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noob top balancing - over charged my cells to 4.2v

gordonshumway

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Sep 13, 2021
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Hey everyone, I goofed and over-charged my LifePO4 cells to 4.2 volts while top balancing and am wondering if I ruined them or if my mistake is salvageable.

I was top balancing my 280ah cells as Will Prowse says to do on his 'how to top balance' YouTube video, but I must have messed something up along the way because when I checked on progress this morning my voltage on my charger said 4.5 and the cells were reading 4.2 volts! Last night before bed the charger was set to 3.6 and was pulling about 10.5 amps, cells were at ~3.35v. About 10 hours later they were at 4.2. Did I ruin the cells? Is there anything I can / should do now that they're sitting there at 4.2 volts? (obviously I stopped charging them when I found them, and they've just been sitting on my workbench since).

Charging was taking a loooong time, so at one point this weekend I created new leads using 10ga wire. The charger settings must have gotten out of whack when I turned it back on with the new leads connected to resume charging, but I don't think I touched the voltage dial. I don't quite understand how I managed to shoot past the 3.6v the charger was set to, but regardless here we are ?.

These are going into a camper and I'm really not wanting to sleep next to a ticking time bomb if I've compromised the cells. Thanks for any help / advice!
 
If you were using a power supply you have the set the voltage and amps without a load and then connect it to the batteries and wait many hours until they come close to your set voltage input. I would put a small load on them to bring them down to your desired voltage.
 
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Another case of lack of patiences and/or touching supply voltage limit while in current limit mode connected to batteries.

If cells did not get above 4.3v and do not show signs of bloating you will be okay.

I recommend calculating approximate time to top out based on total AH's needed and power supply current. It sets your expectation of time.
Also good to disconnect power supply leads periodically to verify it is still set at desired voltage limit.
 
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Put a discharge on them now. Taking them that high involves releasing inorganic materials in a secondary reaction, clogging the SEI layer and reducing capacity you'll never get back.

Good lesson though - if you change your charge connections mid-stream, then a reset / start-over cal of your variable supply voltage is in order.
 
I'm starting to think we need to implement some sort of HVD when doing parallel top balancing.
 
Put a discharge on them now. Taking them that high involves releasing inorganic materials in a secondary reaction, clogging the SEI layer and reducing capacity you'll never get back.

Good lesson though - if you change your charge connections mid-stream, then a reset / start-over cal of your variable supply voltage is in order.

Doing that now. They dropped to 4.02 during the day, and tonight I wired them in series (with BMS) and hooked up a small inverter and my 12v fridge to it - it's down to 13.2v now and dropping. I'm planning on letting the battery draws drain the battery until the BMS turns it off, but then what? Let them rest for a while (days?) and then try top balancing again?
 
As has been said many times - parallel top balancing is a bad idea full stop.

Thankfully even on this forum where parallel top balancing has a weird cult following - a lot of people are realising this.
 
You don't need to discharge them all the way. Just enough to get them away from the overcharged top.

Even though they have fallen in *rested* voltage naturally, doesn't mean they don't need to have a discharge to stop the plating that is happening.

But yeah, no need to discharge all the way. Just a bit to restore to a normal state.
 
As has been said many times - parallel top balancing is a bad idea full stop.

Thankfully even on this forum where parallel top balancing has a weird cult following - a lot of people are realising this.
I think the reason top balancing is sometimes bad is precisely what happened here. But putting a set of batteries in parallel to charge for a while would balance their internal resistance forces. Im not sure why people like to balance all the way to the top. It can be dangerous if you accidently touch your voltage dial on a power supply.
 
Well if nothing else I've learned what not to do! I can't believe I did that... after waiting like 2 months for the cells to show up, and the first thing I do is potentially ruin them ?

Like I said last night, I put the cells in series w/ BMS and took the pack down to 13.24v, applying moderate compression. The swelling has reduced a lot and they're currently just sitting under compression and resting. Should I let them sit for a while before I do anything else, or can I get right to it?

Assume I need to top balance again (this time the correct way - set voltage first, hook up pack, don't touch anything!). After that, assemble as usual and carry on? Should I test the cells for damage first?

Thanks everyone!
 
Should I let them sit for a while before I do anything else, or can I get right to it?
You are a little bit on the bleeding edge. Most folks either bloat their cells to where they cannot be recovered and recompressed back near normal. And the ones who have recovered, i believe they were able to continue as is and suffered insignificant consequences.
I think you're good to go but do keep an eye on things until you are confident they are fine.
 
Assume I need to top balance again (this time the correct way - set voltage first, hook up pack, don't touch anything!). After that, assemble as usual and carry on? Should I test the cells for damage first?
How much did your cells swell? Some expansion is normal when a cell is fully charged.

Since you have the cells connected in series I would test them before doing anything else. Run the cells down until the BMS's low voltage disconnect kicks in and then charge until the high voltage disconnect kicks in. This will test your BMS to make sure it's working properly and you will be testing the capacity of the cells.

If you are using your BMS to test the capacity the cells will need to be fully cycled at least once before the BMS will report the correct capacity. Keep an eye on the cells delta at both the low end and the top end. If it's not too bad at the top then there will be no need to do another top balance.

After the cells have been tested, keeping the cells between the knees will prolong their life.
 
As has been said many times - parallel top balancing is a bad idea full stop.

Thankfully even on this forum where parallel top balancing has a weird cult following - a lot of people are realising this.
Well, that is an interesting position. I guess I'm part of the "cult" as I think it is the right thing to do for Solar, where the cells will spend more time near the top than the bottom.

Since you have such a well informed position, would you care to share your logic?
 
Since you have such a well informed position, would you care to share your logic?

Sure, as i have done many times before.

Top balancing is best done one cell at a time, using a high voltage disconnect on that cell and charging to 3.65V, stopping at 0.05C tail current.

When all cells are charged this way, connect cells in series to form pack, and use an appropriate BMS.

It’s never a good idea to connect a charge source to a cell without a HVD.

If you have no idea of the SOC of your cells before you parallel them, you will be causing damage to the higher charged cells if you hold them for an extended period at full charge while top balancing.
 
Ok. I'm sorry I haven't seen your many posts along this line. I don't want to make you repeat any of that, but let's talk about it some.
Top balancing is best done one cell at a time, using a high voltage disconnect on that cell and charging to 3.65V, stopping at 0.05C tail current.
The implication is that you don't trust your charging source to be at 3.65V. I guess that's OK, but...
It’s never a good idea to connect a charge source to a cell without a HVD.
If you can put a HVD on a single cell, why can't you put it on a group of cells wired in parallel? You know that they are all at the same voltage, right?
If you have no idea of the SOC of your cells before you parallel them, you will be causing damage to the higher charged cells if you hold them for an extended period at full charge while top balancing.
This requires a bit of discussion. Maybe you have some sources you can point to.
  1. What I have read is that it is bad in repeated cycles to charge up LiFePO4 cells to a full 3.65V and hold them there.
  2. Even if the cells are way out of balance when you hook them up in parallel, by the time they get to 3.65V there is not likely to be a big enough difference that the current doesn't drop precipitously down to .05C (which by the way, for 4 280Ah cells in parallel is 56A, or if you want .05C on one cell, that's still 14A). And remember, if you are using 3.65V, none of the cells are going over 3.65V. Especially if you want to put an HVD on it.
How short of a time do you think can cause damage to cells holding them at 3.65V?

I have not seen any literature saying this, or anecdotal reports of people seeing failures or reduced performance due to parallel top-balancing. If you have, I'll gladly accept your point.

When I've top balanced, I've watched the voltage of the parallel cells climb very slowly until the knee in the curve, then it rises quickly. Even with my measly 30A charge, once it got to 3.65V the current dropped so fast that I think it was down to 5A within a few minutes. Do you really think this causes harm to the cells?
 
Sure, as i have done many times before.

Top balancing is best done one cell at a time, using a high voltage disconnect on that cell and charging to 3.65V, stopping at 0.05C tail current.

When all cells are charged this way, connect cells in series to form pack, and use an appropriate BMS.

It’s never a good idea to connect a charge source to a cell without a HVD.

If you have no idea of the SOC of your cells before you parallel them, you will be causing damage to the higher charged cells if you hold them for an extended period at full charge while top balancing.
Just curious what you are using for HVD? Anything you would recommend?

I suspect there are more people accidentally overcharging their cells while top balancing than we know .... it is just too easy for that to happen.
I agree that adding a HVD .... that has also been tested .... would be a good recommendation.

With all threads already in play about top balancing .... it may be hard to get the word out. Maybe we can talk Will into doing a video utilizing one.
 
I use my batrium BMS.

I set up the pack in its final configuration, with the Batrium set to stop charging when a cell reaches 3.65V.

Then i attach the single cell charger to each cell in turn via the charge cut relay until each cell cuts off at 3.65V.

That way you always have 2 protection circuits.

You could series charge first until one cell is full, but you are then relying solely on the BMS.
 
which by the way, for 4 280Ah cells in parallel is 56A, or if you want .05C on one cell, that's still 14A

There is your key. Any time you are in the upper knee voltage at low current you are damaging the cell.

There is a difference in the chemistry at low and high currents.

I gather you are looking for a reputable manufacturer to publish a paper saying that for every minute at 3.65V and less than 0.05C tail current you will reduce the lifespan of your cell by x%.

I haven’t seen that information online.
 
I gather you are looking for a reputable manufacturer to publish a paper saying that for every minute at 3.65V and less than 0.05C tail current you will reduce the lifespan of your cell by x%.

I haven’t seen that information online.
Ok, do you have something from an disreputable manufacturer? ;)

Seriously though, if you are rushing into a gymnasium full of people top balancing their cells and telling them the sky will fall, you must have something you can share with us. I am not questioning your knowledge, but it does seem to go against my own experience and what the majority of people I respect here have said. I only want to understand better.

There is your key. Any time you are in the upper knee voltage at low current you are damaging the cell.
(bolding is mine) So a few seconds? A few minutes? It doesn't matter, it should just be avoided?

Unfortunately, there have been a number of people posting here that have accidentally charged their cells to over 4V. Time will tell what the long term effects are, but it seems like they are always talked off the ledge by experienced folks who tell them to calm down, discharge some from the cell(s), and go back to proper management of the cells. Their follow-up posts seem to indicate their cells recovered and performed fine.

I'm not advocating charging anything to 4V. On the contrary, I think it is important to respect the published max charge voltage for these cells. The I say that after doing a top balance, avoid the tops and bottoms altogether: Charge a pack to 3.5V per cell, and discharge to 3.0V per cell.

I think 3.65V is a defined limit for a reason. But that reason is not that it must be avoided at all costs. It is simply the 100% max charge value.
 
All i can offer is a longer term perspective than most, having commissioned my first LiFePO4 pack in 2012. The LiFePo4 world was smaller then, and even the manufacturers were rapidly revising specs as their cells failed under certain conditions.

I’m not saying the sky is falling. I am sharing my experiences in the operating conditions of cells that have lasted over a decade.

The problem with LiFePO4 is that the lithiaton reaction changes with current, so accelerated testing isn’t possible.

I know that sustained low current charging in the upper knee causes cell degradation - it is well published. As there is no reason to go there - i think it’s a bad idea.

When someone publishes a conclusive document showing the damage is negligible - i will gladly revise my opinion.

As it is now - parallel top balancing at low end current with no HVD poses an unnecessary risk. There are just so many ways to top balance i can’t see a reason for it.
 
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