You may have seen my recent blog on Connecting EV Batteries. This generated a lot of interest, particularly in what was not covered- what about connecting smaller Lithium batteries?
In the EV world, Lithium batteries (specifically, individual cells) are rather large, hulking items that require large connectors both for mechanical security as well as the high current demand. Bolted joints are frequently used in conjunction with large bus bars. Other cable connectors are similarly large to deal with the high currents and keep voltage drop and power losses low.
But that’s for EVs and very large batteries. What about smaller (much smaller) ones? There are several alternatives that are readily available, all with their own advantages. All of these options originated well before Lithium cells, coming primarily from earlier rechargeable technologies such as NiCd (Nickel Cadmium) and are found available for most battery chemistries.
Perhaps the most common method of connecting Lithiums is using solder tabs. Solder tabs are very versatile; you can
connect the solder tabs together to form a pack of multiple cells, you can solder them directly to the board, you can solder on wires and use a connector to attach to a PC board or cable assembly. Their versatility drives popularity, especially with Lithiums in applications that do not need much in the way of serviceability. That is the chief downside- soldering these tabs to other tabs, PC boards, or wiring typically is a manual process due to the very high heat requirement (batteries are great heat sinks). In high volumes this process can be automated, but will require a specialized process to do so (excepting some smaller coin or watch cell holders). Also, the battery or cell you may be looking for may not come with solder tabs on them, and attaching them to a battery requires spot welding. Once you have this process, though, tabs can be spot welded to each other, making assembly of a pack much more reliable and much faster.
Yes, that’s right, a good old-fashioned battery holder. This is by far the easiest option for field replacement. It’s also a handy solution if board space is at a premium (but overall enclosure space is not). It’s also a pretty mindless solution
that doesn’t require a lot of planning. It’s great for quick and dirty projects. Batteries are plentiful in these sizes, assuming that you stay with relatively common sizes (typically cylindrical, such as the familiar AA, AAA, C, D, or various coin/watch flat cells) or slightly less common (such as less familiar A, N, sub-C, etc.). The downside to this connectorization is, of course, cost. While not expensive in an absolute sense, in consumer electronics or other low-margin, cost-sensitive products, pennies count. Reliability also is only as good as the unit itself, and how it is mounted. This can be spotty in lower-cost units that can be flexible or have poorly plated contacts. Once batteries are in a holder, though, connecting them to a PCB or a harness is a simple task of managing the cabling with a good wire-to-wire or wire-to-board connector.
While these two are the chief methods of connecting smaller lithium batteries, there are several others and variants on this. Watches are a great example. Typically they will rely on pressure or friction to hold the cell in place, much like a
traditional battery holder. Consider this a custom holder, built into the final device packaging. There are also adapters of all sorts and sizes to adapt various cells to fit in various other cell holders. While this may not be cost effective in production, it shows that with a little ingenuity and packaging effort, you can adapt a wide variety of cells to different applications.
Keep in mind when designing a battery pack or connecting your battery, that the size of tabs, straps, cables, and connectors need to be rated for the fuse that you end up using on-board. This is critical with batteries as they are usually MUCH more capable of driving into a short circuit than you would think. Also, remember that when using Lithiums proper battery management to control charge and discharge current, temperature, and over/under voltage conditions is critical to ensure the safety and reliability of the cell.