In February of 2022 I began using my first prototype of a remotely powered receiving loop antenna system. The unit worked, but I could see that it needed improvement, and it possibly had a lot of potential. So in March 2022 it was time to start working on the next prototype, an antenna array system which will have a number of changes and new features. I am referring to this project as my ESOLA project, standing for Electrically Steerable Orthogonal Loop Array. This is part one. The entire project table of contents is below, including planned future articles.
- 1. A Compact High Current AA Battery Pack For Mobile or Outdoor Applications [This Article]
- 2. Charge & Discharge Battery Management Circuit
- 3. Wideband Loop Antenna Amplifier Design, Build & Test [being written now]
- 4. Loop Receiving Antenna Array Combinatorial Linear Switching Network [Future]
- 5. Loop Receiving Antenna Array Optimization, Testing and Performance [Future]
- 6. ESOLA Microprocessor-based operator interface [Future]
The first step is to build a 6.8V battery pack, one that is large enough for extended operation. It needs to be compact enough to fit inside this nice waterproof plastic box that I bought. AA NiMH batteries looked like the right choice for this, even though the project might have been done with Lithium Ion batteries such as the 18650 size. One reason was that the ideal voltage range for the RF transistors was easier to get (5.7-7.2V) with the NiMH batteries — a single Li-Ion battery wasn’t enough voltage, and two in series were too much.
With NiMH AA batteries, 4 parallel sets of 5 in series gives 8 AH (if you can believe the manufacturer’s 2000 maH rating, which I don’t) which at 1/2 amp drain will nominally run 16 hours, but with conservative charging and discharging, should give practical use of at least 8 hours. That’s 20 batteries, and with them in a very compact arrangement they could fit into the lid of the box. So, I ordered these battery holders:
These are terrible! I left a bad review for them. These are obviously meant FOR TOYS. The wire is so thin that they amount to a half ohm in series, so high current is impossible with these battery holders. Furthermore, the button end is a brass rivet, cheaply holding the thin red wire to the contact, and in most of these it is NOT STRAIGHT, such that the pressure of the anode spring is enough to “pop” the battery’s cathode off the button with the slightest vibration (with the exception of the singles, which hold the battery on the sides with the plastic shell). These can’t be used for any serious application requiring a secure battery connection.
Instead, I set about to try to design a battery pack using single sided 0.065 inch copper clad board and Keystone 590 battery clips, a part that I happened to have a bag of 100:
The clip isn’t perfectly suited to this application because there would be no holes for the contacts, so it needs modification to use on an undrilled PC board. If there were room, the contacts could just be bent at right angles and soldered that way, but the closest possible spacing was desirable, so I used wire cutters to cut off the contacts flush with the bottom. In addition, these contacts are not ideal for the cathode end, so it was necessary to remove the spring leaf for the cathode ends by gripping it with pliers and bending up — the metal fatigues and it snaps off easily leaving a flat surface perfect for the cathode button, having a “lip” which contributes to battery retention.
It’s important to measure carefully for the PCB planning, allowing for closest packing. As they say, “Measure twice, cut once.” The interconnection can be done almost entirely by cutting grooves in the PCB using a rotating tool such as a Dremel, so the first step was to mark the places to cut using a marker. Setting the distance between the clips is CRITICAL because that determines the holding force. By experimentation, I found the ideal distance and then cut a short brass strip to use as a template for mounting the clips. Soldering was tricky, because the clips had a tendency to walk away while waiting for the solder to wet their surface. The clip sides did better by running them over a piece of sandpaper before soldering, so that the solder would run up the metal quickly. Using tweezers to hold the clip snug against the brass template while soldering did the trick and gave very consistent results, as measured by the battery insertion force.
- Battery to battery width spacing: 0.565 inch (14.1 mm)
- Battery end-to-end spacing with clips, approximate: 2.6 inch (66 mm)
- Brass template for clip spacing: 2.084 inches (52.93 mm)
Below is the unfinished battery pack, showing the layout, the cutting marks, and the destination box lid. Each 5-battery series set is labeled 0-9 for the order in which the batteries stack, with 0 volts at “0” and 6.8V at “9”. The odd numbers are the cathodes, and they need the clips which have the spring bent off.
Finally, here is the completed unit. In building this, I had to use two boards since my raw PCB sizes were not large enough. The two boards are splinted together with some short brass strips soldered to both at three points. Completing the connections to join the 4 sets of batteries in parallel required two extra wires.
Overall, this took quite a bit of manual work trimming the battery clips and doing the precision soldering, but I enjoyed it a lot. Sometimes doing things the hard way is the most satisfying. This home built battery pack has a secure grip on these AA-size batteries and the unit is as compact as possible. The next part of the project will be to design, build and test the battery management circuit.