Like most SOTA activators I started with a 3S style pack to power my radio’s. These packs are a great choice, affordable and light weight. They are however a little harder to pack and carry for aircraft travel and require a specialist (balance) charger when recharging thus adding bulk to any extended holiday plans including SOTA or Parks.
Two things caused me to re-visit the options around battery packs, firstly the KX2 uses a 3 cell Li-ion pack which is difficult to locate an equivalent for in VK, and the failure of my 3S style pack.
After the usual amount of consideration, I felt there were two options that fitted my expectations.
The first one would use a bog-standard USB battery bank with an external DC-DC inverter. These inverters (or Boost regulators) are commonly available. In the interest of getting a unit with a known performance I went for a named brand (Pololu S18V20AHV) rather than selecting something from ebay. When assessing the required parameters, I settled on an output of 10v. This was primarily to simplify the math, the ATS and MTR will happily run on 10 volts and when considering the available current, it was just a matter of 5V@2A = 10W so at 10V I could draw a maximum of 1A from the standard 2A outlet of the USB bank. This comfortably covers the key down current of around 600mAh of the MTR or ATS
The second choice centred around the 18650 cell, these have great capacity, using 3 would allow me to run all of my portable rigs not just the MTR and ATS. The solution for transporting the loose cells was overcome when I found a single cell USB battery bank which was a BYO battery version from ebay. By using three of these I could simply pop the loose cells into their own battery bank for transport. Thus making them much safer to carry, less likely to attract attention at luggage check-in plus would allow me to charge the cells from a commonly available usb charging socket, or even use them as back up power for the phone etc.
I built the DC-DC version first, during build I discovered some battery banks would switch off as the standby current of the ATS and MTR were simply too low to hold the output on. The solution I chose was to wire a blinking LED across the 5V input of the boost regulator, the additional current was low and it provided a simple indication of supply volts (i.e. I had some). I provided no fusing as I expected the battery bank to look after over-current and it is very unlikely any fault with the rig would cause excessive current draw. Performance in the field was fine, although I did note an increased noise on 10Mz due to the switch mode inverter.
The 3 x 18650 cell version provided different challenges whilst in theory these cells are a standard size, not all that are sold meet the 65mm length as the protected version are often longer. So I approach this build by starting with the 3xCell holder and then visiting my local battery supplier and purchasing 3 of their normal 18650 (unprotected) cells after checking they fitted comfortably. Of course, the maximum current of a 2600mAh Li-Ion cell is very high so I included over current protection in the form of a simple blade fuse. I considered other options, but I wanted something that had a manual reset and a fuse that was readily available. To keep size down I used a mini blade, in line fuses holders are quite bulky when compared to the rest of the gear so I used a 2.5mm female “phone type” crimp (Jaycar Cat# PT4520) to make the connections.
Whilst the ATS and MTR have internal voltmeters some of my other rigs do not and nobody wants to run their Li-Ion cells below the recommended minimum voltage. To overcome these short falls with the 3xcell version I built the DC Beeper kit from Jackson Harbor. This happily protects under-voltage, allows me to check the current voltage plus has the timed off function which I felt was a handy bonus.
So which do I prefer, well the USB bank version is very neat but lacks the current to run the higher powered rigs (eg KX2) and is a little noisy on some bands. The 3×18650 has a few more wires and plugs, not as robust for travel but will happily run the KX2 with its higher output, ~11V v’s 10V and 2A v’s 1A. However, both have more than enough capacity to run the ATS and MTR which needs around 300mAh of capacity to achieve one hour of activation. As a bonus I also purchased a Nitecore LC10 single cell charger in addition to charging a single 18650 (or other sizes) it can also be used “backwards” to become a usb battery bank to recharge a phone etc.