Another top balance, real quick question.

[Gould]

I’m finally getting around to top balancing using the method we all love and trust. I’ve set the power supply to 3.65V, connected the terminals on the cells in a parallel configuration and away we go.
Things look to be work fine but within an hour the voltage on the power supply has dropped to 3.60 with the amperage dropping about .30 over the same period. Is it normal for the voltage to drop as well? The cells have gained .01V in that timeframe.

 

[Diysolar123]

first, get a trusted volt meter and make sure the voltage is what you think it is.
next, is your power supply a CC/CV type supply?

what should happen is that the voltage will drop while the batteries take the max current you have set on your supply (this is the constant current phase). Then, as the batteries start to take less current, the voltage will rise to what you set (this is the constant voltage phase).

 

[Gould]

Okay, that makes sense.
Yes, this is a cc/cv power supply. Certainly not the highest quality - looks awfully similar to the one in Wills videos but this one has micro adjustments.

Process was, with the leads disconnected, set the voltage and max the current. Connect the leads to the cells and the amps being pulled registered at 2.5a. The amperage and voltage have been slowly dropping.
I have confirmed with a voltmeter that the voltage matches - I set the power supply to 3.62 thinking that’s leave me a bit of a buffer until I trust it.

 

[Ampster]

I don't understand why the voltage would drop when connected to a power supply set to 3.62. Usually voltage increases then current drops. If the voltage of the cells are 3.60 then you are done anyway. Then the cells will settle after you have disconnected them. They should settle at 3.35 but that might take a day or two.

 

[Gould]

Cells are at 3.27. I’m going to make some beefier terminals, the alligator style leads that came with the power supply are 24awg at best.

 

[Gould]

Wow, that’s a difference. With 10awg leads it’s now drawing 10amps and the voltage has dropped to 3.45.

 

[Ampster]

These discussions are much more informative with actual facts. Thanks for the update. It also illustrates the value of good connections and wire sized for the load. I am still not clear why the cells would go from a resting voltage of 3.27 to 3.45 when you apply 10 Amps of current? What size are these cells?

 

[Bob B]

Just don't adjust the power supply once it is connected to the cells .... the power supply voltage will come back up as the cells get charged.

 

[Gould]

Seeing how these cc/cv power supplies work now I can see how adjusting the voltage at this stage would shoot you above your original target.

Appreciate the help

 

[Gould]

I can see how folks get a little anxious and crank up the output. Plan is to only charge while my eyes are open. A quick relocation of a security campera and at least I can keep an eye on it remotely.

 

[Gould]

Thought I’d just bolt onto this thread with another question. I’m now into day 5 of my top balancing. I’m only charging while I’m at home and awake, about 12 hours per day. The cells arrived at a consistent 3.27V so it shouldn’t be much longer.

That said you can see in the picture that the voltage is at 3.49V but the voltmeter attached to the cells is reading 3.33. Just wanting to confirm this is normal.

 

[Gazoo]

Thought I’d just bolt onto this thread with another question. I’m now into day 5 of my top balancing. I’m only charging while I’m at home and awake, about 12 hours per day. The cells arrived at a consistent 3.27V so it shouldn’t be much longer.

That said you can see in the picture that the voltage is at 3.49V but the voltmeter attached to the cells is reading 3.33. Just wanting to confirm this is normal.

Yes it's normal. It's voltage drop and there is no way to avoid it.

Once the voltage of the power supply reaches 3.65 volts the power supply should go into CV mode and the current will begin to taper off as the cells voltages continue to increase to 3.65 volts. This step took about 2 hours when I parallel top balanced my 8 EVE cells. When all is said and done the cells voltages will reach 3.65 volts and the current displayed on the power supply will be zero.

 

[Gould]

Perfect, thanks for confirming. This process is not for the faint of heart!

 

[Diysolar123]

Seeing how these cc/cv power supplies work now I can see how adjusting the voltage at this stage would shoot you above your original target.

Appreciate the help

...warning will robinson, warning...
Good thing you sorted this out early; I suspect many good cells suffered an early death due to this one little change.
it is very very important to adjust your CV/CC settings with NOTHING connected...
If you change your settings once its connected, well, you could certainly damage your cells; quite quickly actually.

 

[GSXR1000]

Thread 'Top balancing why proper wire' https://diysolarforum.com/threads/top-balancing-why-proper-wire.17168/

 

[Gould]

Definitely learning a lot through this process!

 

[mikefitz]

that the voltage is at 3.49V but the voltmeter attached to the cells is reading 3.33. Just wanting to confirm this is normal.

Which reading is correct?
The multi meter will not be accurate, from the specification it could read +/- 100mV error on the actual voltage. In reality the voltage reading of a 3 digit low cost meter may be even higher.
Unless you have a calibrated quality meter the readings cannot be relied on.
The power supply may be better at indicating the correct voltage.

You also have volt drops to consider in the cables, due to cable resistance.

Use the meter direct on the power supply to establish any error between the two.

Mike

 

[Gould]

I can’t speak to the calibration of the meter, I suspect it is not based on the $25 it cost me.

I’ll take a reading at the power supply and report back.

 

[Gazoo]

I can’t speak to the calibration of the meter, I suspect it is not based on the $25 it cost me.

I’ll take a reading at the power supply and report back.

You are doing fine. It would be better if you had a 4 digit meter but I would bet your meter is within 10mv's accuracy. Same with your PS. Even if they are both 100mv's off, and I seriously doubt it, it isn't going to matter...you won't damage your cells charging to 3.750 volts.

 

[Gould]

Perfect, thanks. I did confirm the voltmeter matches the power supply exactly.