Hi,
In regards to the coin placed in the Box, as with many things it has a lot to do with the small details. There are three perspectives of interest: 1) what happens electrically, 2) what happens in the infra red (IR), and 3) what happens with the heat. Mostly, this is a review of the relevant physics.
When a coin is placed in the chamber as described, a new circuit path is created. The resistance of this path is highly variable, mostly due to the points of contact between the coin and the screen/rails on either side of the trench. The resistance of the coin itself is fairly low whereas that of the interface junctions is typically much higher -- all three resistances are effectively in series. The interface resistances are largely defined by the degree of surface oxidization of both metal surfaces and the pressure (weight and inertia) associated with the contact. As such, it is hard to define in general what is going to happen electrically since all of these variables are very specific to the degree of coin polish, newness of the unit, the true area of contact, and the contact pressure. (For example, 2 lbs of difference in surface pressure can result in a 80% variance in overall conductance).
What we can say is the following: the current demand of the Box is already fairly stressful on the battery -- adding any more additional current paths is definitely not recommended. IF the light is normally observed to not flicker and the addition of the coin causes it to change -- ie causes the light to be any dimmer or to flicker at all, then the coin is most likely having an adverse effect on the health of the battery. Conversely, if the light is not affected at all by inserting the coin, then it is probably 'ok this time' (although perhaps there is still a risk). Since you say you are observing things happening with the light, I would advise against the coin. Conductance through the coin itself is not a significant source of heat -- most of the heat generated would arise at the contact points -- not really in the right locations to do any good initially.
In regards to the IR effects, I can absolutely agree that the coin is most helpful in this regard. A clean copper surface acts as an excellent reflector of IR radiation and improves the balance of the criticality zone within the Box significantly. Overall, this would make the Box even more able to warm up quickly and cook the load more evenly. Partly this is due to its close proximate position and the near path imaging characteristics of its placement. The IR improvement is affected by the degree of surface corrosion of the coin and to a lessor extent, the specific alloy composition of the coin. More polished coins and coins with a higher copper, silver, or gold content would tend to work better. Unfortunately, these are the exact same characteristics that make the risk of electrical problems significantly higher and much more serious.
Finally, in regards to the heat, the coin will act as a substantial increase in the apparent load volume. By itself this is not a problem except that the coin also has the effect of significantly redistributing that heat. It means that the Box once heated will stay hot much longer. One concern is that when attempting to reload the Box, the coin will likely still be hot. It also means that the heat will be placed at locations for which the Box was not previously expecting it -- ie, outside of the design envelope. This also is not recommended as the specific locations where heat would accumulate were carefully mapped out for long term safe operation. Adding the coin has the effect of making these aspects less certain.
Therefore and in conclusion, the overall recommendation is to not use the coin. It may help with some characteristics of the heating (rapid onset, improved critical balance), but it does so at some risk to the batteries and with some other performance disadvantages as well (heat transfer in time and space; no visibility to the load, compromised airflow).
One alternative approach to achieve some of the same beneficial IR effects without any risk to the battery would be to add a thin copper foil reflector to the inside surface of the lid (or alternately, for us as the manufacturer to have make the lid completely out of copper/brass altogether). While the reflector position is somewhat less advantageous, it also does not interrupt the airflow path nor does it prevent the potential of stirring/shaking in the Box. While this approach was considered in the design, it also has the unfortunate effect of making the cover opaque -- people would not be able to see the load without opening the Box first. It was deemed preferable to have visibility -- especially when needed for shaking/stirring -- hence this aspect was not included in the build.
So therefore, the main question is this: how many people here would prefer to have a metal cover and not see the Box contents, knowing also that sometimes the cover could get a little warm to the touch?
-- Magic-Flight
