Using PV Panels to Recharge Small Nicads Richard Perez An appropriately sized photovoltaic panel can easily recharge those AA, C, or D nickel-cadmium cells. These "flashlight" sized batteries can be recharged hundreds of times and here's how to recharge them simply and cheaply using sunshine as the only power source. A quick look from the battery's point of view. The small nicads used in flashlights, radios, tape machines, and many other battery powered devices are made in standard sizes: AA, C, or D. The AA model in nickel-cadmium contains about 500 milliAmpere-hours (that's .5 Ampere-hours) of power storage. C cells contain about 2 Ampere-hours, while D cells hold 4 Ampere-hours of power. All of these nicad cells have the same voltage operating range- 1.0 to about 1.3 Volts (rated at 1.2 VDC nominal). These small, sintered-plate nicad cells are differently made from the large, pocket-plate nicads we've been discussing in Home Power recently. When I speak of nicads in this article, I am referring to small, sintered-plate nicads in AA, C, or D sized packages. I am not refering to the larger pocket-plate types. The standard recharging process for small sintered plate cells is to recharge them for 15 hours at the C/10 rate. C/10 means that the amount of current being fed the cell is one tenth of the cells capacity. For example, a C/10 rate for a AA cell is 50 mA. (that's .050 Amperes), the C/10 rate for a C cell is 200 mA. (that's .200 Amperes), and the C/10 rate for a D sized nicad is 400 mA. (that's .400 Amperes). This process is called constant current recharging. This is because the cell is fed a constant amount of current for a specified amount of time. Most nicad flashlight type cells use the C/10 rate for current and 15 hours as the recharge duration. Two, three,or more nicads can be charged in series. See the illustration below of series cells making up a battery. Since the battery is made up of only series cells, the amount of current flowing is the same for each cell. Thus, a series string of say, eight AA cells will be recharged at the same 50 mA. current rating as a single cell. The only difference is that one cell has a nominal voltage of 1.5 under recharging, while eight cells in series has a nominal voltage of 12 VDC while under charge. INSERT SERIES BATTERIES Well, if all the nicad cells really need is constant current, then we need to introduce them to PVs which are constant current devices. Photovoltaic cells as constant current rechargers. The amount of current (Amperage) that the cell produces is directly proportional to the cell's size (surface area). Small PV cells make small amounts of current and large PV cells make large amounts of current. They all, regardless of size, make this current at the SAME voltage, that is one-half of a Volt. See the article on concentrator PV panels in this issue for more techie data on the relationship between voltage, current, and sunshine in PV cells. Since the current (Amperage) output of a PV cell is determined by its size, it is possible to produce constant, limited, amount current by choosing the right sized PV cell. Just what we need to recharge nicads! Now the series interconnection scene with PV cells as with batteries. Elements added in series produce a higher voltage of the array, but at the same current. Since PVs are constant current devices, if a PV module is loaded at a lower voltage, then the voltage of the module drops to the load voltage (in this case, a nicad pack) and the current still remains the same. A marriage made in Heaven We went to work finding a suitable PV panel (that's a series connected set of PV cells) for recharging AA nicads. AA nicads require 50 MA. of current, so the PV cells used in the panel must have just enough area to produce about 50 mA. of current. This means small PV cells made into a small PV panel. Voltage is really immaterial, just as long as there is enough voltage to do the job. What we found was a PV panel of 36 series cells, each producing about 50 mA. The small PV panel will recharge anywhere from one to eight series AA nicad cells at the same time. Since the sun must be shining, it takes the JetSki about two days to recharge a AA nicad pack. The return of the JetSki These PV panels are encased in weatherproof plastic. Their exterior dimensions are 5.4 inches, by 3.5 inches by 1 inch. It comes with about three feet of flexible, color coded wire firmly attached to the back of the module. The JetSki PV is as cute as a bug's ears! These panels were originally made by Kyocera for a JetSki manufacturer, hence their rugged waterproof case and name. The PV cells used in the panel are the same polycrystalline type used in the famous J48 Kyocera modules. Same stuff, but the cells are much smaller. The JetSki has cells that measure .33 inches by .75 inches, while the J48s have cells that measure 4 inches by 4 inches. With 36 tiny PV cells in series, the JetSki is designed to work with 12 Volt systems. This means it can recharge between one and eight AA nicad cells in series. That's right, the very same charger will recharge one cell, two cells, three cellsÉ up to and including eight cells at the same time. No controls, and no adjustments necessary. And it takes the same amount of time, two sunny days regardless of the number of nicads in the series pack! That's what we get from introducing a battery that requires constant current recharging to a power source that is truly a constant current source- simple, reliable, and easy performance. INSERTJETSKIPHOTO The Recharging Process The JetSki comes with two perfectly good wires, one black and one red. The black wire is the PV panels negative connection and the red wire is positive. I soldered two alligator clips to the wire ends. I then inserted some empty AA cells in my Radio Shack AA battery holder (RS# 270-407 for $1.29), and clipped the PV panel to the battery pack. I have used this circuit to recharge everything from one to eight series AA cells. Note that the circuit doesn't use a blocking diode. It doesn't need one. The PV cells making up the JetSki panel don't leak enough current at night to be measurable by our very sensitive Fluke 87. Hence, no blocking diode needed. INSERTSCHEMATIC I propped the JetSki up in the sun using the battery holder as a stand. This worked out well since the JetSki shaded the nicads and kept them cool. I separated the nicads from the JetSki in the photo so you could see each element clearly. Now, the JetSki is the PV panel for recharging AA cells because it produces 50 mA. A perfect panel for C cells would produce 200 mA. The perfect PV panel for recharging D cells would produce 400 mA. The voltage of the panel is not important provided it has enough voltage to recharge the nicads. I like using a 12 Volt panel (that's 36 series PV cells) because it allows recharging of any number of series nicad up to and including eight. If a six Volt panel is used then only four series nicads can be recharged. I leave the nicads connected to the JetSki overnight and let them finish recharging the next day. The nicads really appreciate have their recharging cycle broken into tow segments with a night to cool off between recharging periods. If you space the cells out for an extra day and leave them under charge, no big deal. Most nicads can handle this gentle constant current overcharge with no problems. Conclusion This technique provides very trouble free nicad recharging. It can recharge any number of nicads from one to eight at the same time. No complicated or limited recharging apparatus needed. Just the right PV panel for the specific job at hand. And the whole setup is very portable, providing flashlight, radio, or what have you recharging in the field. JetSki PV Panel Access The JetSki PV panel is available from Electron Connection Ltd., POB 442, Medford, OR 97501 or call 916-475-3401. Cost is $20.00 shipped free UPS within the 48 states. Dealers wishing to stock this PV module should contact the JetSki's maker: Kyocera America, Inc., 8611 Balboa Ave, San Diego, CA 92123 or call 619-576-2647.