Bobier's LCB40 Sun Selector tested by Chris Greacen & Richard Perez The LCB40 (Linear Current Booster) is a high current electronic switching DC to DC converter which allows our 24 Volt wind generator to deliver full power to our 12 Volt battery pack. The LCB40 increased our wind generator's power output by over 50% compared with direct connection to the batteries. It can also work the same electronic magic on photovoltaic modules and microhydros. The Voltage Mismatch Blues The wind generator at Home Power is an Australian-made Survivor S5000 rated at 500 Watts, with 800 Watt maximum output. She uses a rectified, three phase permanent magnet alternator, designed to charge 24 Volt batteries. From September through March we ran the Survivor connected directly to our 12 Volt battery pack. This didn't hurt our batteries, or the wind generator. But it put the electrical brakes on her, forcing her to run slower than she would like. The wind generator produced her rated current, but with the voltage clamped, she couldn't spin fast enough to produce the power she was capable of. During these winter months her maximum output was 38.8 Amperes at 14.8 Volts = 574 Watts. It was a respectable amount, and a terrific boon when it was cloudy and the PVs weren't producing. Quest for the Holy Maximum Power Point We needed a machine which would allow the Survivor to run closer to her "maximum power point" Ð the combination of voltage and current which produces the greatest possible power. This was a job for a high current DC to DC converter (or Linear Current Booster - LCB). We chose the Sun Selector LCB40, a giant cousin to the LCB20 and LCB3-4-8 units widely used for solar water pumping and long distance power transmission. The LCB40 uses high speed power switching circuitry to efficiently convert high voltage, low current input to low voltage, high current output (up to 80 Amperes). You can think of it as a "DC transformer". Set Yer Own Power Point A knob on the front of the LCB40 lets the user set the input voltage threshold. Whenever the input voltage is reaches this threshold, the LCB converts the incoming power to lower voltage, higher current output. Whenever the input voltage is below this threshold, the LCB looks like an open switch, and no current flows. In this way, the user sets the "number of windings" of the LCB40 "DC transformer". The ideal is to tune the voltage threshold to the maximum power point of the power source. See page 34 in this issue for more on maximum power points. We knew the Survivor was built to charge 24 Volt battery packs, so her maximum power point is a little higher than 24 Volts, with considerable variance with windspeed. After installing the LCB40, we set the voltage threshold to near 27 Volts. When it's really windy we raise it to around 30 Volts. PV Examples Most folks will probably buy LCBs for low-loss, long-distance PV power transmission. Wiring panels in series wastes less power to resistance wiring. See "Long Distance Power Transmission" by Paul Cunningham in HP #28. How many panels in series? This is limited by the open circuit voltage of the string. Multiply the open circuit voltage (Voc ) printed on the back of each panel by the number of panels in series. This must not exceed the voltage rating of the LCB. LCB40s come in 50 Volt, 100 Volt, and 250 Volt models. A number of series strings may be wired in parallel as long as the final output from the LCB does not exceed 80 Amperes. Set the voltage threshold on the LCB to your best estimate of the maximum power voltage (Vpmax). For PVs, multiply Vpmax (printed on the back of each panel) times the number of panels in series to get Vpmax for the series string. If it's hot out, subtract 10% to 20%. (See "Home Power Measures PV Performance" for Vpmax at 50¡C for various panels, HP #24, p.26.) A properly adjusted LCB will help reduce line losses, and will cajole more power from your PVs. However, you probably won't see the 50% increase we see out of our wind generator. Our wind generator was originally operating far from its maximum power point, while your solar panels probably aren't. Battery Charging Voltage Regulator The LCB40 can be outfitted with a battery charge controller, making it the LCB40CC. The charge controller has a toggle switch for 12 or 24 Volt mode to match the nominal voltage of your battery pack. You can order a lead acid regulator (13 Volts -15 Volts in 12 Volt mode, 26 Volts - 30 Volts, 24 Volt mode) or an alkaline regulator (extended to 18 Volts in 12 Volt mode, 36 Volts in 24 mode). You can also choose three different regulator types. The ECM1 ($99) is a switching series regulator, which gives pulsed charges to the battery. The OVL regulator ($50) is a constant voltage regulator. Both the ECM1 and the OVL are for PVs only. For wind or hydro turbines, a charge diversion (shunt) regulator option will soon be available for around $100. Packaging, Documentation, and Installation The LCB40 arrived well packaged in nasty styrofoam peanuts. The documentation for the LCB40 is short but adequate. It tells you what the LCB40 does and what you need to know to install and operate the unit within its operating range. Installation is very straight-forward. The LCB40 is 10" x 11" x 3" and mounts on a wall with two screws. There are four wires to hook up, positive and negative to the battery, and positive and negative from the charging source. We praise Sun Selector for providing large electrical terminals. These terminals, however, are exposed from the front, and present an electrical shock danger. To conform to the NEC¨, the LCB would need to be encased in an additional metal enclosure. The Results We now have a pronounced increase power. During the windy winter months the Survivor put out a maximum 38.8 Amperes at 14.8 Volts = 574 Watts. In two months since we've installed the LCB40, we've seen a maximum current of 67.6 Amperes at 13.8 Volts (919.4 Watts). On a windy day, we regularly see currents in excess of 55 Amperes. In our situation, the LCB40 delivers 60 to 90% more current out than is put into it, as shown below in the graph. Input and output currents were measured simultaneously with Fluke 87 multimeters using 100 Amp, 100 mV shunts. INSERT GRAPH LCB40chartPICT caption: Current out verses current in for the LCB40 at two different voltage threshold settings. The current charged our nominal 12 Volt alkaline battery pack (13.5 to 16 VDC). With a producer as large as a wind generator, this makes a tremendous difference. We're now cooking extensively with the microwave, and occasionally running electric heat in the office. We know this is disgusting for a renewable energy system, but we've got lots of power. What to do? Soon, the hydrogen electrolyzers... Efficiency There are two different ways we can look at efficiency here. Most important to us is systemic efficiency. The wind generator/battery system is now at least 50% more efficient than it was when the wind generator was directly connected to the batteries. But with the wind generator operating closer to her maximum power point, how efficient is the LCB in transferring this power to the batteries? Even before we took measurements, we knew the LCB40 was efficient at its job because even when cycling maximum power, the unit was barely warm. Inefficiency in electronics shows up as heat. Efficiency was calculated as Ef =(Vout X Iout)/(Vin X Iin). Measurements were taken with three Fluke 87s and a Beckman 2020. As an individual component, the LCB40 is over 90% efficient in our application. The Bottom Line The LCB40 is a very flexible piece of equipment. The LCB will enable you to optimize the energy output of your RE power producers by operating at closer to their maximum power points. The LCB40's suggested retail price is $560 for a 50 Volt model, and the 100 Volt model's suggested price is $680. The 250 Volt model's price will be announced soon. They're not cheap, but consider this: we're getting 50% more power out of our $3,000 wind generator. In situations where a substantial power producer is already in place, adding an LCB can be by far the cheapest way of putting more electrons in your battery. The LCB40 is especially appropriate for: 1) Long distance power transmission in photovoltaic systems 2) Enabling 24 or 48 Volt, or higher, wind or hydro generators to provide more power for RE systems with lower battery voltages. Access Author: Chris Greacen, c/o Home Power, POB 130, Hornbrook, CA 96044 ¥ 916-475-3179 Maker: Bobier Electronics, Sun Selector, POB 1545, Parkersburg, WV 26101 ¥ 1-800-222-3988