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Is this just purely convention, or is there some kind of technical reason behind it?

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    \$\begingroup\$ Not always...I've seen a number of kids toys with a plain metal strap to connect between cells on one end of the battery tray and two springs with soldered leads on the other end. \$\endgroup\$ – HikeOnPast Nov 21 '12 at 6:01
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    \$\begingroup\$ @DeanB - same here, there are many battery holders that doesn't use the spring. Those who use it seems to always use the negative side though. I guess it's because combined with the fact that the flat end always is the negative side and the most useful side to have the spring. \$\endgroup\$ – Trygve Laugstøl Nov 21 '12 at 12:08
  • \$\begingroup\$ @trygvis, my point was that there are battery holders with both springs on one end of the tray - one for the positive end of one cell and the other for the negative end of another cell. In other words, while the "spring on the negative side" is certainly most common, there's nothing that prevents using springs on the positive end of a cell. \$\endgroup\$ – HikeOnPast Nov 21 '12 at 15:17
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Assuming you're talking about round-type (cylindrical) batteries, such as D, AA and AAA, it's to ensure maximum contact with the flat end of the battery, which is the "negative" terminal as described in the ANSI standard. You'll commonly see leaf-spring contacts as well as coils. The side with the "nub" will automatically provide a solid contact if enough pressure is provided by the contact on the flat side, so no second spring is needed.

I can only assume that the flat and "nub" design that the ANSI standard describes for round-type batteries was chosen because it provides a clean contact mechanism as well as a clear indicator of polarity to consumers.

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  • \$\begingroup\$ The spring could easily position around the nub for exactly the same contact. Furthermore, when the spring is compressed, perhaps the nub could touch additional metal. The nub would also help keep the spring from sliding off. Furthermore, the opposite, flat end would make better contact. So, the common arrangement is actually suboptimal. \$\endgroup\$ – Kaz Nov 21 '12 at 16:42
  • \$\begingroup\$ I think the battery design comes from technical reasons. The flat end is part of a metal can, and the nub covered a carbon rod of about the same diameter before alkali batteries became popular. (I used to take batteries apart in the seventies, when I was a child.) \$\endgroup\$ – starblue Nov 23 '12 at 8:21
  • \$\begingroup\$ @Kaz I'm not convinced by that argument. Having a spring touch a nub limits the types of springs you can use (need one that'll fit "around" it), causes problems with batteries getting stuck in the springs, and just plain looks odd. Have you ever tried putting a battery in backwards? It might be a psychosomatic thing, but it just feels wrong. \$\endgroup\$ – Polynomial Nov 23 '12 at 8:26
  • \$\begingroup\$ @starblue I think today's batteries have a positive can. That is, the flat end is actually an insulated "button" that sits in the otherwise open end. The nub is the closed end. \$\endgroup\$ – AaronD Jun 26 '18 at 3:44
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It is quite common to use the physical properties of the NUB to provide polarity reversal protection in a battery. So the NUB will fit between two plastic shoulders and make contact with the plate. If there were a spring this end you would need to more tightly control tolerances in the spring and its mounting compared to a flat plate. For this reason it makes economical sense to place the spring at the flat negative end and push the positive nub onto the plate.

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