Designing a strong electronics holder

Question

For a yearly running event at our university we have developed a lap counting system relying on Bluetooth-chips passing by small embedded computers. The chips are these. To power them, we use 4 AA batteries.

Last year, we devised a solution which involved cutting open a single AA battery holder, gluing it to a small plastic rail to hold four batteries, soldering this to our chip, and glued it on the plastic rail. During the event we soon realized this was a bad idea, as we had to deal with a large amount of running batons failing due to the plastic rail being broken, the connections breaking off or the batteries not making a good contact with the holder anymore.

It didn't help our weak design that on such a event batons are not handled with care, they are thrown, fall on the hard ground and get shaken around terribly. (You can read a full report here.)

This year we want to learn from our mistakes and produce an improved version of the relay baton. We were wondering on some design decisions to be made: what precautions should we take to build a robust holder for our battery and chip that can withstand extensive shaking? Are we better off looking for one battery of the right voltage instead of four AA's? Are battery holders with integrated springs a good or a bad idea? Ideas for putting (and keeping) it all together?

I'm not sure if this is the right place to ask but we could use some advice on this subject from people with a bit more practical technical knowledge.

EDIT: We're thinking about using battery holders like these. Would they still require additional enhancements to provide a steady power supply when being swung around? Being dropped is not that big of a problem, detection quickly recovers when power is restored.

Answer 1

I'm going to address the larger issue of making the system robust and reliable, rather than just focusing on the batteries. The main issues that I see are: ruggedness, waterproofness, battery/charging, and the "chassis".

If I were building these things myself, I would use PVC pipe as the chassis. But more on this in a moment.

To increase ruggedness, I would encase the PCB in "casting resin". Just google "casting resin". Essentially it is an epoxy that you can pour into the PVC pipe to encase the PCB's to both waterproof them and support the PCB against shock and vibration. Casting resin is available from many hobby/craft stores like Michaels and Hobby Lobby. Just put your electronics in the PVC pipe, mix up the resin, and pour it into the pipe. Important Note: casting resin comes in 2 parts and the ratio of the two parts effects how long it takes to harden. The faster it hardens the HOTTER it gets during the curing process. You want it to harden as slowly as possible, otherwise it might get hot enough to damage the electronics. Experimentation with the resin is important to getting this right.

Casting resin will work best if your batteries are rechargeable and fully encased in the resin. However, I wouldn't do that. Batteries behave weirdly when charged. Best case your batteries could get hot and not be able do dissipate the heat due to the resin. Worst case, your batteries build up some internal pressure that can't be dealt with due to being encased. As an alternative, you could use some super-capacitors. The usefulness of super-caps will depend on your power consumption and a variety of other issues, but I've used them for several applications and they work quite nicely. Essentially, supercaps behave like rechargeable batteries except that they don't hold as much power but they can be charged and discharged almost an unlimited number of times.

If you can't use supercaps, then rechargeable batteries with tabs/pins/wires already attached would be your 2nd best choice. 3rd choice would be standard or rechargeable AA's. With AA's, I personally wouldn't spend much time making the spring keep good contact. That is a massive waste of time because whatever you do, it won't be good enough! Instead, your design should take that into account. The best way to do this is to simply put large-ish caps in your circuit so that if the batteries do momentarily loose contact then the circuit will remain powered up.

If you use supercaps or rechargeable batteries then next comes the charging system. You could simply have a connector that goes to a charger. Of course that isn't very waterproof. A cool way would be to have a non-contact inductive charger. Imagine a transformer with two coils of wire. AC goes into one coil and comes out the other coil. An inductive charger is the same, except that one coil is in the base of your baton and the other coil is in the "charging station".

In the best case everything-- including super caps, charging coil, and PCB's-- could be encased in casting resin. With no seams there is no way for water to get into the circuitry! And with everything encased and fully supported the whole thing is very mechanically robust. With a little bit of work, you wouldn't even need end-caps on the PVC pipe. That way your baton would be a simple and smooth rod. I'd bet that you could take this and throw it out of a low-flying plane and it would survive.

Also, Casting resin is water-clear. Your circuit can have status LED's that can be seen through the resin.

Answer 2

Of the following methods, using a tube with 4 untabbed cells compressed within it, with cooling holes if needed, would probably be the cheapest and easiest solution. Strong foam or springs at each end only - preload then latch in place, should work well. "Latch" can be as simple as a disk pushed into place then a pin pushed across outside it. Or a stiff wie (say paper-clip thickness+ which can be pushed through cross holes then bent.

Springs are a bad idea unless you can guarantee a holding force which exceeds the spring force under all situations.

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4 in a tube: If you place 4 batteries in series in a tube and then apply some force at both ends you should be able to make a battery that can withstand any sensible applied force and which will fit inside your baton.

If you find a plastic or even perhaps cardboard tube just slightly greater than the battery diameter you can if necessary cut holes in the tube sides at numerous locations to allow cooling. This should not be necessary at modest discharge rates.

Standard "nipple ended" untabbed cells should be able to be used this way. Batteries can have a spring or stiff foam wad at each end. Batteries are pressed into tube with spring etc at one end then an end disk added then a pin or wire or screw across the whole tube to keep batteries in place and maintain spring compression.

Diagrammatic illustration of scheme below. Contacts not shown - add end disks with wires.
No inter-battery connections required (friction contact).
Foam or spring pressure must resist acceleration on impact when dropped longitudinally.
Tube can be as strong as desired

Black - body tube.
Orange - foam or springs to provide compression.
Red - disk, fitting body diameter. Non conductive probably.
Blue - pins or wire that push through holes in opposite sides of tube.

Insert foam + disk + pin at one end.
Insert contact (not shown).
Insert 4 batteries. Insert contact + 2nd foam + 2nd disk.
Push down until 2nd pin can be inserted

Bend pins over for security OR use screw + nut - say 2mm to 3mm.
Nylock nuts for extra points.

Tube needs to be strong enough to withstand foam compression forces + dropping + ...
Tube MAY need to be ventilated. Probably not.

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Tabbed cells?: Using AA batteries with tabs will allow you to solder a pack in the shape of your choice. Tabbed cells are usually (not invariably) rechargeable. From the bluetooth module datasheet it seems that 4 x NimH would be too low. BUT if you can bypass the BT onboard regulator you MAY be able to run on 4 x NimH.

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Lithium Ion: Energy content in 4 x 1000 mAh cells (see below) is about 4 x 1000 mAh x 1.2V (say) =~ 5 Watt.hour. A single LiIon 18650 cell delivers about 2000 mAh x 3.6V = 7+ Watt hours. IF you can use the voltage that a LiIon delivers then a single 18650 rechargeable would do well.

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Premade: You can buy premade NimH packs of 3 and 4 cells made for cordless telephones.

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Your bluetooth module needs 5V at a current which varies widely with mode. Bluetooth datasheet here.

Your report says that the cheapest batteries lasted 50 hours.
At 2000 mAh, 50 hours = 40 mA average.
At 1000 mAh (more likely for cheapest batteries) = 20 mA average.

4 Alkaline or Zinc batteries will NOT provide 5V when flat (fully discharged). 5V/4 = 1.25V = 50%-60% discharged.
The Bluetooth module has a linear regulator. Vout is not specified BUT if it is 3V3 then 3 batteries could be used with an LDO down to about 1.133 V cell. (3 x 1.1333 = 3.4V = 3.3+ 0.1V headroom worst case. (Very tight).

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PCB Mounting:

@David Kessner 's advice re potting PCB etc sounds good and in earlier years I would have agreed that that sounded the best way to do it. My experiences of the last few years indicate that you can get superb results more easily.
If you need better than superb, try potting.

We have found that a semi floating PCB works wonders.
We have a light with the PCB along the long axis and mounted in slots with an end cap, ability to move slightly fore and aft and some modest padding at ends. It has a few SMD ICs and a small ferrite core inductor on it plus discretes and some glue components. You can drop th ABS cased light 100 x from 1.5metres onto concrete all axis, or say 20 x from 2 to 3 metres or say 5 x from 5 metres without damage, in many trials. The 5 metre test reliably breaks part of the housing but nothing else - fall energy must be enough to reliably impact it past breaking point. Light still functions but you can no longer hang it up ;-).

Answer 3

It's just an idea, so you're free to skip over it.

If you transform that problem of shaking in an advantage? You can buy a charging device with a coil and a moving core (there are also torches with it, so the effort to search is reduced). One example is this.

Then you need a buffering battery, that you can solder to ensure that it's stable, and you obtain a perfectly autonomous tool.

About batteries, take into account this: the more power you save, the smaller batteries you will need, and smaller batteries means less inertial energy, and less risk to break the support.

You could also be interested in this(energy harvesting from vibration) or, more in general, to this site about energy harvesting.

But thinking about it, if it's a device that's used once a year, maybe it's not the case in which you will spend for an harvester, and even for sophisticated batteries; in that case, I can only suggest to try saving as much power as possible and make a good case for 2 batteries.

Answer 4

Here is a hack that is cheap and easy to try.

Put the Bluetooth module and the battery holder in the baton tube and fill with expanding insulation foam. (Probably after wrapping the circuit with something to protect it).

That should protect it from shocks and prevent anything from moving around inside the tube.

You could do tests with a similarly sized circuit board and see how much abuse it can take.