Battery contacts: An underappreciated challenge

Article By : Bill Schweber

There's an aspect of batteries which doesn't have the glamour that these electrochemical energy-storage units have: their physical contacts and connectors.

Batteries are getting a lot of attention and R&D effort these days, for many reasons I don’t need to explain to this audience. The focus is on both primary (non-rechargeable) or secondary (rechargeable), with chemistries ranging from the venerable and still important lead-acid type to more-exotic lithium mixes, and in size from tiny button cells to much larger packs used in EVs and grid-scale storage.

But there’s an aspect of batteries which doesn’t have the glamour that these electrochemical energy-storage units have: their physical contacts and connectors. Whether it’s microamps and milliamps from a tiny cell to hundreds of amps from an EV  large-scale backup system, how you connect the batteries to the circuit the battery is critical.

In some cases, it’s a matter making a tiny-scale mechanical fit; in others, it’s an obvious mechanical connection that must reliably and consistently carry hundreds of amps with low resistance in challenging circumstances of vibration, extreme temperatures, and more. Ohm’s Law says it all: when you have a few hundred amps going through a few  milliohms contact resistance, you’ve just wasted a non-trivial fraction of voltage and power from a battery that may be delivering a relatively modest nominal voltage, and you have also dissipated tens of watts or more as heat.

Some battery connectors appear to be fairly straightforward such as the ones traditionally used on 12-V lead-acid car-battery posts, whether single cable or multi-cable, Figure 1.

Figure 1 The basic 12-V car battery clamp on connector is critical in passing hundreds of amps to the car’s electrical subsystem, and with low contact resistance, for many years despite a harsh operating environment, whether it’s a basic single-cable version (top) or a multi-cable variation (bottom). (Image sources: National Automotive Parts Association/NAPA; National Luna/South Africa)

(Note that I said “straightforward” and did not say “trivial,” as one of the many things I have learned over the years is that there is no component which is trivial. Regardless of how simple that component may be in its appearance or function, there’s a lot of technology, materials science, design subtleties, and manufacturing know-how needed to make it work reliably at a reasonable cost.)

It’s not just “heavy duty” connectors that have challenges, and the battery-connection issues are compounded when you need to connect multiple batteries in series or parallel, even a very low voltage or current levels. For example, I recently opened a cheap, 20+ year old “give-away” scientific calculator that was still working with its original batteries; admittedly, the display was dimming, but I was still impressed that it worked at all.  (Why did I do this? Just the usual engineering curiosity, coupled with the always-welcome opportunity to take something apart which does not necessarily need be reassembled into working unit.)

Inside I found two button cells (AG12, equivalent to 386 size) in series which were slightly corroded but still able to provide enough power for the calculator. What I found most interesting  was how the two cells were electrically connected. If I had been doing the design, I probably would have taken two standard button cell holders and wired then in series via a discrete wire, or with a PC board track if I used  surface-mount cell holders.

Instead, this calculator ha a button-battery holder unlike any I had seen previously, and I have “dissected” many button- and coin-powered devices. The holder was designed to not only hold the batteries but also provide the needed series connection, Figure 2. To do this, there was one small metal strip (punched, I presume) doing multiple duty of connecting the rim (+ terminal) of the battery on the left side to the bottom (- terminal) of the one on right side without need for interconnecting wires, solder joints, or other pieces.

Figure 2 The series-connected button-cell holder (in the highlighted oval): note how the circumferential contact for the left-side battery also becomes the interconnection to the right-side battery and the contact for its bottom side as well. (The yellow-white areas are not corrosion or discoloration but are a reflection from the light.) (Image source: author)

Even nicer, the bottom-side connector end had tiny prongs which reach up and make contact with the battery, and can presumably pierce through any surface oil from the user’s skin, oxidation, film, or similar. To me, this seemed better than the flat tab of most button/coin cell holders, which provides relatively large surface area (good) but often doesn’t make full contact (bad)due to inadequate pressure.

Since using two cells in series is not an unusual configuration, I assumed that this simple hybrid-role holder with series connection was a standard item. It may be, but I didn’t see it in the relevant catalog pages of the “go to” reference source I use when researching small but essential pieces that complete a system (Keystone Electronics Corp.). Of course, this may be a standard item at other vendors, but my curiosity was overcome by the time the research would take.

Have you ever seen or used a battery connector – whether small, medium, or large – that impressed you with its cleverness beyond its basic functionality? Conversely, have you ever been frustrated trying to find a mate for a battery connector of undetermined model or vendor, and even had to improvise one?

This article was originally published on EDN.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.


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