The lifespan of the battery according to available info is approximately 400 charging cycles. We typically replace them about once a year. As the battery ages it will still take a full charge but won't hold the charge as long. Instead of 12 hours of useable charge you only get 10, then 8, etc. Each time you place them in the charger and shut the lid you are running a charging cycle. I remind people who take them out and put them in the charger while they take a nap to not shut the lid each time, only once a day, usually overnight. They take about six hours to fully charge. Siemens (now called Signia) has a newer rechargeable product called the Cellion with a sealed Lithium Ion battery that will run a minimum of 24 hours on a single charge and should last at least three to five years.
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