Battery Technology Comparisons Richard Perez Electric vehicles and home power systems have batteries in common. Whether it's going down the road, or lights at night, the battery is where the energy is stored. It would be nice to recommend the ideal battery, but there isn't one. What we have instead is a confusing array of dissimilar choices. Here's a quick graphical look at how today's battery technologies stack up. The Players There are over twenty battery technologies that could be used in an electric vehicle. I narrowed the field down to eight current players. Three lead acid (LA) technologies, LA starting, LA Traction, and LA Gel are represented. Four alkaline technologies, nicad (Ni-Cd) sintered plate, nicad (Ni-Cd) pocket plate, nickel metal hydride (NiMH) sintered plate, and nickel iron (Ni-Fe) pocket plate, are represented. And one technology on the horizon, the sodium sulfur (Na-S) battery is also charted. Paying for a new battery If you go shopping for batteries based on price, then you will buy an automotive starting battery. Lead-acid starting batteries are cheapest because they are mass produced and designed for very light duty service. This battery has only one mission in life Ä starting a car. When you consider that the average EV will store over 20 kiloWatt-hours of power, then you understand why battery buyers are cost conscious. A 20 kWh battery made for LA starting batteries will cost about $1,400. The same battery composed of traction cells will cost about $2,000. The same battery using nicads will cost over $10,000. Or you could spend a whopping $80,000 for sodium sulfur cells. The chart below shows how the different batteries compare as to initial purchase price. The cost is given in dollars per kiloWatt-hour ($/kWh) of power stored. All prices used here are for new batteries. INSERT COST CHART Cycle life Both electric vehicle and home power service are brutal deep cycle punishment for batteries. Some batteries technologies survive thousands of daily deep discharge and recharge cycles. Other batteries only last a few hundred deep cycles before failing. The chart below shows how the different battery technologies accept deep cycling. Car starting batteries fail after less than several hundred deep cycles, while other technologies go on for several thousand cycles. INSERT CYCLE CHART The real costs Cheap batteries are false economy if they don't last very long. The chart below shows the cost of a battery in dollars per kiloWatt-hour per deep cycle ($/kWh/cycle). While the car starting battery was cheap to buy, we see that it is expensive to operate because it dies so quickly in both EV and home power service. In fact a lead acid starting battery is over three times more expensive to use in deep cycle service than a lead acid traction battery. INSERT Cost/CycleChart Energy density While home power folks don't really care how much their battery weighs, the subject is of intense interest to EV drivers. Battery size and weight must be kept to a minimum if an EV is to accelerate quickly and have a long range. The two charts below show how much power can be packed into a battery by weight and volume. INSERT TWO (BOTH) ENERGY DENSITY CHARTS Batteries and temperature Home power systems and EV drivers share a common interest in battery performance at low temperatures. Home power systems often keep their batteries in unconditioned spaces. EVs are parked in a variety of cold places during the winter. The same battery that powered everything just fine last summer, suddenly seems smaller due to the winter's cold. Almost every battery technology exhibits some apparent decrease in stored capacity when it gets cold. The only battery not charted here is the sodium-sulfur which must be kept at 300øF or more all the time, which is one reason (aside from astronomical price) for its limited popularity. INSERT TEMP CHART Bottom line time After a through study of these charts you will probably reach the same conclusions as Shari Prange did in her article on page 48 of this issue. Lead-acid traction batteries are currently the best deal for electric vehicles. They are inexpensive, long lasting, and have necessary high power density required EV service. In home power systems, the freedom from weight and space restraints give the user more selections in a battery technology. While the nickel cadmium and nickel iron technologies are still initially more expensive, they offer greater longevity and better low temperature performance. Buying batteries is a like getting married, both are not to be rushed into lightly... Access Author: Richard Perez, c/o Home Power, POB 520, Ashland, OR 97520 ù 916-475-3179