Inexpensive coin- and button-cell holders of stamped metal may work for a while, but their longer electrical and mechanical integrity is questionable while their user convenience varies widely.
Over the years I have replaced coin- and button-cell batteries in countless products and have had to fix or improve the marginal battery holders used in many of them. It’s understandable but frustrating that many low-end products use cell holders with an absolute minimum of electrical-contact and mechanical-attachment surface area, which is never good for long-term reliability as well as resistance to normal wear-and-tear.
The cell holders for these products are often just stamped metal pieces for the PC board. They connect to the battery bottom for one contact and the top (or edge rim) for the other, Figure 1. Every time you remove and replace the battery, you put stress on the tiny contact area between the metal stamping and the board.
Figure 1 Coin cell holders are often just basic stamped-metal contacts soldered directly to the PC board. (Image sources: diyleyuan.com, PCBoard.ca)
It can easily be twisted inadvertently twisted and ripped off, especially when there is little clearance to reach the battery and you have to sort-of twist the contacts to do the removal and replacement. I’ve tried to resolder those minuscule contacts with mixed success, since access is limited, and the area is pretty tight.
Let’s face it, those stamped-metal battery holders are very inexpensive. Perhaps the product manufacturer assumes that the overall product is so cheap that no one will bother to replace the battery, which itself costs between $2 and $4. Maybe, but maybe not.
That’s why I act with caution when I need to replace a coin or button cell in one of these products. If I see one of those cheap battery holders in there, I take it slow and gentle with the battery removal and replacement.
But it doesn’t have to be like this. There are plenty of widely available, fairly solid battery holders for coin and button cells available from many sources, including ones with real bases plus tabs to solder to the board; some even have a retaining clip holding things in place, Figure 2. Yes, they are a few cents more and take a tiny bit more room, but it would be nice if even inexpensive products used them…hey, we can all dream, right?
Figure 2 Better-quality cell holders include a discrete base for better mechanical and electrical connections. Some even have a top-side latch which retains the battery but can be released for battery replacement. (Image sources: West Florida Components; MTI Corporation)
Sometimes, the battery holder is a source of surprise. I needed to replace the CR2032 coin cells in a small, inexpensive (~$25) Salter 2001 kitchen scale that is at least 20 years old (and still available used at Amazon and eBay), Figure 3. As with most such scales, the unit is “closed up” fairly tight to maintain the mechanical integrity of the cantilever-weighing beam, load-cell, and platform assembly.
Figure 3 When it came time to change the batteries in inexpensive kitchen scale, I was expecting a cheap battery-holder right on the PC board.
I expected the batteries would be hard to reach, but instead they were located under a small door on the underside. I opened that door not knowing what to expect for battery placement and connections, and assume it would a cheap, barely reachable arrangement.
Instead, however, I found a truly elegant approach. The two coin cells were in their own somewhat “elegant” removable holder in a pocket away from the PC board, and connected to the board by a pair of modest-gauge wires so they are somewhat independent, Figure 4.
Figure 4 The two CR2032 batteries are in a removable case.
This holder, in turn, had a fully removable slide-out “drawer” which held the cells in place, making them easily accessible yet full protected, Figure 5. All I had to do was take the battery holder out of its pocket on the scale’s bottom, slide out the drawer, remove and replace the two cells, and reverse the procedure.
Figure 5 To remove/replace the batteries, you simply slide their holder out from the case—truly elegant in its own small way.
Not only did it take just a few seconds for the entire process, but there was no stress on the internal circuit board, as the wires between holder and board had enough slack. Whoever designed this scale and approved the bill of materials (BOM) was thoughtful enough to make it easy yet rugged, even if it added a few cents to the BOM cost—and I was grateful to that unknown designer or team.
Too often we see where cost pressures result in a design, which is barely adequate with respect to mechanical and thus electrical convenience and reliability, in consumer products such as these kitchen scales, low-end remote controls that come free with TVs, or other mass-market devices. Often, we pay the price, as the unit is tossed out too soon when what should be an easily replaceable part isn’t. From an engineering perspective, when I see one that’s made “right” with that little bit of extra consideration, I give silent shout out even while knowing they will never hear me.
Do you give the coin/button cell holder in your design much attention or consideration? Have you ever used a slightly better and more costly part to achieve a significant increase in product life or user ease, only to have your BOM ripped apart at review by the cost team? Were you able to make the case and win, or did a few cents get in the way of a more functional and longer-lasting component or design approach? Have often had to asked yourself “what were they thinking here?” or “why didn’t they choose another way?”
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.