Cherry67
Member
- Joined
- May 13, 2018
- Messages
- 518
I have been babbling around IR (Inner Resistance) for awhile, in several threads now.
I should come to the point.
As an introduction, i see discussed, regarding usage of the cells,
- Capacity,
- Self discharge
- and inner resistance.
The first two are described quite well from many users here, and im rather sure that most agree in taking these in check and account.
The third one, i assume, is well understood as a factor to be taken in account on packs with higher discharge rates, for instance but not only as a factor of incresed losses on higher currents.
But, i dont see a common understanding on checking IR, and taking it account on building packs.
That may either be the case because of measurement problems for accurate IR, or IR is considered less important.
The Basic problen on IR is, that it is very low, and that requires a 4 wire measurement. Resistance may be 20 to 50 mOhms for a cell, and that is much to low to be measured 2 wire via contacts. It is to be understood that contacts itself have resistances in the 10 to 20 milliohm range, and that adds (two times) to th measured IR, plus the cabling, plus another 2 contacts where the plug goes into the instrument. Additionally, contact resitances are not constant, but change starting fron actual pressure to many other reasons.
There is a thread here already, which discuss proper IR measurement with Kelvin connection, the link is:
https://secondlifestorage.com/t-4-Wire-Kelvin-testing?highlight=kelvin
All the basics of a 4 wire Resintance measurement are correctly shown, but with all respect little seems to be understood.
Especially the video shows all, but what i have seen here and elsewhere is some misundestanding about the REAL implication how to connect the 2 additional wires.
The point is - 4 wire measurement does NOT mean (only) to connect the 2 wires (of one pole) BEFORE the contact to one of the cell poles.
It does mean to use a separate contact for all 4 wires to the battery poles.
This is the Kelvin Clip assembly of a professional 4 wire Bridge (Hioki). In the picture the rubber rings, which close the clip, are removed.
It is to be understood that the 2 brass pieces (of one clip) are NOT connected in their turning point, but isolated, and each has its own wire to the bridge.
When the clip is connected to a wire, it makes TWO contacts to it - one to carry the current into it, one to pick up the voltage there.
And since the voltage pickup does not carry significant current (in contrary to the other contact) its contaxt resistance hardly influences the measurement result.
Example for the well used Opus charger, it does not only suffer fof the wiring between board and contacts to the cell (especially the spring) it suffers of two SEPARATE ADDITIONAL contacts to the cell to pick up its voltage.
Example for the known B6AC charger, here the situation is quite different. This charger usually charges packs, which are soldered/welded together. The BMS connections go between the cells, soldered as well. THIS is a nearly perfect 4 wire connection. Therefor a B6AC has a good chance to supply proper readings, IF you really use a soldered pack.
If you have installed several sockets for easy replacement/remove, you again have the contacts ruining your IR results.
I hope this makes things clearer how to get proper IR Results. If this will result in using them might be another discussion.
I should come to the point.
As an introduction, i see discussed, regarding usage of the cells,
- Capacity,
- Self discharge
- and inner resistance.
The first two are described quite well from many users here, and im rather sure that most agree in taking these in check and account.
The third one, i assume, is well understood as a factor to be taken in account on packs with higher discharge rates, for instance but not only as a factor of incresed losses on higher currents.
But, i dont see a common understanding on checking IR, and taking it account on building packs.
That may either be the case because of measurement problems for accurate IR, or IR is considered less important.
The Basic problen on IR is, that it is very low, and that requires a 4 wire measurement. Resistance may be 20 to 50 mOhms for a cell, and that is much to low to be measured 2 wire via contacts. It is to be understood that contacts itself have resistances in the 10 to 20 milliohm range, and that adds (two times) to th measured IR, plus the cabling, plus another 2 contacts where the plug goes into the instrument. Additionally, contact resitances are not constant, but change starting fron actual pressure to many other reasons.
There is a thread here already, which discuss proper IR measurement with Kelvin connection, the link is:
https://secondlifestorage.com/t-4-Wire-Kelvin-testing?highlight=kelvin
All the basics of a 4 wire Resintance measurement are correctly shown, but with all respect little seems to be understood.
Especially the video shows all, but what i have seen here and elsewhere is some misundestanding about the REAL implication how to connect the 2 additional wires.
The point is - 4 wire measurement does NOT mean (only) to connect the 2 wires (of one pole) BEFORE the contact to one of the cell poles.
It does mean to use a separate contact for all 4 wires to the battery poles.
This is the Kelvin Clip assembly of a professional 4 wire Bridge (Hioki). In the picture the rubber rings, which close the clip, are removed.
It is to be understood that the 2 brass pieces (of one clip) are NOT connected in their turning point, but isolated, and each has its own wire to the bridge.
When the clip is connected to a wire, it makes TWO contacts to it - one to carry the current into it, one to pick up the voltage there.
And since the voltage pickup does not carry significant current (in contrary to the other contact) its contaxt resistance hardly influences the measurement result.
Example for the well used Opus charger, it does not only suffer fof the wiring between board and contacts to the cell (especially the spring) it suffers of two SEPARATE ADDITIONAL contacts to the cell to pick up its voltage.
Example for the known B6AC charger, here the situation is quite different. This charger usually charges packs, which are soldered/welded together. The BMS connections go between the cells, soldered as well. THIS is a nearly perfect 4 wire connection. Therefor a B6AC has a good chance to supply proper readings, IF you really use a soldered pack.
If you have installed several sockets for easy replacement/remove, you again have the contacts ruining your IR results.
I hope this makes things clearer how to get proper IR Results. If this will result in using them might be another discussion.