Battery Rooms Ä a cellular home Richard Perez Everything needs a home. Electrochemical cells are not any different from anything else Ä they need a home. And like any home it needs to suit its inhabitants. Battery Housing Requirements Electrochemical cells allow us to store electric power in reversible chemical reactions. A collection of cells is called a battery. Cells have very specific housing requirements for safe, efficient, and long lived service. Providing a good home for your battery is not difficult. Building a containment for cells starts with understanding the battery's requirements. This information applies to small batteries located in battery boxes and large batteries housed in their own rooms. The battery containment is called a "room" here, regardless of the containment's size. The containment may or may not be within an existing building. Much of the info here comes from building a battery room here at Home Power Central. We now house 160 nickel-cadmium cells in their own room. The main system's battery has 150 cells and the radiotelephone system has 10 cells. We learned much during this experience. Location The battery usually has one specific location in the system where it functions best from an electrical view point. The rule is minimize the voltage loss in low voltage transmission lines. This means short low voltage wire runs and sizing the conductors properly. From a human point of view, locate the battery away from areas where you spend much time. While a battery can effectively store power for us, we don't need to invite it into the living room. The location of existing buildings and their ability to accept a battery room addition also determine the battery room's location. In our case, we built our 7.5 foot by 3.5 foot battery room as an addition to our existing building's west wall. While this located the battery further from the PV arrays, it worked better from a human standpoint. The battery and inverter rooms are as far as they can get from our high traffic areas. Security The materials used within electrochemical cells are highly reactive. If they weren't chemically aggressive, then these materials would not store enough power to use in cells. Cells are inherently chemically nasty and must have a secure home away from living things. Even a car battery can store enough power to be dangerous when short circuited. Many home power systems are now storing twenty times more power than a car battery. A short circuit can vaporize even a heavy wrench, burn the hand holding the wrench, and possibly touch off an explosion. Protect your family and environment by giving your battery a secure home. This means a locked box or a locked room. Mark the battery room door with a sign reading, "DANGER Ä Battery Charging Area Ä No Smoking." Children, animals, and casual observers must not be allowed easy access to your cells. Keep the battery room locked! Safety Equipment The following pieces of safety equipment must be inside, or next to, every battery room. This means even small systems with a battery in a small box. Fire Extinguisher Ä Use an extinguisher rated to handle both wood and electrical fires. Size the extinguisher to match your battery size. We use a First Alert model FE2A10. This unit is rated A (for wood fires), B (for liquid /grease fires), and C (electrical fires). It is a dry extinguisher containing 4.5 pounds of Foray(r) pressurized with nitrogen gas. It is ozone friendly and large enough to deal with a large battery room. It cost $32 at a local discount store. Smoke Alarm Ä Place a smoke detector in the battery compartment to warn you at the first sign of fire. Rubber Gloves Ä Use elbow length, industrial thickness rubber gloves. Don't fool around with lightweight dish washing gloves sold in supermarkets. Safety Glasses Ä Use safety goggles which fit around the face firmly and protect from splashes. These are under ten bucks at any hardware store. Neutralizing Agents Ä Use a chemical to neutralize any electrolyte spills. Neutralization means adding a chemical to the electrolyte that renders it relatively harmless and stops it from eating holes in the floor. If you are using lead-acid cells, then keep baking soda (sodium bicarbonate) on hand. It takes two pounds of baking soda (sodium bicarbonate) to neutralize a quart of sulfuric acid electrolyte. The most available, effective, and inexpensive neutralizing agent for alkaline cells (nickel-cadmium or nickel-iron) is Muriatic acid (a 31% solution of hydrochloric acid in water). It takes slightly less than one quart of Muriatic acid to neutralize one quart of spilled alkaline electrolyte. It takes over ten quarts of vinegar to neutralize a quart of alkaline electrolyte. Our local swimming pool supply store sells Muriatic acid for $13.95 for four gallons. Muriatic acid is dangerous in its own right and needs to be stored where it is safe. This means in unbreakable containers and locked away from children. Be careful not to add too much Muriatic acid to the spill, or you will have an acid spill instead of a caustic spill. Litmus Paper Ä Use litmus paper to determine if the electrolyte is really neutralized (pH 7). You can buy litmus paper at a well- equipped drug store or a chem supply house. Litmus paper turns a red color in an acid and a blue color in a base. Ventilation When recharging, all types of cells evolve a potentially explosive mixture of hydrogen and oxygen. All battery containments must be ventilated. If you are using a battery box inside an existing structure, then provide a ventilation tube to the outside. If you are building a battery room, then provide an exhaust vent at the top of one end of the room. This vent moves air outside the building and may be either passive or active. If you add a fan to exhaust the battery room's air, then use a fan with a sparkless motor. See Amanda Potter's Homebrew in this issue for a detailed discussion of Home Power's battery room fan setup. If the battery room has a door into the building, then arrange the venting so that the battery room sucks air from the building and exhausts this air to the outside. This assures that battery room vapoury wind up outside instead of inside your home. Temperature Electrochemical cells don't like it too cold or too hot. Ideally all the cells we are likely to use in a home power system want to operate at 78øF. If lead-acid cells are operated below 32øF, then they lose effective capacity and efficiency. While alkaline cells have a much wider operating temperature range (-50øF), they too are happiest at room temperatures. All cells also dislike operation at temperatures above 120øF. The bottom line is some degree of temperature control in a battery room. Insulation is a very good idea. Provide some form of winter heat input. In our case we insulated the exterior walls, floor, and ceiling of our battery room to R-19. We did not insulate the interior wall. Our main building loses heat into the battery room during the winter which keeps the cells warm. During the summer's heat, the cells have their own insulated room which is the coolest place in the building. There are over 3,000 pounds of cells in a very small insulated space. Our battery room contains a tremendous amount of thermal mass in relation to its highly insulated volume. If the battery room is a stand alone building, then consider solar heat. A superinsulated (R-40+) building with adequate solar access can keep lead-acid cells from freezing in most locations. The cells themselves act as thermal mass. Superinsulation keeps in the solar heat received through double glazed south facing windows. Since a battery shed is not large, neither is the price tag for the superinsulated building with a couple of high tech windows. Electrical Networking The reason we have assembled all these cells together is to store electrical power. Here we are at the mercy of Ohm's Law and topology. From the standpoint of networking electrons, we need to provide all cells with equally low resistance electrical paths. Working against cell electrical equality is the physical universe. If the battery contains many cells, then not all cells can possibly have the same length electron path (i.e. wire resistance). If you are not familiar with the series and parallel connection of cells, then see the article "Battery Basics" in HP#27, page 30. Series connection means connecting the negative pole of a cell to the positive pole of another cell. Parallel connection means connecting the positive pole to positive pole and negative pole to negative pole. Cells are connected by series wiring into strings to increase the voltage of the resulting string. For example, a lead acid cell has a nominal voltage of 2 Volts. By wiring six cells in series, we get a resulting battery with a voltage of 12 Volts (2 x 6 = 12). While adding cells increases the voltage of the battery, adding cells in parallel increase the capacity of the the resulting battery. Both series and parallel wiring techniques are used in most large batteries. For example, our battery here at Home Power is composed of 150 nicad cells. Each cell has a nominal voltage of 1.2 Volts. Each cell has an electrical capacity of 100 Ampere-hours. We wire ten cells into a series string to obtain a 12 Volt string. We make fifteen of these ten cell series strings and connect them in parallel to form a battery of 1,500 Ampere- hours at 12 Volts. Each cell has three physical dimensions and two electrical connections. In some types, like 6 Volt lead-acid batteries, several cells live in a single case. In every type, the case has specific dimensions and two electrical terminals. What we have here is a classical problem in network topology. And what's worse, it's different for every collection of cells. Our case is a classic example. We had to series connect ten cells to make a single 12 Volt string. The NIFE cells we are using come with factory made cell interconnect buss bars and nuts. This setup works only in one configuration, a line of cells each with its longest side flat against the next cell. This means that we had fifteen of these ten cell strings (about three feet long, eight inches wide, and one foot tall) to parallel connect. Add to this electrical networking topology problem the physical requirements of our existing building. The only side of the existing building available was the west side. The west side addition was limited to four feet in width by a large old Black Oak. Every application is different. What I am trying to communicate here are the design concepts involved in planning and building a battery containment. It is up to you to apply these concepts for your specific situation. Cell Configuration Here is a diagram that shows the basic configuration we use to connect our cells within our battery room. Please note that we designed the room as a walk in with cells along both sides. The resulting room offers just enough working space for one person. Ideally the cells should have been clustered as a block in the center of the room, but there just wasn't enough room to do this in our case. INSERT BATTERY ROOM DIAGRAMS Stair Step Racks Stair step racks make it easier to check the electrolyte levels and to service the cells. The stair step racks also provide vertical separation for the side by side strings of cells. This separation means less danger of short circuit if a tools is dropped on the cells. Battery Room Size The size of your battery determines the size of the room. If you are using two golf cart batteries, then the room could be a plastic picnic cooler. These coolers make excellent enclosures for small systems. A bigger battery requires a bigger room. Our battery room occupies just under 200 cubic feet of space. A little engineering on configuration of the cells and their connections will make a smaller enclosure. Be sure to allow enough room for maintaining the cells. Make sure that the floor can handle the weight of the battery (our weighs over 3,000 pounds). Buss Bar & Cables & Connections Copper buss bar makes networking large collections of cells easy and efficient. Each series string of cells is connected to the buss bar by short (less than 1.5 foot) #00 gauge copper cables with soldered ring terminals. The copper buss bar is 6 feet long, 1.25 inches wide and 1/4 inch thick. This size bar will just slide into 1.5 inch diameter plastic conduit. Our battery room used about 60 feet of copper buss bar to make the parallel connections. Our design called for walking through the middle of the battery. We used six #00 copper cables to connect the left banks of cells to the right bank of cells. These heavy cables run up the wall and across the ceiling. Every ring connector used in a battery room should be soldered to its cable. Anything else is just temporary. Power Processing Area When designing a battery room be sure to include an adjacent power processing area. Here the inverters, distribution panels, and controls can be close to the battery and away from humans. We ran the buss bars through the battery room wall and into the adjacent power processing room. Here the buss bars bolt directly to the DC power distribution center (in our case an Ananda Power Center IV). A happy home... A battery needs a secure, warm, low resistance home. After years of cells in our living room, I'm sure glad they now have their room. They are happier and so are we. Access Author: Richard Perez, c/o Home Power. POB 520, Ashland, OR 97520 ù 916-475-3179