What does the low battery indicator 'D' mean and what should I do?

When the LCD display shows condition 'D' (Inadequate), the battery voltage is below 3.6V and the alarm gives a low battery warning at least once per minute. Replace the batteries with new LR6 alkaline batteries immediately. Even after the first low voltage warning, the alarm can still function for more than 7 days, but replacement is necessary for continued protection.