Using power generators to recharge Rungu in the backcountry
To recharge batteries away from AC mains (wall socket), you have several backcountry options – gas-powered generator, DC-DC solar panel generators and solar AC generators. Gas powered generators produce 120 VAC (US/Canada) at an acceptable power output to charge one or more Rungu batteries. There are DC-DC solar generators that convert solar power to electricity and can charge batteries directly. These kinds operate at lower power output and take longer to charge Rungu batteries. Solar AC generators are less efficient than DC-DC solar panel inverters. However, they have their own battery backup system, which can continue to charge batteries even at night.
Charging Rungu Li-Ion Batteries – primer
Fast-charging lithium ion batteries requires a lot of power. Rungu uses high-capacity Lithium Ion batteries and high-power chargers. Fully charging a dead battery with a wall socket takes less than two and a half hours. Lithium Ion battery chargers are special in that they charge in two stages. So unlike other battery chemistries, the charger power input (and output) varies over time. For Rungu Batteries, the charger input power averages 400W for the first two hours. Then input power drops below 100W in the last half hour. The total energy/AC power consumption to charge one battery is 910 Wh (or 0.91 kWh). Nominal capacity of a Rungu battery is 780 Wh. This means that Rungu chargers are 85% efficient. The charger loses 15% of the energy to heat when charging a Rungu battery. To make backcountry recharging options work at similar speed, generators need to output similar power to the charger.
Gas powered generators – great energy capacity, all-weather capable
Gas powered generators have the best energy capacity and most flexibility of the three options. A fifty pound, entry-level gas generator has a capacity of up to 5.76 kWh at rated load. That means 115 Wh/lb. Gas-powered generators operate in all conditions (rain or shine, day or night) unlike solar generator options.
Gas-powered generators can be loud, and they need enough gas to produce the power to meet your recharging needs. Though many generators use noise-suppression technologies, they make as much noise as an ATV when operating at rated load. Capacity to recharge batteries, on the other hand, depends on how close to rated load you operate the generator.
Battery charging capacity varies depends on power draw
Gas generator charging capacity varies depending on how much power you draw from the generator. As an example, the Honda EU2200i generator has a rated load of 1800W (15A @ 120VAC). According to the generator’s specifications, it has a 3.2 hour run-time per tankful at the rated load; so 3.2 hours times 1800W calculates to 5.76 kWh of charging capacity per tank of gas. The specifications also state that the generator will run for 8.1 hours per tankful at ¼ rated load. That means 8.1 hours times 450 W, which calculates to 3.645 kWh. That amounts to 37% less charging capacity per tankful if you operate the generator at ¼ rated load. So, if you draw less power than the rated load, you lose overall power capacity. This happens even though the generator will run longer on a tank of gas.
How to calculate the number of batteries a gas generator can charge
Gas generators charge batteries more efficiently the closer the total charging power is to the rated load. To fast charge a Rungu battery, the charger needs up to 400W of input AC power. The generator from the last example can charge four batteries simultaneously and be just below rated load. 400W x 4 chargers = 1,600W, which is 200W or 11% less than the 1,800W rated load. Fast charging a single Rungu battery requires 910 Wh of energy to complete a full battery recharge. The gas-generator output capacity required is 3.64 kW. At rated load, one tank of gas still has 2.1 kWh to spare after charging all four batteries. On the other hand, if you charge four batteries in sequence you may run through a full tank. The output required (3.64 kWh) is very close to the capacity available at ¼ rated load (3.645 kWh). The generator may run out of gas before completing the recharge of the last Rungu battery.
We recommend using these five steps to calculate the number of batteries you can charge with one tank of gas:
- Multiply the hours of operation for a tankful at rated load by the rated load of the generator. This is the high capacity estimate.
- Multiply the hours of operation for a tankful at the fractional rated load for the same generator from manufacturer specifications. That’s the low capacity estimate.
- Count the number of chargers you have and multiply them by 400W. If the power required exceeds the rated load of the generator, reduce the number of chargers to use at one time. Then recalculate so that the total power required is less than the rated load of the generator.
- Extrapolate the generator capacity or choose the high or low capacity estimate based on the power required in step 3.
- Take the capacity number from step 4 and divide it by 910Wh. The result is the estimated number of Rungu battery recharge cycles for the generator using one tank of gas.
DC-DC solar generators – the most efficient solar charging method
DC-DC solar generators and solar panels don’t make noise and require no gas to operate. These are big advantages if you have a lot of sun and crave quiet!
The big drawbacks of DC-DC solar panels are availability, solar panel size and the sunshine requirement. As noted above, Rungu batteries fast charge using up to 400W for a total of 910 Wh. Rungu can source DC-DC solar chargers. However there are no complete kits designed for charging Rungu batteries on the market. To build a DC-DC solar generator, you also have to source solar panels. The DC-DC charger is approximately 80% efficient. This means the converter needs the panels to generate 500W to get 400W for fast charging. 500W of solar power can require a large array of solar cells. Expect to source 35-40 sq. ft of the highest quality solar panels, which is a lot to carry. Batteries can also charge at half the power input to let you reduce panel size. However, charging the battery will take twice as long. Finally, you need the sun. Do not plan for a DC-DC solar generator to work if you expect rain, snow or clouds in the backcountry.
Solar AC generators and solar panels – the most practical solar combination
Solar AC generators come in two configurations – with and without a battery backup. AC generators without battery backup provide power only as long as the sun shines. AC Generators with battery backup continue to operate when the sun isn’t available. These generators are very quiet, though they may have cooling fans that produce some white noise.
Generators with battery backup have less energy capacity than gas generators. An example solar charging station with a 1000 Wh battery weighs 22 lbs. This calculates to an energy density of 45 Wh/lb. This is less than half the density of a gas generator. And that’s not counting the weight of the solar panels.
These generators still need sunlight most of the time. These generators need the sun to operate beyond the battery capacity (assuming a full charge to start). Some versions can be recharged from the 12 VDC output from a car’s cigarette lighter. Remember to keep the engine running to avoid fully draining your car battery.
Solar AC not as efficient as Solar DC
They aren’t as efficient as DC-DC solar generators. This means AC solar generators need more solar panels to get similar output to DC-DC generators. Power efficiency suffers because of multiple voltage conversions. For a battery backup version, it takes three conversion stages to charge a Rungu battery. First, the AC generator needs an internal DC-DC converter to charge its internal battery. Then, the generator has to convert internal battery power into AC power. Finally, the Rungu charger converts power back again to charge the Rungu battery. Each stage loses 5-20% of power to heat, so expect solar power conversion efficiencies in the 60-70% range.
For Solar AC generators, make sure that the output power for the plugs and capacity are adequate for the job. The Solar AC generator from the last example has nominal output power of 1,000W distributed over three sockets. Since the Rungu battery needs around 400 W for charging, the generator should accommodate two batteries simultaneously. But the battery inside the solar generator has 1,000 Wh and two batteries need 1,820 Wh. There is not enough capacity. To charge two batteries, the Solar AC generator will needs some help from the solar panels.
What’s best for you?
This blog evaluated three backcountry electricity generation alternatives for recharging Rungu Batteries. Gas-powered generators have high power capacity and don’t rely on the weather to operate. However they are noisy and they need gas. DC-DC solar chargers are the most efficient charging system. But they rely 100% on sunlight. And unless you have large solar panels, DC-DC generators can take much longer to charge a battery. Solar AC generators with battery backup are the inefficient middle ground. With a battery they have some ability to charge Rungu batteries if the weather’s not sunny. If it is sunny, they’re less efficient than DC-DC chargers. Solar AC generators will take longer to charge Rungu batteries than DC-DC generators.
If you still have questions about backcountry recharging, please email info@riderungu.com.
