anelok: considering a battery holder

Werner Almesberger werner at almesberger.net
Sat Jan 4 17:15:08 EST 2014


Right now, the battery is held in place by the following contraption:

http://downloads.qi-hardware.com/people/werner/anelok/tmp/case-bot-clip.jpg

The idea is that the case bottom presses against the clip which in
turn presses against the battery which thus makes contact on both
sides. This work, but not very well. In particular, since the two
case halves do not have a lot of vertical pressure, contact is easily
lost when handling the current Anelok prototype.

Joining the two case halves with screws may improve this. A
production version could perhaps also use a custom-made steel clip
that doesn't depend so much on external pressure.

But to play it safe, I'm thinking of using an industrially made
battery holder in the next prototype. Advantages:

- holds the battery firmly,
- does not depend on the case for support, and
- can be globally sourced.

Disadvantages:

- larger than my current contraption,
- thicker, too,
- cost (see below),
- sourcing risk (hopefully minor).

Now, there are many designs to choose from ...

http://downloads.qi-hardware.com/people/werner/anelok/tmp/bat-holders.jpg


The simplest type I know is a single metal clip that presses the
battery onto the PCB, a concept not too dissimilar from what I have
at the moment:

http://www.digikey.com/product-detail/en/BK-913/BK-913-ND/2329371

It's also pleasantly inexpensive (USD 0.22 @ 1000). It has three big
drawbacks, though:

- it exerts an enormous pressure on the PCB which also makes it hard
  to remove the battery,
- the battery is accessed on the side so there has to be a free path
  in that direction which also limits the possible orientation, and
- it's through-hole so there have to be sufficiently sturdy mounting
  holes.

There is also an SMT version of it but that one doesn't even stay on
the PCB if not bolted down in addition to soldering.


There are various types with a round plastic shell and a "crane" type
of clip but they're all quite tall. The ones at 5 and 6 o'clock in my
collection are quite nice when it comes to battery removal (you press
down on the little plastic tongue to release the battery) but are
also rather bulky.


Now, the one in the rear at 1 o'clock is also a full holder but more
slender. One of its drawback is that it holds on to the battery with
surprising strength - while you can pry it loose just with your
fingers, a screwdriver will make this a lot easier.

There are now even smaller variants of the same concept (at about
USD 0.60 @ 1000):

http://www.digikey.com/product-detail/en/LP2032SM-JJ-GTR/LP2032SM-JJ-GCT-ND/4246309
http://www.digikey.com/product-detail/en/BLP2032SM-GTR/BLP2032SM-GTR-ND/2077832

Using one of these would make Anelok thicker by about 1.0-1.2 mm.
With a little luck a small cavity could be made in the bottom plate
to accommodate the center pin of the wheel, which would allow moving
most of the battery holder under the wheel.

As an added benefit, traces could be run under the holder since.

- Werner



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