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Hi! Newbie on the forum, but driving my electric bug since 2013. My battery setup is 78x prismatic GBS 3.2v nominal 100ah. No BMS, all cells have a top balancing chip that prevents them from overcharging (as long as Amperage is not too high during CV charge). I'm monitoring the charge and no cell goes any higher than 3.7x during charging (that's when the chip kicks in). I'm using the Elcon PFC2500 that charges with ~9 Amps in the beginning and much less at the end of the charge curve.

Now sometimes (but not all the times) there is this sweet smell of electrolyte when I step in the car right after charging? What can this be? No cells are being overcharged (I've even set the charger to a lower voltage, so it stops earlier) and no cells are too low or too high.

What I was thinking of:

1. A certain cell is damaged and off gasses when charging
2. I have to let the pack rest a bit after a drive (50% SOC), before charging them again (does this make sense?)

If a certain cell is damaged: is there a way to define which cell(s) are offgasing. I don't hear or see anything (only smell).

Any help is most appreciated :)
 

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there is this sweet smell of electrolyte
The "sickly sweet" smell is lithium electrolyte or something mixed with the released vapors as you've discovered. It means one of your cells is leaking.

It's certainly damaged.

no cell goes any higher than 3.7x during charging
3.7v I presume?

Are these LiFePO4s or regular Lithiums?

A 3.7v lithium is only about 50% charged.

If a certain cell is damaged: is there a way to define which cell(s) are offgasing. I don't hear or see anything (only smell).
There sure is.

Sniff them.

Seriously. Just go from one to the next and give them a sniff, you'll find it pretty quick. Helps if they're warm from charging.
 

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Discussion Starter #3
Thanks MattsAwesomeStuff! They are LifePo4, max 3.65v. I think the smell is coming from the pack behind the back seat. It’s small, a VW bug, so basically the whole car smells. I hope to determine which cell(s) is/are off gassing by just using my nose. I’ll give it a go.

How likely would it be that the damaged cell has a different/lower Voltage after a drive? Are there any other characteristics that I can take into account?
 

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Thanks MattsAwesomeStuff! They are LifePo4, max 3.65v.
Oh, okay, that's a normal max voltage then.

I think the smell is coming from the pack behind the back seat. It’s small, a VW bug, so basically the whole car smells. I hope to determine which cell(s) is/are off gassing by just using my nose. I’ll give it a go.
Air it out, then get close. 95% chance you can tell which it is on your first pass through them with your sniffer.

Also, don't go huffing it more than you need to. It's not fatal but I doubt it's any good for you. Weirdo chemical esters and all.

How likely would it be that the damaged cell has a different/lower Voltage after a drive? Are there any other characteristics that I can take into account?
Well, it's not going to outperform the other batteries, so, yeah, you might be able to narrow it down by voltage testing, but, your nose is going to be a lot quicker.

Chances are it will have dripped some electrolyte, clearish or crusty white eye-goobers.
 

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The pressure in some cell(s) has caused the vent valve to release electrolyte vapors. The pressure goes up due to the temperature increase during overcharging.

The 100% SOC open circuit (resting) voltage of LiFePO cells is about 3.33 volts. In my opinion 3.7 is too high to be trying to hold the pack during charging, so is 3.65 and likely 3.60. Without a BMS you don't know the individual cell voltages, so a weaker cell will be overcharging even more than the rest of the pack.

If you have a voltmeter and can measure each cell voltage then you will find the culprit(s) as they will be lower voltage than the good ones.

How well do you know that the "chip" is calibrated for voltage? and what is the tolerance range for measuring voltage?

The Elcon chargers have been know to hold the cells too high for too long, and cause venting such as you have.

Have you ever run the car to empty or lower and continued to try to drive? If a cell gets pulled too low during discharge then it can puncture the internal separator during the subsequent re-charge, resulting in a damaged cell.
 

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Discussion Starter #6
The pressure in some cell(s) has caused the vent valve to release electrolyte vapors. The pressure goes up due to the temperature increase during overcharging.
Yes, that's what I thought, however the smell is there when charging and pack is nowhere near full: NO cells are higher than 3.4v. That's what's odd about this...

The 100% SOC open circuit (resting) voltage of LiFePO cells is about 3.33 volts. In my opinion 3.7 is too high to be trying to hold the pack during charging, so is 3.65 and likely 3.60. Without a BMS you don't know the individual cell voltages, so a weaker cell will be overcharging even more than the rest of the pack.
I have been monitoring the charging process with a volt meter. Plus: every cell has a chip (this one) that prevents the cells from being overcharged. I found it strange however that the red LED on this chip (the one that says "balancing") lights up only around 3.75V, so these chips may not be too reliable. There are 4 cells in my pack of 78 that light up fastest, so these are the cells that are suspected.

If you have a voltmeter and can measure each cell voltage then you will find the culprit(s) as they will be lower voltage than the good ones.
Yes, good idea! Will check that out later this week (together with doing some sniffing :))

How well do you know that the "chip" is calibrated for voltage? and what is the tolerance range for measuring voltage?
Good question. These chips have been shipped with the batteries when I bought the pack, but also online it says that they shouldn't let the cells being charged higher than 3.65V, so they may be faulty (or some of them).

The Elcon chargers have been know to hold the cells too high for too long, and cause venting such as you have.
I don't think this is the problem. Is see the Amperage of the charging process going down towards the end of the charging process and the charging stops when the pack has reached the voltage that I have setup. But will keep an eye on this. However: the smell is there way before overcharing...

Have you ever run the car to empty or lower and continued to try to drive? If a cell gets pulled too low during discharge then it can puncture the internal separator during the subsequent re-charge, resulting in a damaged cell.
May have happened once and/or I have had some cells replaced by the provider but these new cells were (ab)used. I suspect some of them may be the culprits.

Thanks for all your input though, super! I have something to work on the upcoming days.
 

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as others have said, it is likely you have one or more cells damaged and 'venting'. usually occurs if the pack was run into the ground, or if your top balancing failed at end of charge and one or more cells exceeded 4.0v too many times.

best way to zero in on which is to catch the pack at end of a charge cycle as the pack voltage starts rising close to 'end of charge' voltage.

- first calculate your target 'end of charge' voltage per cell
(i.e. a 120v nominal pack (38x cells) might have the charger pre-set to
terminate charge at 3.65vpc = 138.7 volts for LiFePO4 cells.

- measure each cell as quick as you can toward the end of charge, and look for a cells more than .05 volts off, indicating that your balance is off.

- you CAN bypass the damaged cell, rebalance the pack, and consider lowering your charger end-of-charge voltage

...the pack will work, but is likely damaged and will have reduced capacity as well as more 'sag' under load.
 

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Discussion Starter #8
I found 4 cells to be having a different Voltage so I created some bridges to remove them from the pack. Now the smell is much less, however I found indeed that my Elcon charger (at least one of them, haven't tested the other) never stops charging (as kennybobby mentioned). Although it was putting in only 0.4 Amps for the whole pack (still 74 cells) I found the charger still ON the next morning with all balancing chips in balancing-mode. Pack should have been full around midnight :eek:

So if (a) a cell is not healthy anymore, or (b) a balancing chip is broken or can't pull away enough Amps, this causes cells to be overcharged (and venting). This may have happened before :(

Will be checking the other charger later this month and keep a close eye on everything (like: set a timer when charging should be finished). The next Electric Bug that we're building will definitely have a BMS and BMS-managed charger!

Thanks everyone sofar for all the input.
 

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There is very, very little additional capacity put into LiFeP04 cells beyond 3.5 Volts per cell. Be nice to your cells, increase the number of charging cycles overall and terminate at 3.5VPC.
 

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I found 4 cells to be having a different Voltage so I created some bridges to remove them from the pack.
Hi klikhier,

If you have removed cells from the pack the charger output voltage needs to be adjusted to match the new pack (total) voltage @ 3.65V per cell.
The is why the charger doesn't stop. it's still trying to reach the original pack voltage.

Eric
 

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Hi, For some reason, today I have received an update to say there is a new message on this thread.
I cannot find one, but I did research the "Balancing" modules that you are using. Note, that these are only balancing modules and will not prevent overcharging. To prevent overcharging you need a proper BMS system which shuts the charger off when the first cell reaches 3.65V(for LiFePO4).
 
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