An electronics firm produces calculators at a rate of 100 per week (5-day week and 50 weeks per year). Each calculator requires two nickel-cadmium batteries (10,000 per year total), which are also produced internally at a rate of 500 per day. Setup cost for battery production are estimated at $230 per run, and the production analyst estimates carrying and storage costs for each battery at $20 per unit-year. What is the most economical run length for batteries?