18650 voltage sag

[Glennn]

Hello,


I'm just creating my first diy 18650 pack, after reading hours and hours here on this great forum.

I did a test on the first (brand new) batteries I received today in the mail: samsung ICR18605F.

I did a charge at 0.5A to 4.2V, went great.

Not so great:
A discharge at 1A immediatly creates a voltage sag to 3.8V.
Once it reaches 3.0V, discharging stops and the battery jumps back to 3.6V...


Is this normal for a 18650 5.2A rated battery or did I buy fake batteries that even can't handle 1A discharge? I'm used to lipos on my other (rc) projects and have never seen such a voltage sag.


I would love to get some insights before I actualy start building this 18s9p pack.

 

[Manny]

can you post a picture of your test setup?

Resistance can have a big influence on the test results.

 

[Hillhater]

see if you can find that cell test on this site..
http://lygte-info.dk/review/batteries2012/Common18650IndividualTest%20UK.html
here is a typical 18650 test result for various loads....your cell may be very different though.

And this is a similar test on a Samsung 25R cell, a known very "strong " cell

 

[Glennn]

can you post a picture of your test setup?

Resistance can have a big influence on the test results.

I've been using these balance chargers for my RC lipos for years, no problem there.
it might have to do with the magnets I'm using to connect the battery to the crocodile clamps?


free image hosting

 

[Glennn]

see if you can find that cell test on this site..
http://lygte-info.dk/review/batteries2012/Common18650IndividualTest%20UK.html

It's this one here: http://lygte-info.dk/review/batteries2012/Samsung ICR18650-26F 2600mAh (Pink) UK.html

 

[Manny]

there is resistance between the crocodile clamps and the battery. maybe you can add a wire between the battery and the magnet and measure the real cell voltage.

 

[Glennn]

You were completely right.

I used a wire to make a better connection and now I see a sag to 4.09V instead of 3.8V.

Thanks!

 

[Matador]

No wonder.... Neodynium magnets have very high resistivity, hence high resistance means high unecessary voltage sag.