Lino VK3EI Portable Power
Getting sufficient power when portable is a challenge at the best of times. It is nice to be able to use 100W when you need it but this has to be balanced against battery life. While we are all used to carrying Sealed Lead-Acid (SLA) batteries around when we are portable, we have to be judicious in our use of high power if we want to have any sort of reasonable operating time before necessitating a recharge.
Recently, I came across a new (to me, anyway!) battery technology called Lithium Polymer (LiPo), which is used extensively in the remote control of model planes and cars.
It did not take long to be convinced that the LiPo battery was worth experimenting with for portable power use.
What eventuated was a battery pack that has turned out to be a very significant improvement over the SLA solution that I had been using.
Figure 1 – Completed battery case.
Figure 1 shows the completed battery pack in the closed position. I used a small plastic instrument carry case that just happened to be exactly the right size for four LiPo packs that I had chosen to use.
Inside the case, I have fitted four cell packs, each rated at 5 Ah with a nominal voltage of 14.4V. I say “nominal” because the storage cell voltage of a LiPo is 3.7V but the fully charged cell voltage is 4.2V.
By placing four of these packs in the case, I achieved a total capacity of 20Ah at 16.8V (Figure 2).
Figure 2 – Carrying case neatly holds four 5Ah cells.
The packs come with polarized connectors that are common in the RC model market, however I have converted all of my equipment to Anderson Powerpole connectors so the same connectors were fitted to the LiPo packs.
An interesting characteristic of LiPo batteries is their phenomenal discharge current capabilities. My assembled pack is capable of delivering a continuous discharge current of 20 times the rated capacity or 20 x 20Ah, that’s 400 Amps!
The impressive part of this discharge characteristic is that the voltage remains very flat during discharge under full load. Referring to the typical LiPo curves in Figure 3, you can readily see how the curves stay flat until the battery is essentially fully discharged whereupon the voltage drops off very quickly.
Figure 3 – Typical LiPo discharge characteristics.
While the discharge rate will lower the cell voltage, the curve shape is consistent with varying loads.
To achieve a total desired capacity it is a simple matter of paralleling the appropriate number of battery packs. You will notice that the LiPo battery pack has the high current leads fitted with the Anderson plugs and a separate multi-wire lead terminated in a white connector; this is the charge balance connector. During the charging process, the special LiPo battery charger uses this connector to monitor the voltage on each individual cell in the pack to ensure that they all receive an equal charge.
To make up the total battery capacity, you simply connect all of the packs in parallel. It is also necessary to parallel all of the charge sense wires so that all of the packs can be charged at the same time from a single charger.
To connect the packs together, I made up a couple of adapters. The high current leads of each pack are connected together with the adapter shown in Figure 4.
Figure 4 – Multi-headed adapter that connects all the cells in parallel.
This adapter is simply five sets of Anderson plugs wired in parallel with heavy gauge wire – I used 5mm2 cross-section copper wire. The second adapter (Figure 5) consists of four 5-pin connectors (also wired in parallel on a 100mm strip of Veroboard and covered with shrink tube) that mate with the white charge balance cable on each pack. A lead with the same charge balance type of connector is wired in parallel to enable all of these packs to be connected to the mating charge balance connector on the charger.
Figure 5 – Adapter to allow the charge balance connectors to be paralleled.
The carry case has sufficient space in the top to allow all the cables to fit and remain connected. Figure 6 shows the completed package with all cells in place and connected together with the adapter cables. With lid closed everything is securely held in place and the batteries are well protected from accidental damage.
If we compare the LiPo battery pack discussed here with the 115Wh that we can get from a similar size SLA battery, we get the following:
16V (nominal) x 20Ah (total pack capacity) x 0.5 (DOD) = 160Wh.
That is almost 50% more energy from the LiPo pack and, in addition, we get a better (flatter) discharge characteristic in a very much lighter package. The LiPo battery pack weighs in at only 2.7 kg compared to 8.5 kg for the 24 Ah SLA battery.
There is an issue with the higher output voltage of 16.8V compared to SLA batteries. Most radio equipment is rated to run at 12V nominal (meaning 13.8V) but a quick check of my radio specifications showed that the upper limit is 15% higher, or approximately 15.8V. The LiPo pack gives 1V higher than this upper limit and pushes it outside the manufacturer’s specifications.
I decided to leave it at the higher voltage because I reasoned that many radios used in mobile operations are used in vehicles where they are subjected to voltage surges and spikes that are probably a lot higher than 15.8V.
Figure 6 – All components in place and connected together.
The LiPo pack has been used on many occasions with five different radios (FT-857D, IC-706Mk2G, TS-50, IC-718, FT-897D) with no problems. The advantage with this pack is that I can easy operate at full power for quite reasonable periods. The average radio current draw on receive is about 1A so this gives me 5-6 hours of continuous operation from a single charge with about an hour of talk-time at 50+ watts output.
I am very pleased with the new addition to my mobile operations kit as I now have a reliable power source that lasts, is easy to carry around and, if necessary, can be recharged in less than 2 hours from my car, a solar panel, or any other source of power.