DOUBLE THE FUN WHILE QRP
by KE4WMF
Project Corner,  September 1998

      Have you ever wondered how you can best conserve battery power while operating portable or QRP? I know I have. I primarily operate QRP/Bicycle Mobile (Riding the Airwaves, CQ VHF Feb 98), so battery conservation is a major concern. Many of today's newer handheld transceivers (HTs) run on 9.6 volts and are unable to accept 13.8 volts directly. Getting 9.6 volts into these HTs without the dedicated NiCad means using a DC-to-DC Adapter, a device that converts one voltage (13.8V) to a desired voltage (9.6V). The problem with even the most efficient adapters is that they consume power during operation, effectively reducing the duration of your battery. For example, my YAESU EDC-12 consumes an additional 20mA... even with the HT disconnected. Given that my YAESU FT-11R HT consumes a miserly 16mA in standby, over half of my battery's available power is being used to operate the adapter, effectively cutting my operating time in half! Even worse, the battery should be disconnected from the adapter when you are away from your rig or it will slowly discharge.

      There are options available for this problem. I'll cover three. Since most HTs can operate on as little as 4 volts, my first option is to operate directly from a 6-volt 4 amp-hour (Ah) gel cell (one came with my high-power bicycle lighting system). With 6 volts, transmitter power is reduced to 2 watts, and maximum current is 900mA, instead of 1.5A. In addition, the battery-draining adapter is eliminated. Four amp-hours is enough to modestly operate most HTs for days; so I'm happy with that kind of endurance. However, five watts is occasionally needed to use a distant repeater. For this, 12 volts and an adapter are required. This dilemma compelled me to find a way to have the best of both worlds - maximum HT power AND battery longevity; hence the next option.

      This option uses two 6-volt batteries instead of one 12-volt battery. Battery conservation begins with using the minimum power necessary to maintain communications, but continues by using the minimum voltage necessary to operate the radio. Using two 6-volt batteries is in keeping with this philosophy, but it allows 12 volts to be immediately available (for high power). To minimize expenses, I decided to use items already on hand: two 6-volt 4Ah gel cells, a YAESU EDC-12 adapter, a handful of DC coaxial jacks/plugs, and my current homebrewed HT mount. The HT mount holds all components except the gel cells, which fit snuggly in an under-seat pack. The idea is relatively simple: Create a system that allows convenient changing of the operating voltage from 6 volts to 12 volts. At first, I simply wired the batteries to have two independent 6-volt outputs and one 12-volt output (series-parallel basics). While I thought success was had, I decided I did not want to physically unplug and plug connectors to change the voltage supplied to the HT while I was riding my bicycle. Also, a method for bypassing the adapter is necessary while operating on 6 volts. I decided a switching network would be much more convenient and safer to operate while riding. Here's the plan: A double pole four throw (DP4T) switch is needed. Since size is important and small DP4T switches are hard to find, I opted to use two double pole double throw (DPDT) switches. Refer to the schematic for the explanation:

     For 5-watt/12-volt operation, both switches are placed in the "1" position. Switch S1 is used to switch the batteries from parallel to series. The S1B contacts shorting battery V1's negative terminal to V2's positive terminal does this. V1's positive terminal, which now has 12 volts, is routed to S2B via jack J1. S2 either includes or bypasses the DC-DC Adapter in the circuit. In the "1" position, voltage is sent to the DC-DC Adapter via S2B. V2's negative terminal is routed to the DC-DC Adapter via J2. The output of the DC-DC Adapter is sent the HT via S2A and J3. When economic 6-volt operation is desired (most of the time for me), place both switches in the "2" position. S1 places V1 and V2 in parallel. The positive terminals of V1 and V2 are routed to S2B. The negative terminals are routed directly to J3. With S2 in the "2" position, operating voltage bypasses the DC-DC Adapter and is routed to J3. The system is easy and convenient to use. The switches are mounted close to one another for simultaneous operation. Both switches have a center OFF position that helps prevent accidental battery discharge or voltages where they are not wanted (12 volts to the HT, or 6 volt to adapter). The switch handles can also be physically connected with plastic and epoxy for 2P4T action... further avoiding accidents. Fuse protection is abundant. With the vibration my connections receive while riding off road, the last thing I want is a short circuit that results in a fire under my seat! Another benefit of this setup is that switching from 12-volt series to 6-volt parallel DOUBLES my operating time (from 4Ah to 8Ah). With that kind of endurance available, I could operate on a bicycle tour for several days without recharging!

     Perhaps the most efficient option is to substitute the 6-volt gel cells in the schematic with 1.2-volt Nickel Metal Hydride (NiMH) "AA" batteries. I would've used this plan, but I didn't want a new investment. NiMH AAs cost about $3 each; plus I normally carry 6-volt batteries for my lighting system, anyway. In the schematic, V1 and V2 are each comprised of a bank of four or eight AAs in series. This makes 4.8 volts and 9.6 volts the available voltages... exactly the same voltages provided by the manufacturer's batteries! The DC-DC Adapter can be eliminated altogether since voltage step-down is not necessary, further increasing endurance. To make this change in the schematic, eliminate the DC-DC Adapter and S2. Then wire S1A-0 to the center conductor of J3. Finally, wire S1B to J3's shield. To charge the batteries, plug the manufacturer's charger into J3 and place S1 in the position appropriate to the output of your charger (4.8-volt use position "2" or 9.6-volt use position "1").

     Is this switching system overkill? Perhaps. But my endeavor is to make my bicycle mobile station as safe and efficient as possible. To illustrate the gains of my setup, I offer the following calculations: Using a 5-5-90 duty cycle (that's 5% transmit, 5% receive/signal, and 90% standby/squelched... the same figures used in manufacturers' ads), a 12-volt 4Ah gel cell would power my HT at 5 watts for 30 hours (total two-way talk time, or TT, is 3 hours, or 10% of total time). The same battery would power my HT at 1.5 watts for 40 hours. By eliminating the adapter and operating on 6 volts at 1.5 watts, that time is extended to 60 hours. Remember that when I switch to 6-volt parallel operation my battery size is doubled to 8Ah, further extending my time to 120 hours. The key to battery conservation while QRP is using a dual voltage setup. As stated earlier, most HTs can operate on as little as 4.8 volts. Why operate on 9.6 volts or 12 volts if the same results can be achieved with less voltage, especially while listening? By using two lower voltage batteries (either 4.8 volts or 6 volts), the benefits of parallel longevity and series maximum power can be had with no drawbacks other than the initial labor involved to make it work. The difference in size and weight between two 6-volt 4Ah gel cells and one 12-volt 4Ah gel cell is negligible, but the endurance gains are undeniable!

73,

Scott