Batteryless Flashlights - The Good and the Bad
Best bet in an emergency
Hand-crank or shake flashlights provide illumination without the need to frequently wrangle with batteries - which means they are great to have in case of an emergency. Not all of them are strictly battery-free, though. Some use batteries to store the charge produced by the user. But at least there's no worry about frequently replacing the unit's power-storage device or waiting hours for a battery to fully charge. Waiting for a recharge could be downright impossible, if, for example, the emergency involves a power outage.
Planning ahead for a blackout no longer includes keeping a bulky rechargeable flashlight plugged in at all times. Throw a manual flashlight in a drawer or nightstand and power it up when it's needed.
Because of their long lives and lack of waste, crank and shake flashlights appeal to the environmental and money-conscious alike. Some manufacturers claim their brands last "forever." In reality, most flashlights with LED bulbs are expected to last at least 10,000 hours, (some even claim up to 100,000) or approximately 6 to 10 years, before they need to be replaced. That's still a significantly long time in the life of a flashlight. As for battery life, many contain storage devices that will never suffer the fate of becoming too weak to hold a charge.
One big downfall is the majority of shake-powered flashlights take about 30 seconds to power up, and once they are, they last for just a few minutes, sometimes 15 minutes before the light fades or dies altogether. Some shake or crank flashlights claim to last over an hour, but in that case a lot more manpower is needed. If extremely strong lighting is required, batteryless flashlights may not be the best choice. Ask if you can try out a "no-battery" flashlight before you make a purchase, or if you're guaranteed a return if you aren't satisfied.
In flashlights with no batteries, illumination can be less than stellar. As the charge is used up, the light often compensates for staying on by dimming. For basic needs, or for emergencies, they make a good back-up. Such a light can sit unused for years and almost immediately begin working again. On the other hand, a flashlight that runs on batteries can have them corrode if it's left unused for a long time.
A good batteryless flashlight can last as long as the frequency in which you use it. So it's safe to say that if it's a quality product and kept strictly for emergencies, it could last until the day you lose it.