Having completed and glued together the battery holder to the prototyping board I have now created a functional working prototype. The cable from the E14 battery pack has been plugged into a micro USB breakout PCB to gain access to the power connections. I have not cut the micro USB plug off the battery pack so that I can still use it as a mobile phone charger should I want to at some time in the future. In order to make the battery pack work continuously a minimum current of approximately 50 mA must be taken and as the battery output is about 5 V this means a resistive load of 100 Ohms. As I do not have any 100 Ohm resistors I have used two 220 Ohm resistors in parallel which gives me the 100-ish Ohm resistance, with the added benefit of twice the heat dissipation. As this load has to be taken when in use then an ON/OFF switch is also required otherwise the battery would not last very long. The two resistors and the slide switch have been soldered together on a piece of stripboard which isn't that pretty but it is functional. The switched battery connection is then made directly to the input of the DC-DC convertor with soldered connections - mainly because the micro USB connector fell off when I was playing around with it - a drawback of surface mount micro USB connectors. The circuit diagram is shown below.
The working prototype is illustrated next. The output voltage can be varied from a low of 0.5V all the way up to 25.8V although I'm not sure of the maximum output current at these points. There is also going to be some power loss as heat in this circuit, especially in the two 220 Ohm resistors which become warm to the touch after only a few seconds. I will have to keep an eye on that issue.
One of the stick on rubber feet fell off the prototyping board as I was doing the filming; old glue and hot temperatures I expect. I'll have to glue it back on with better glue, or even with a small screw.
I have an objective of designing and 3D printing a casing for all the electronics on the top of the BattPackHack to make it look nicer but the DC-DC convertor is an awkward shape to cover so this might be delayed. I also want to add some sort of built-in voltmeter as well so I might concentrate on that rather than the 3D printed case. If the weather's nice I'll be out in the garden although it is 28 C today which is a bit too hot for me.