Cell voltages during slow charge

[rio]

Hi everyone,

I am building a 16s system with CALB 100Ah cells and an Overkill 16s BMS.

The new cells arrived at about 40% SoC (according to the seller). Before top-balancing, I decided to first slowly charge them up in series (at 3A) until one cell reaches 3.55V, and then I will put them in parallel (also at 3A) to top-balance. (I'm limited to 3A because that's the limit in my benchtop CV/CC power supply - slow, I know, but that is what I have available at the moment).

Right now I'm in the series-charge process. My power supply is set to 56.8V, and it has been going steady for about 20 hours (not continuous, but maybe over 4-5 sessions). The voltage per cell started at about 3.23V and now it's just about hitting 3.38V. The Ah estimate of the Overkill app thinks I've already reached past 100Ah (which makes sense based on the time*current, though I'm sure there is no way for it to accurately have known the initial SoC).

Should I be concerned that my cell voltages are still so low? The pack is still accepting the full 3A, so charge must be building up still. Based on the charge voltage curve of LiFePO4, I am expecting a sharp rise after 99% SoC or so; I just thought I would have seen a faster rise by now. In fact, over the past 30 min or so it seems that some cell voltages have actually gone slightly down, not up.

Thanks!

 

[Scph9002]

Maybe the cells came with less than 40% soc. Just put your bench supply on 58.4v go to bed and let your bms cut the charging when a cell goes high voltage.

 

[Morgan360]

I had my 16 cells in parallel and charged them with my bench power supply also. They arrived at 3.26 volts and took roughly 2 weeks to get up to 3.6 each. The current started around 11A and slowly declined over the last few days. Once they got to 3.4V, it was about a day until they were finished.

My power supply also has a meter. Supposedly it put about 12KWh into the cells over that time. Not sure how much I trust it though. They are new 277AH GFB cells.

 

[Bud Martin]

OP, How many hours did you charge the cell?
If it has I.E. 50% full, then 50A/3A = about 17 hours to full charge.

 

[rio]

Thanks for the responses. You guys were right, it was just a matter of patience. It finally took a turn after about 5 more hours and voltages started to rise much faster. So total charging time was close to 25 hours, which means their original SoC was closer to 25% than 40%. One cell hit overvoltage about 150mV ahead of the other ones, so pack is clearly out of balance. Now they are all in parallel sipping on 3A which I suspect might take another couple days.

 

[rio]

Here's another question (or 2): Once the current starts dropping when top balancing in parallel, does the current drop to zero at any specific time rate? Mine left the 3A limit about an hour ago and is now at 2.5A. Will the current eventually plummet as the cell voltage rises sharply?

If it's still not at zero by bed time, would you guys recommend leaving it hooked up overnight, or is it better to shut it off and resume in the morning while it can be supervised?

 

[fafrd]

Here's another question (or 2): Once the current starts dropping when top balancing in parallel, does the current drop to zero at any specific time rate? Mine left the 3A limit about an hour ago and is now at 2.5A. Will the current eventually plummet as the cell voltage rises sharply?

If it's still not at zero by bed time, would you guys recommend leaving it hooked up overnight, or is it better to shut it off and resume in the morning while it can be supervised?

Eve’s 280Ah cells have a standard charge current cut-off of 0.05C (14A)

Can’t say what Calb specifies, but for 100A cells, an 0.05C cutoff would mean you should charge until current drops under 5A (0.05C)…

 

[Bud Martin]

The problem is that OP only has 3A charger, so I do not know if OP just have to wait until the current drops down close to zero will work or not.

 

[rio]

It's down to 1.6A and still slowly dropping. From what I've read, I should let it get down to zero or near zero from them to be considered top-balanced. Cell voltage right now reads 3.45V (and still slowly rising). Just not sure if it's a good idea to leave them unattended overnight. The writeups I've seen don't recommend leaving the cells unattended during the last portion of the top-balance, but it's not clear to me why.

 

[TomC4306]

Whatever you do, do not go to bed with them hooked up to the power supply

 

[TomC4306]

It's down to 1.6A and still slowly dropping. From what I've read, I should let it get down to zero or near zero from them to be considered top-balanced. Cell voltage right now reads 3.45V (and still slowly rising). Just not sure if it's a good idea to leave them unattended overnight. The writeups I've seen don't recommend leaving the cells unattended during the last portion of the top-balance, but it's not clear to me why.

Because from 345 to 365 might only take an hour

 

[rio]

Whatever you do, do not go to bed with them hooked up to the power supply

Thanks, I won't.

Because from 345 to 365 might only take an hour

Yes, I get that. But what happens if this occurs while I'm not looking? What is the risk?

 

[TomC4306]

Thanks, I won't.


Yes, I get that. But what happens if this occurs while I'm not looking? What is the risk?

For me, I don't trust the cheap power supply. I have too much money in those cells. And when your top balancing they're hooked up with no monitor. ie bms

 

[fafrd]

It's down to 1.6A and still slowly dropping. From what I've read, I should let it get down to zero or near zero from them to be considered top-balanced. Cell voltage right now reads 3.45V (and still slowly rising). Just not sure if it's a good idea to leave them unattended overnight. The writeups I've seen don't recommend leaving the cells unattended during the last portion of the top-balance, but it's not clear to me why.

Wait a minute - I thought you were charging at 3.65V. If you’ve got a 200mV drop between charger and battery at 1.6A, that a 125mOhm resistance in the connection - WTF???

If your charger was limited to 3.0A, waiting until current drops to 10% of that level (300mA) is probably good enough.

But what you really want to see is cells held at 3.65V and current dropping to that level.

There is something wrong with the connection between your charger and the battery…

 

[rio]

Wait a minute - I thought you were charging at 3.65V. If you’ve got a 200mV drop between charger and battery at 1.6A, that a 125mOhm resistance in the connection - WTF???

If your charger was limited to 3.0A, waiting until current drops to 10% of that level (300mA) is probably good enough.

But what you really want to see is cells held at 3.65V and current dropping to that level.

There is something wrong with the connection between your charger and the battery…

Actually, I'm charging at 3.55V first. Going for a two-step approach. I'm measuring an 80mV drop between power supply and battery. But this drop is getting smaller as the current keeps dropping. I'm using a banana-to-alligator lead between PS and battery, which I know is not ideal but given that I was limited to 3A I was not too concerned about - it should just take a bit longer to top-balance, no?

 

[rio]

For me, I don't trust the cheap power supply. I have too much money in those cells. And when your top balancing they're hooked up with no monitor. ie bms

Gotcha, that makes sense. I'm hoping it will hit close to zero in the next hour otherwise I'll resume tomorrow.

 

[fafrd]

Actually, I'm charging at 3.55V first. Going for a two-step approach. I'm measuring an 80mV drop between power supply and battery. But this drop is getting smaller as the current keeps dropping. I'm using a banana-to-alligator lead between PS and battery, which I know is not ideal but given that I was limited to 3A I was not too concerned about - it should just take a bit longer to top-balance, no?

Settling in two steps makes little sense.

An 80mV @ 3A means 27mOhms - still pretty lousy. But it is what it is.

If you started charging at 3.0A @ 3.55V and you are now down to 1.5A and 3.45V on the cell, there is little to gain by watching that ‘tail’ down to 300mA @ 3.55V.

If you are too-balancing to 3.65V, I’d increase your PSU to that voltage, current will increase, possibly all the way back to 3A for a time, possibly to less than that, but in either case, waiting for the current to drop under 300mA at 3.65V would be a much quicker and more direct way to get where you are trying to go…

 

[rio]

Settling in two steps makes little sense.

An 80mV @ 3A means 27mOhms - still pretty lousy. But it is what it is.

If you started charging at 3.0A @ 3.55V and you are now down to 1.5A and 3.45V on the cell, there is little to gain by watching that ‘tail’ down to 300mA @ 3.55V.

If you are too-balancing to 3.65V, I’d increase your PSU to that voltage, current will increase, possibly all the way back to 3A for a time, possibly to less than that, but in either case, waiting for the current to drop under 300mA at 3.65V would be a much quicker and more direct way to get where you are trying to go…

Thanks, you make a good point. I was trying to play it safe but as long as I'm monitoring the process I see that I might as well get more current and get there faster.

 

[RCinFLA]

Going t

Here's another question (or 2): Once the current starts dropping when top balancing in parallel, does the current drop to zero at any specific time rate? Mine left the 3A limit about an hour ago and is now at 2.5A. Will the current eventually plummet as the cell voltage rises sharply?

If it's still not at zero by bed time, would you guys recommend leaving it hooked up overnight, or is it better to shut it off and resume in the morning while it can be supervised?

More likely you have voltage drop on the leads from battery to the power supply and the power supply has begun to enter constant voltage limiting. If cables are too small the voltage drop will allow power supply to start CV early which reduces charging current prematurely and greatly extends the top off time.

Many of the cheap power supplies do not have 'crisp' transition from CC into CV. This also backs off constant current early extending top off time.

Now would be a good time to disconnect power supply from batteries and verify its unloaded voltage limit is still set to 3.65v.

 

[Smokin]

The balance circuit on the bms will kick in at some point between 3.5 to 3.65. This may foil you observation. What I do is run them up to 3.65 x number of cells. Being charger,power supply cutoff. Stop charger and leave Battery sit and settle for 12 hours. Then measure each cell. They should all be the same. If some are low recharge these one by one to 3.65 and let them settle. You are good to go.