if enough water is lost, the cell fails. Button cells have a limited current drain; for example an IEC PR44 cell has a capacity of 600 milliamp-hours (mAh) but a maximum current of only 22 milliamps (mA). Pulse load currents can be much higher since some oxygen remains in the cell between pulses. Low temperature reduces primary cell capacity but the effect is small for low drains. A cell may deliver 80% of its capacity if discharged over 300 hours at , but only 20% of capacity if discharged at a 50-hour rate at that temperature. Lower temperature also reduces cell voltage. Cell types Primary (non-rechargeable) Large zinc–air batteries, with capacities up to 2,000 ampere–hours per cell, are used to power navigation instruments and marker lights, oceanographic experiments and railway signals. Primary cells are made in button format to about 1 Ah. Prismatic shapes for portable devices are manufactured with capacities between 5 and 30 Ah. Hybrid cell cathodes include manganese dioxide to allow high peak currents. Button cells are highly effective, but it is difficult to extend the same construction to larger sizes due to air diffusion performance, heat dissipation, and leakage problems. Prismatic and cylindrical cell designs address these problems. Stacking prismatic cells requires air channels in the battery and may require a fan to force air through the stack. Secondary (rechargeable) Rechargeable zinc–air cells require zinc precipitation from the water-based electrolyte to be closely controlled. Challenges include dendrite formation, non-uniform zinc dissolution, and limited solubility in electrolytes. Electrically reversing the reaction at a bi-functional air cathode, to liberate oxygen from discharge reaction products, is difficult; membranes tested to date have low overall efficiency. Charging voltage is much higher than discharge voltage, producing cycle energy efficiency as low as 50%. Providing charge and discharge functions by separate uni-functional cathodes increases cell size, weight and complexity. A satisfactory electrically recharged system potentially offers low material cost and high specific energy. As of 2014, only one company has commercial units for sale, as described in a Dept. of Energy produced video at the ARPA-e Energy Innovation Summit in 2013. Fluidic Energy has apparently covered hundreds of thousands of outages in Asia at distributed critical load sites. EOS Energy Storage has deployed a 1MWh system for a microgrid at a New Jersey wastewater treatment plant and has previously tested grid-scale backup applications. AZA Battery has announced development of pilot production of