The busbar to cell connectors need to have:
- low electrical resistance
- mechanical integrity
For some applications they need to also have good thermal conductivity.
The busbar to cell connections are physically different for each cell format. However, all have the same requirements around low electrical resistance and good mechanical integrity. That mechanical integrity includes the joint with the cell.
With all of these parts the following needs to be considered:
- venting
- the cell connection should not obscure gas venting from a cell in thermal runaway
- the busbar and connection should not act to direct gases venting from one cell onto another cell
- shock and vibration loading
- just of the part
- due to relative movement of joined parts
- corrosion
- between dissimilar material joints
- from environment – even from materials designed into the pack
- mechanical fatigue
- work hardening in manufacture or assembly of the part
- due to vibration or relative movement over lifetime
- due to thermal cycling in use
- heat generation in the busbar
- conducting into the cell
- resulting in mechanical fatigue of the busbar
- part manufacture and shipping
- surface corrosion
- damage
- assembly
- pushing parts together to weld or form connections
- local heating
- removal of local plating to form joint
- compounds formed in weld
- repair
- in original manufacture can a second joint be formed if first fails
- does joint need to be able to be repaired if it fails in service
- recycling
- ease of removal
This is not an exhaustive list, but hopefully will start a process of considering all aspects of the design. (Please do contribute: nigel@batterydesign.net)
Examples
Therefore, these busbar to cell connectors come in many different forms and hence we wanted to share examples, features and further information:
Anti-Shunt Tab Design
The busbar for a cylindrical or prismatic cell has to be welded blind. Meaning you can only weld from the topside of the busbar as the cell itself is a sealed system.
The anti-shunt tab design shown on the left welded to the top of a cylindrical cell.
Three different busbar to cell tab designs for cylindrical cells.
A regular repair on these battery packs is to strip out the bus bars and replace the connecting plates and nuts on each battery module. Corrosion can be caused because the air used to cool the battery comes from the cabin that can be hot, cold, moist etc. When rebuilding always use a drop of Stabilant 22A on the nut/stud.
Andy Latham, Salvage Wire
Image supplied and copyright of Salvage Wire.
Cell Joining Techniques
There are many cell joining techniques. When we say joining we mean external electrical cell joints to busbars, some of these will be specific to certain cell formats. They basically fall into the main categories:
- Soldered
- Welded
- Mechanical