Battery terminal better design

Question

We are a Tier 1 automotive locks and key manufacturing company.

We are in the process of designing a remote key system and are considering one of two design strategies. Production volume is expected to be about 120,000 units a year.

• Design 1: Press Joint in the +ve terminal of the battery which is pressing against the rectangular pad on the PCB once the housing is closed with battery.

• Design 2: +ve terminal soldered on PCB and battery is placed with the help of a top cover.

We wish to achieve best reliability and ongoing functionality.

Which of these designs will give better long term performance, and why?

Answer 1

Both should work well "often enough".

I would trust the soldered version more, because the press fit version relies on a contact which in turn relies on proper case closure and fitting, and also on contact integrity.
In most cases the cases are clip together or screw together and contact pressure will be maintained well enough.

If I was building something using this style of battery and reliability was of utmost importance, I'd consdier having the battery case guide physically fixed to the PCB.

In some holders the battery slides sideways into the PCB mounted holder so that negative terminal contact pressure is maintained when the case is open.
In most cases this is not strictly essential as the positive terminal circumferential ring maintains enough grip to stop the negative terminal contact pressure reducing too much.

If designing from new, consider ensuring that the positive case contact cannot be "folded" by clumsy insertion so that it either malforms and reduces pressure or, worse, shorts out the battery.

I recently inserted batteries into a device which provided for two coin cells connected in parallel (a common arrangement in some products). The device started operating when the first battery was inserted. It stopped when I tried unsuccessfully to insert the second battery. It took a few moments for me to realise that the first battery was now being shorted by the bent positive contact on the second battery holder :-( .

---

It is worth looking at what known competent manufacturers do.
HP "they don't make no junk" - or they didn't used to 20+ years ago when the HP 10BII calculator was introduced.

Shown below is the battery compartment of an HP 10BII calculator. (I bought it in well used condition from an Op-shop for a $1 , earlier this year). It is not certain which method HP use here because they use a small extra piece of plastic to maintain the integrity of the battery box regardless of the state of the battery door.

In your example the case halves are clipped/screwed shut when the door is open - but that is true of the HP too, and they saw fit to design it so that the case closure state is unimportant.

As a general rule, doing what HP did in their designs before they split in 2015 is usually a good idea. After that date it varies.

Answer 2

Personally, I wouldn't use design 1 without validating it to death, including at extreme temperature and vibration environments. The cost of validating this may well be higher than the added cost to production for design 2 for many years. My choice is clearly colored by decades of devices I've owned where the battery cover gets loose.

You certainly need to think about your failure modes as well. Design 2 might work if you lose the battery cover, but design 1 is much less likely to work if you lose the cover. Design 1 might call for a more complicated battery cover to hold the battery in place, and the nature of the mold will also be important -- if the fins pushing the battery down have any sort of "crush", the connection for a replacement battery may not be as reliable as the placement of the first battery. Of course, you can glue a foam pad or something to the battery cover to try to maintain contact over time, but that will decay over time. Also, you might need a thicker battery cover to avoid bending, and that has a cost. You might even need to use a different polymer than what you plan to use. I think that the overall construction issues associated with design 1 may go well beyond what you decide to stick on the PCB, and if you decide to go that route, you should discuss all these issues with the appropriate people in your organization to prevent silly surprises associated with an "over-the-wall" design strategy in which you do your part and then pass your portion of the design to the next team. If you don't think that the people who will handle that aspect of the design are up to that task, that would push me to design 2. Certainly, you should have a conversation at this point with those members of your team to see if they think that they can keep the battery case tight for years and multiple battery replacements. If they don't feel they can, your choice is straightforward.

Only your company can decide how much risk you're able to accept, but somebody needs to think about what "failure" means to you. If it means a one-star Amazon review, that's one situation. If it means warranty replacement, that's another. If it mean you'll lose major OEM contracts, that's a different situation. Obviously, somebody should also assess what going with the more robust design does to your price point -- but that should include any demands that going with the less robust design places on the manufacturing of your casing.