suggestions but I don't remember.

I think the reasoning is partially the same as with the method you are considering. Minimizing time in the knees/time at near-full SOC. But I think the other benefit is convenience/ease. You can set the voltage to 3.4 or 3.45 and forget about them without risking any immediate damage, or charge them in series for the first step to speed things along, and then do the last step or two in parallel.

I don't recall a rest time being mentioned, just that current should taper to near zero. I think this is the central piece of this method, long absorption times at the lower steps, so the final step is minimized.

Relevant excerpts:

Spoiler: MarineHowTo: Parallel Step Method

Updated Cell Balance Process: Parallel Step-Method Top Balance

My goal when balancing cells is always the following:

Keep the cells in the upper-knee for the shortest amount of time and still net a perfect balance

Trough 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. You will need a variable power supply capable of low voltage (3.6V) to do this. You will also want a model with the highest amperage you can source. Keep in mind that when we wire the cells in parallel the bank capacity grows tremendously. Four 400Ah cells become a 1600Ah 3.2V pack! Getting to 3.40V will take quite some time! The key with the PSMTB come from the fact that the cells are essentially full when you get to 3.40V and 0A. This 3.40V threshold is a perfectly safe voltage for the cells so no matter how long it takes to get there will not be causing damage to the cells. Once at 3.40V this means our steps to get to 3.5oV and then 3.60V are much, much shorter than the first step getting to 3.40V. The step to 3.50V is longer than the final step to 3.60V, which happens pretty quickly.

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.

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

Spoiler: Nordkyn Design: Parallel Step Method

Method 1: Charging and Balancing Cells Using a Regulated Power Supply Unit

There are a few options available for first charging and balancing the cells. Using a regulated bench top power supply unit (PSU) is the commonly promoted approach and also the least practical and accessible for a one-off job on board – which is often the context in place when building a DIY system on an ocean cruising yacht. This process is very slow, inefficient and requires a regulated power supply unit and mains power for several days.


In some cases these constraints don’t apply or this method can be combined with the second method to “finish off” the cells, so the process is explained below, but you should prefer the second method described.


Never use a common battery charger: its output is unregulated and, even if it is able to hold without overloading and tripping, it cannot limit the voltage as the cells charge up. The guaranteed outcome will be a totally destroyed set of cells at best, or a fire. Don’t imagine for a second that you will be able to “see it coming” and prevent it. The voltage seems to remain constant forever and then rapidly rises without any warning.


You need an adjustable, regulated power supply unit to follow this process.

Parallel charging and top-balancing cells using a regulated power supply unit (PSU).
Voltage regulation is essential to ensure the target voltage cannot be exceeded.

First of all, power the PSU before connecting anything to it and never interrupt the mains for as long as there are batteries connected to it. Some PSUs are not well protected against reverse current flow and not intended for use with large capacitive loads!


If possible at all, use a PSU that is explicitly suitable to charge a battery; in doubt, use great caution as a mishap can easily damage it. If smoke escapes from it, you will never get it back in.

1. With the output disconnected, set the voltage regulation limit at 3.40-3.45V and preset the current limit (if any) to a value that won’t overload the PSU. Refer to the manual as required. In doubt, always start with a low current limit and never exceed 80% of the rated output.
2. With all the cells wired in parallel, connect the PSU, bulk charge and absorb until no current flows any more. The voltage will stay around 3.3V for a very long time before starting to rise. Charging this way can take several days. This will near-fully charge the cells without stressing them unduly, but don’t hold them at that voltage indefinitely. Keep checking up on them at least a couple of times each day. Briefly disconnect the cells and recheck the voltage limit setting on the PSU: better safe than sorry. Avoid charging the cells individually, or in batches; the whole process would take just as long, but would also result in some fully charged cells lying around for several days.
3. Once the voltage has reached the PSU output regulation limit and there is no apparent charging current any more, disconnect the cells from the PSU and increase the output voltage regulation limit to 3.60V.
4. Then, while standing by only, reconnect the cells and allow the voltage to rise up to 3.60V and stabilise for a few minutes; this normally takes little time and additional current, provided the cells were fully absorbed at the lower voltage. Whether you target 3.60V, 3.65V or even 3.70V is of no consequence or interest if you are actively monitoring the process, because these values are often reached seconds apart only.
5. Disconnect the PSU from the cells again and wait. The cells should hold above 3.50V for at least 30 minutes. If not, bring them up again and hold them for a little longer until they do. At 3.60V, you may need to insist a little more than if using 3.65 or 3.70 volts; that’s all.

As with all unattended charging of lithium batteries, some very careful thoughts must be given to the potential consequences of a failure somewhere

Using a regulated PSU, a failure – no matter how unlikely – of the unit cannot be entirely excluded and there is no other line of defence in place. Hopefully, it would just trip, but if it didn’t, it could lead to a battery fire. Someone could also come past and interfere with the equipment during charging with the same outcome.

Anyways I am most likely going to top off my cells individually. While rare it makes sense to me a cell could be damaged parallel top balancing. If one or more cells is at a high SOC compared to the others I think it could be damaged. Of course that's just my thoughts, and what do I know? I read the document upnorthandpersonal posted.

A point that should at least be considered before doing the single cell method:

Avoid charging the cells individually, or in batches; the whole process would take just as long, but would also result in some fully charged cells lying around for several days.

 

[Alfalfameister]

Still sorta disagree with you in theory (I think you can set it to 3.6 and just charge it all the way there), but the funny thing is, in practice and reality, I'm doing the step method (but just 3.5+, then 3.6v after) - though the reason behind is not the same as yours.

I'll get back to those papers mentioned (esp the GWL one) later; just gotta go somewhere now.

Cheers!

 

[Dzl]

Still sorta disagree with you in theory

Its not really me you disagree with (but I get your meaning), I don't have enough info/insight to have formed my own opinion, I'm just trying to articulate what I see as the best practices for top balancing as explained by those with more expertise and much deeper knowledge than me and most here. There is a good chance in my mind that either method is perfectly fine. But I don't see a good reason not to choose the step-method if its potentially a bit safer and a bit better for the battery, but if you do, I'm curious to learn more and explore it. I'm always interested in learning more balancing logic is one of my weak points for sure.

(I think you can set it to 3.6 and just charge it all the way there), but the funny thing is, in practice and reality, I'm doing the step method (but just 3.5+, then 3.6v after) - though the reason behind is not the same as yours.

Funny how that works sometimes
What is your reason? expedience?

I'll get back to those papers mentioned (esp the GWL one) later; just gotta go somewhere now.

Definitely worth taking a look at all three resources. I believe Will has said the marinehowto article is where a lot of his info originally came from.

 

[Alfalfameister]

What is your reason? expedience?

I just want to be there when they hit 3.6. Also, I figured it's very safe to leave it at 3.5+, and just look at it twice a day (by the way, no change at all today; several hours ago, about 17.8A was being pumped into it; maybe 6 hours later, it was still the same - I'll look tomorrow if, maybe, it's down to, say, 17.5 or whatever).

 

[zorlig]

I think people are playing with fire by sitting above 3.4v for so long. Whatever you do, your bms is going to rebalance around what you have it set to and your perfect 3.6v parallel charge will be lost. Unless your bms will only balance above 3.6 or you don't have one.

 

[HRTKD]

I think people are playing with fire by sitting above 3.4v for so long. Whatever you do, your bms is going to rebalance around what you have it set to and your perfect 3.6v parallel charge will be lost. Unless your bms will only balance above 3.6 or you don't have one.

The specification sheet for the EVE 3.2v 280 Ah cells calls for a charge/discharge cutoff of 3.65V/2.5V. Charging above 3.4v shouldn't be bad, it's normal.

 

[Ampster]

I think people are playing with fire by sitting above 3.4v for so long. Whatever you do, your bms is going to rebalance around what you have it set to and your perfect 3.6v parallel charge will be lost.

I assume we are talking about LFP. Literally or figuratively fire doesn't enter into the equation. Nothing in any manufacturer's data does it talk about risks above 3.4 volts.
No doubt the BMS could to that but with a large pack that could take months.
The real issue is that most of the time these cells come to us at various states of charge but the voltages are all very close. 10 to 40 Ahrs difference in SOC can make a big difference in how much of the pack is usable.

 

[zorlig]

Not fire, just cell damage. Floating up to 3.6 is not supported by the battery specs, which only allow that voltage when charging down to a specific rate. To do it according to spec would require following their standard charging instructions.

 

[Dzl]

I just want to be there when they hit 3.6. Also, I figured it's very safe to leave it at 3.5+, and just look at it twice a day (by the way, no change at all today; several hours ago, about 17.8A was being pumped into it; maybe 6 hours later, it was still the same - I'll look tomorrow if, maybe, it's down to, say, 17.5 or whatever).

That's a good reason, and one of the reasons I tend to like the step method as well (Its also one of the reasons Nordkyn and MarineHowTo give for the step method).

 

[Dzl]

I think people are playing with fire by sitting above 3.4v for so long. Whatever you do, your bms is going to rebalance around what you have it set to and your perfect 3.6v parallel charge will be lost. Unless your bms will only balance above 3.6 or you don't have one.

The specification sheet for the EVE 3.2v 280 Ah cells calls for a charge/discharge cutoff of 3.65V/2.5V. Charging above 3.4v shouldn't be bad, it's normal.

Not fire, just cell damage. Floating up to 3.6 is not supported by the battery specs, which only allow that voltage when charging down to a specific rate. To do it according to spec would require following their standard charging instructions.

Zorlig is right about the standard charge profile, see my earlier comment re: EVE standard charge model and low current power supplies:

EVE defines end of charge as when current drops to 0.05C @ 3.65V. With 4 x 280 Ah cells, the max a 10A benchtop supply can supply is 0.009C and 16 cells in parallel the max rate for a 10A benchtop supply is 0.002C, Even with just a single cell, the max a 10A supply could deliver is 0.036C. Point being, the common chargers everyone is buying can't even supply the current that EVE considers 'end of charge' cutoff point (orders of magnitude less if parallel balancing). So at a minimum, cells will be held near 3.65v for longer than they should, while the power supply trickle charges the large cells. Stepped balancing, is one way to minimize the time spent near 3.65v, if you make sure to be present and attentive during that last step.

I don't know how harmful or benign this is (worth keeping in mind we are talking about a single cycle here), but I understand the logic of minimizing the time the cells are being charged at or near 3.65v. And I do agree that trickle charging for hours or days at or near 3.65 is not within EVE's specs.