Recharging Batteries With A Gas Generator by Windy Dankoff Many photovoltaic systems begin as gas generator charged battery systems. As the system evolves with your needs and finances, the generator may be semiretired and used only for occasional back up. Either way, an efficient system must be used to deliver DC power to charge the batteries. There are three good methods of charging batteries from a generator. DC GENERATOR A gasoline powered DC generator/alternator is a direct approach for battery charging. None are available ready made to the best of my knowledge. Many people build their own using a common gasoline engine of 3 to 5 HP belted to a car or truck alternator. Get an alternator WITHOUT a built in regulator (do not use an auto regulator). A 5 HP engine will drive a 100 Amp alternator to full power (reduced some at high altitudes). Use a rheostat (25 ohm, 25 watt) to regulate the current manually. See Home Power #2, pgs. 23 through 26, for details on construction of this type of unit. AC GENERATOR with BATTERY CHARGER This is the most popular method because AC generators are so common and versatile. Many of our customers run an AC generator periodically to pump their water, wash their clothes or run tools. At the same time a battery charger is plugged in to make best use of the generator's capacity by charging their batteries. LP gas is a preferable fuel to gasoline, for cleaner burning and easier starting. Slower, 1800 RPM generators are longer lasting and more efficient than faster 3600 RPM models, but are heavier and more expensive. Many AC generators have 12 Volt charging capability but its current capacity in Amperes is small. The 12V output and the output of a battery charger may BOTH be connected to the battery bank at the same time! WELDER/GENERATOR A "portable welder" is a generator built to supply arc welding power. A welder puts out approximately 30 volts, at a current of 200 Amps or more. Perfect for charging a large 24 volt battery bank! On a low setting it will charge at 12 volts. If you KNOW you need to rely largely on your generator (like if you live in Alaska) or if you like to weld AND have a large battery bank (at least 1000 Amp hour) this may be the best way to go. Most portable welders also have AC output for tools etc. Get a DC welder. If you already have an AC welder, a rectifier may be built to convert to DC. BATTERY CHARGERS Battery chargers are common devices, available everywhere. Here are a few things to know about them: (A) Any automotive charger can charge a very large battery bank; it only "sees" the voltage, not the battery's Ampere-hour capacity. If the battery is very large, it will take longer to recharge. (B) More than one charger may be connected at once, and so can other charge sources such as PV. Each regulator responds as battery voltage rises, regardless of the charge source. (C) Automatic (regulated) battery chargers tend to regulate too soon for the fast charging that is desirable with a generator AND they may regulate to a low current suited to a small (car) battery. Buy a "manual" charger, or switch your charger to manual. If you will be charging from utility power, use an automatic charger. (D) Battery chargers are expensive. They contain only one expensive part though, the transformer. Transformers rarely fail. Big garage type battery chargers may often be found in scrap metal yards, beat to hell by such abuse as someone driving away with the charger connected. A little rewiring and replacement of small parts will almost always restore a charger to reliable use, however ugly. (E) Truck and industrial battery chargers are available for 24 volt charging. If you're a junkie, check scrap yards. (F) Two 12 volt chargers of equal capacity can charge 24 volts by connecting each one to each half of the bank. (G) Industrial electric vehicle chargers are of higher quality and efficiency than automotive, but are expensive. INVERTERS WITH "STANDBY" BATTERY CHARGING OPTION Trace Engineering and Heart Interface inverters have an optional battery charging function, which works well. This option costs far less than a separate battery charger of equivalent capacity. This inverter's transformer is used "backwards" to step down the voltage. (Remember, the transformer is the expensive part of a battery charger). The Trace inverters are particularly good in this regard because of their "programmable" regulating response, which you may set optimally for your particular system. Trace features an additional advantage, although not yet clearly documented....charging is accomplished thru pulses of very high current which slightly vibrate the battery plates and knock off inactive sulphate crystals. This tends to restore some capacity in older batteries, and to extend battery life. To obtain the full effect, you need a generator with at least double the wattage of the inverter, and high (over 165 vac PEP) peak voltage output. FAST CHARGING Beware.... service station attendants and many auto mechanics do not know what it MEANS to fully recharge a battery. Less than an hour of high current charge on a dead car battery will allow a car to start, but cannot charge the battery anywhere near full. If you take batteries to town to have them charged, be sure they are kept on the charger at least 8 hours. A hydrometer reading is the best way to assure that they are "topped off". Failure to top off or "finish charge" batteries at least every two weeks will reduce their life greatly. Beware of excessive gassing when fast charging.... it's hard on the batteries and creates a very real explosion hazard! Don't make a habit of it. Photovoltaic CHARGING AS SUPPLEMENT TO GAS CHARGING Finish charging, as mentioned, is extremely important to long battery life. Any cell that tends to lag slightly behind the others will be brought up to full, instead of falling further behind. This requires long periods of low current "trickle" charging, which is very wasteful use of a generator BUT perfect use of a small PV array. Although a minimal PV array may contribute relatively little energy to a generator system, it will pay for itself by extending battery life. If your generator system is used only seasonally, a small PV module should be used to maintain your battery bank at full charge during the months you are gone. A typical deep cycle battery (in GOOD shape) will discharge half way in 6 months, and sitting in a low state of charge is VERY DAMAGING. Whether you consider your fossil fuel generator to be a luxury, a convenience, or a necessary evil, making the best interface with your battery bank deserves your careful attention. Windy Dankoff is the owner of WINDLIGHT WORKSHOP and FLOWLIGHT SOLAR PUMPS, POB 548, Santa crux, NM 87567 (505) 753-9699. Windlight's 1988 CATALOG AND HANDBOOK is available for $6. For information on DC well and booster pumps, inquire.