[Tyn245GL]

Four weeks ago I have built one of my sub-packs of 10 CALB 130Ah cells, connected them in series, and installed the Lithiumate BMS boards on them. Each cell measured between 3.26 and 3.27 V at that time.
Today I checked the voltage, and 9 cells are still at 3.27 V, however 1 cell is now at 3.20 V! What happened? The cells are new, never been charged, never been discharged...
Can I just leave this pack as is, and will the initial charge of the fully assembled pack pull the lowest cell of this sub pack back up again to an equal level with the other cells? If not, what should I do?

 

[rwaudio]

Four weeks ago I have built one of my sub-packs of 10 CALB 130Ah cells, connected them in series, and installed the Lithiumate BMS boards on them. Each cell measured between 3.26 and 3.27 V at that time.
Today I checked the voltage, and 9 cells are still at 3.27 V, however 1 cell is now at 3.20 V! What happened? The cells are new, never been charged, never been discharged...
Can I just leave this pack as is, and will the initial charge of the fully assembled pack pull the lowest cell of this sub pack back up again to an equal level with the other cells? If not, what should I do?

It's often recommended to put the cells in parallel to initially charge/balance them. I did this with some headway cells, then assembled them into a series/parallel pack, left them for a month or so and came back, there was a 0.05v difference between a few of the groups, however after putting a light load on them (5amps) they instantly became the same voltage under load, I left the load there for a couple hours and every group stayed the same. I wouldn't worry about the difference, but it's always a good idea to get them to the same level before use, either top/bottom balancing depending on the method you believe in.

The other thing you could check is swap the BMS from the 3.2v cell to a different one perhaps it's drawing more current for some reason?

 

[EVfun]

You need to charge some cells, certainly the one that reads 3.20. I don't know how much is left in it but if my TS cells are resting at 3.26 volts they are near completely dead, based on the number of amp hour I have to put back in. You shouldn't have the BMS connected until you are set up to charge. The BMS system will have at least a small parasitic load on the cells (in my case about 3 milliamps) and you need to be ready to recharge anytime it is attached to them.

 

[Tyn245GL]

You need to charge some cells, certainly the one that reads 3.20. I don't know how much is left in it but if my TS cells are resting at 3.26 volts they are near completely dead, based on the number of amp hour I have to put back in. .

My cells are CALB cells and when they shipped from the factory (25 January) they were at 3.30 V. Is 3.27 V really that bad? According to the factory they should be capable of 12 months storage without charging.

 

[EVfun]

As soon as you install the BMS boards they are no longer in storage, they are in use. The BMS board is placing a small, continuous, load on the cells.

I have 2 cells that have not been installed and have not had BMS modules on them since new. The cells where made in February 2010 and where 3.30 volts when received. The cells are currently sitting at 3.29 volts each. My cells in the car with the BMS are being drawn down by about 2 amp hours per month just from the BMS.

My preference would be to recharge the block and then move the cell BMS modules. This is to see if you have one BMS module that draws more than the others. You should be able to add some amp hours to blocks of 4 with at a time with a cheap 12 volt charger. The Lithium cells tend to be really flat when charging until near full. So you can charge and sit there watching them. Stop charging when the first cell hits 3.45 volts and then check the resting voltage the next day.

 

[Tyn245GL]

You should be able to add some amp hours to blocks of 4 with at a time with a cheap 12 volt charger. The Lithium cells tend to be really flat when charging until near full. So you can charge and sit there watching them. Stop charging when the first cell hits 3.45 volts and then check the resting voltage the next day.

Just a regular automotive accessory battery charger? I have one that charges at 3 Amps, it is for 80 Ah regular lead acid battery.

 

[EVfun]

A dumb one is better, like an old 10 amp charger that is usually a rather heavy little metal box with a timer knob. Also, 3 amps is very slow and I recommend you remain present to watch the voltage.

What I would do is grab a cheap 10 amp charger and drop 2 hour on the cells (or until the first one hits 3.45 volts), then see what they read the next morning. It would be great if you could add the same amount to all your cells so they stay pretty closely matched in state of charge. Moving that one regulator to a different cell to see if it draws more than the rest would also be a good idea.

I haven't played around with a resting voltage below 3.26 volts, but I don't think there is a lot of difference in SOC between 3.26 and 3.20. The reason I say this is because my cells seem to sit unchanging at 3.28 to 3.32 volts (these are resting voltages.) From 3.33 up and from 3.27 down they seem to move much more quickly. I think at least 60% of the total charge is between 3.28 and 3.33 volts, perhaps as much as 80%.

I have found that Lithium cells are real amp suckers. Like a good AGM battery the cell voltage barely moves as you charge, until the charge is nearly complete. Then it starts shooting up quickly. The effect is even more pronounced with my Lithium pack than my AGM Optima pack in the Buggy.

 

[Tyn245GL]

Re: New pack, in trouble?

Hi all, many thanks for your helpful responses. Today I have disconnected the 3.20 V cell from the block's series string and disconnected the BMS cell board which was on this cell. As I don't have a 12 V charger (yet) I decided to do the following: I used the LiFePo4 single cell charger that I have (the one offered on this Forum by a Chinese vendor) and charged the troubled cell during several hours. Immediately at the start of the charge the cell's voltage climbed to 3.24 Volts and after several hours it was at 3.285 Volts while charging. After disconnecting the charger the cell rested at 3.27 Volts, just like all other cells. Tomorrow I will check the resting voltage again and if necessary I will charge it a bit more and then put it back into the series string.
I also checked some 16 other cells which I had in direct reach, of which 4 are right out of the box and never had a BMS board on them. All were still resting at 3.27 Volts (except one which had a BMS board on it during the past 4 weeks). My cells have been on this 3.27 Volts resting voltage since many months, so I assume this is a normal level for out-of-factory CALB cells which are stored since 9 months after production date. Of course I realise that I am nearing 20% SOC soon and that I must charge my total pack ASAP.

Please let me know if you have any more suggestions, your feedback already has been very helpful!

Oh one more question: as suggested by EVfun I will disconnect all remaining 9 BMS cell boards tomorrow, and will only put them back on just before I am ready to charge the total pack. However, these cells are in a hard to reach location, so I am thinking of just disconnecting (and isolating) the red*+ wire of each BMS cell board, and leaving the negative side of each board bolted onto its respective cell terminal. Will that be OK? Will cause no problems?

 

[EVfun]

Re: New pack, in trouble?

It sounds like you have a good plan. I don't have a single cell charger, but that works great too. Your plan to disconnect and isolate the + BMS board terminals is a good one, just remember the isolate part.

 

[Tyn245GL]

Re: New pack, installed in vehicle!

Today the cell measured 3.25 resting voltage so I charged a couple of hours more. It is now resting at 3.28. Will check again tomorrow.
All BMS boards are disconnected for now.
Today we lifted the three big front packs (10, 10, and 12) into their battery boxes, their final location in the Volvo's engine/motor bay. Heavy stuff to lift but the result looks great!! Will post photo's soon.

 

[Tyn245GL]

Cell is fine again!

The cell that was dropped by the too many milliamps sucking BMS board is now back to 3.28 resting voltage again! Same level as the other cells so it's OK again.
To be sure, I will replace the BMS board.

 

[Jan]

Re: Cell is fine again!

Hi Tyn,

And what are you going to do with your BMS, now? Still planning on using it?

 

[Tyn245GL]

Bms

Yes I will use the BMS. It's the Elithion Lithiumate system.
Will try to bottom balance manually before I do my first full pack charge, which will be in some 4-5 weeks from now.

 

[Elithion]

Four weeks ago I have built one of my sub-packs of 10 CALB 130Ah cells, connected them in series, and installed the Lithiumate BMS boards on them. Each cell measured between 3.26 and 3.27 V at that time.
Today I checked the voltage, and 9 cells are still at 3.27 V, however 1 cell is now at 3.20 V! What happened?

Lithiumate cell boards drain about 2 uA in standby.

Could it be that that particular cell board had been damaged (such as having been connected backwards just once)? A damaged board will drain between 1 and 100 mA, depending on how badly damaged. Please see the cell board testing guide to see if the board is draining more than 2 uA.

Sorry that it took me 3 weeks to discover your post in this forum. If you would rather get a very prompt response, please use the free Lithiumate tech support forum instead.

Davide

 

[Elithion]

Re: Bms

Yes I will use the BMS. It's the Elithion Lithiumate system.
Will try to bottom balance manually before I do my first full pack charge.

What? That's doesn't make any sense! The Lithiumate BMS (as well as every professional BMS) does top balancing. If you do a manual bottom balance first, the BMS will have to work twice as hard to undo your mistake.
By all means, yes, do balance the cells manually before you build your pack, but do either a mid balance (no supply) or a top balance (with a 4.2 V supply).

For an explanation of the issue, please see section 3.2.3.1.6 of the Li-Ion BMS book. Google books has that page for your free preview.

Davide

 

[JohnM]

Hi Tyn245GL,

May we know what is the condition now of that one cell that have a lower voltage? Did it come back to the right voltage?
Thanks!

JohnM

 

[DavidDymaxion]

Re: Bms

I have an RC charger with taps. The manual doesn't say anything about how it balances. Do they typically top balance, or do they try to equalize voltage throughout the charging? I guess that would be called middle balancing. Thanks.

What? That's doesn't make any sense! The Lithiumate BMS (as well as every professional BMS) does top balancing. If you do a manual bottom balance first, the BMS will have to work twice as hard to undo your mistake.
By all means, yes, do balance the cells manually before you build your pack, but do either a mid balance (no supply) or a top balance (with a 4.2 V supply).

For an explanation of the issue, please see section 3.2.3.1.6 of the Li-Ion BMS book. Google books has that page for your free preview.

Davide

 

[Elithion]

Re: Bms

I have an RC charger with taps. .... Do they typically top balance?

Yes: top balancing is the only way that a multi-tap charger can do balancing.

Reference: Section 5.6 of the Li-Ion BMS book: Distributed charging.

Davide

 

[DavidDymaxion]

Re: Bms

I agree that top balancing is likely the algorithm, but theoretically the hardware could do bottom balancing -- my RC charger has a discharge mode. It could also pull up low batteries at the start of charging. I have a really low battery, I should test it.

Yes: top balancing is the only way that a multi-tap charger can do balancing. ...

 

[Tyn245GL]

Re: Bms

What? That's doesn't make any sense! The Lithiumate BMS (as well as every professional BMS) does top balancing. If you do a manual bottom balance first, the BMS will have to work twice as hard to undo your mistake.
By all means, yes, do balance the cells manually before you build your pack, but do either a mid balance (no supply) or a top balance (with a 4.2 V supply).

For an explanation of the issue, please see section 3.2.3.1.6 of the Li-Ion BMS book. Google books has that page for your free preview.

Davide

Thanks Davide for your feedback. I will follow your advice. What exactly do you mean with mid balance (no supply)? Is that: Let them, the whole pack connected in series, without BMS boards connected yet, rest for some time, without a charger, and just give them some time (days/weeks?) to equalize? Or do that, but then all in parallel (like many people recently are advising)?