[Karter2]

This independent test is useful.
It reports a 45.9 gm weight for the unprotected version..
http://lygte-info.dk/review/batteries2012/Panasonic NCR18650B 3400mAh (Green) UK.html
..and 48 gms for the protected version ...also a little lonnger .
http://lygte-info.dk/review/batteries2012/Panasonic NCR18650B Protected 3400mAh (Green) UK.html
And if you want to compare other cells....just pick any one .
http://lygte-info.dk/review/batteries2012/Common18650IndividualTest UK.html

 

[Karter2]

Is this the label you have ?..
The capacity sticker is a requirement for export shipping approvals.
http://lygte-info.dk/pic/Batteries2012/Panasonic NCR18650B 3400mAh (Green)/DSC_2683.jpg

 

[Karter2]

The "normal" ( most commonly used) discharge rate for capacity testing, is 0.2C. Hence why the spec sheet states 0..65amps.for those cells.

 

[Karter2]

The classic method for "DC Internal resistance" ! (DCIR), is just as you discribed.
Discharge at a known current (1C, or 2C, 5C, etc)...measure the steady state voltage under load, then remove the load and measure the relaxed voltage .
Accurate measurements needed,..then Ohms law.
Note DCIR varies significantly with temperature, and state of charge , so be careful to standardise test conditions if you want to compare cells.
You may find this useful...
https://endless-sphere.com/forums/viewtopic.php?f=14&t=73701&hilit=Dcir+testing
Another test recommended is the "charge retention" test..
Charge all cells to a common voltage, say 4.17 (easy if you group cells between two contact plates and charge in parallel)
Then remove the charge contacts and allow the cells to stand for several days before checking individual voltages to see if any self discharge or any voltage variation.
You only need one bad cell to mess up a pack.
You should also match cell capacity as close as possible within parallel groups.

 

[Karter2]

Just curious, given a limited number of cells, why not distribute the cell capacities so each level has about the same capacity (within reason)?

Cells in a parallel group cannot be individually monitored for voltage or SOC, or protected against over discharge, so need to be well matched.
Whereas, strings of Parallel groups of different capacity can be monitored and protected by a good BMS.
One reason why its a good idea to assemble a pack with strings of parallel groups.

 

[Karter2]

Yes, agree guys, for the optimum result its "Delta V"..
And it should be a "4 wire" method also
...but i have never noticed a significant difference for normal applications.

 

[Karter2]

What about the individual cell fuse wire for the positive terminal? Are those welded or soldered? What AWG? I am designing for peak 30 seconds 1.5C or 5A per cell. I think 10A for the fuse is the right size?
l]

Tesla use an Ultrasonic welder i believe for their ((aluminium) fuse wires.
There has been much debate as to what rating fuse wire to use, but i believe The Tesla wires blow between 15-20 amps.
Remember, for them, its main function is to protect the cells/pack from a full internal cell short.

Spot welder...this may be of interest..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=89076
https://youtu.be/fCa2QP1jjBE

 

[Karter2]

Not worth making anything when a proven unit like that costs under $100.
.

 

[Karter2]

Its not necessarily the current needed as much as the control, pulse time etc.
Read the full thread, ....and this one also if you are serious about welding those tabs..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=68005

 

[Karter2]

There are many ways to assemble and connect 18650s into packs, ranging from mutliple spring clip cell holders, soldering, welding, various "dry contact" assemblies, even using those tiny neo magnets to clamp wires on ( yes, it works !). There are also various commercial pack assembly kits, and contract build services....the choice depends on many variables, not least of which are budget and available skills.
This "no weld" system is well respected for its integrity..
https://endless-sphere.com/forums/viewtopic.php?f=14&t=57810
However, the majority of DIY pack builds are generally spot welded or some still risk soldering.

 

[Karter2]

If you are using a data logging type charger/discharger, you can simply compare discharge curves at different loads and even directly plot DCIR against dod/capacity.
Are you logging data for future comparason ?

 

[Karter2]

Many RC chargers have logging.
Such as this..
https://hobbyking.com/en_us/0620-20a-300w-balance-charger-version-2.html
Or you could just use a logging MMeter ( expensive) .
PC logging and graphing software can be downlooaded for free.

 

[Karter2]

If you just compare two discharge curves at known loads (0.5A and 5.0 A shown) you can easily see the delta V at any point in the capacity rating.
Knowing the delta I is constant at 4.5A , thn the DCIR is directly proportional to the delta V.
And in the case of the Pana 18650B shown , it "looks" to vary between 0.3v (67mohm) at 1ah, and 0.4v (88mohm)at 3ah toward the end of the discharge.
This is just a "quick and dirty". Visual calculation, but with a logging charger you could plot DCIR completely over the discharge cycle if necessary.

 

[Karter2]

Here is another "no weld" 18650 pack assembly kit.
The Vruzend kit..
$19 for a kit to assemble 50 cells (0.5 - 0.6 kW? Depending on which cellls)
https://endless-sphere.com/forums/viewtopic.php?f=31&t=87434

And the Celllog 8 channel data logger for $35...test 8 cells at once.
http://www.ebay.com.au/itm/CellLog-8S-/301992460197?hash=item4650273fa5:g:aI0AAOSwmtJXabob

 

[Karter2]

What devices and functions would you recommend for this project?
?

......unless i have missed it, i dont think you have actually said what the intended use is for this pack .
Mobile, ?..Car , bike, boat, aircraft, ?
Stationary, ? ..Solar store, emergency back up, etc ?
Intended use is helpful to know , especially for things like housing , cooling etc.