I guess daylight savings time used to be about a 6-month interval. And if one uses general-purpose or heavy-duty battery I certainly wouldn't want to have them go beyond 6 months. But an alkaline battery is so close in price to the lesser types that it makes sense to be sure you have one in your smoke alarm.So pick either the start of daylight savings in the spring, or the end of daylight savings in the fall, and change them yearly.
Or, one might consider buying a lithium battery. Then it might be a good idea to wait for the smoke alarm to let you know when the battery is low. A lithium 9V battery will cost almost $10 and may very well last for 5-10 years depending on the smoke alarm. However, please at least test your smoke alarm every year even if you don't need to change the battery.
Tesla has a new size cell which they, and Panasonic who will actually manufacture it, say is the best size for energy density and manufacturing cost.Previously Tesla was using the ever-so-popular size 18650 cell. This cell was 18mm in diameter and 65mm length as denoted by the name. Their new cell, the 21-70, will be 21mm in diameter and 70mm length, following a similar naming convention.
Elon Musk says the battery will have the highest energy density and also be cheap relative to today's alternate offerings.
Everything is going very well at the battery Gigafactory and we believe quite strongly that SolarCity’s technology on the Silevo front added to Panasonic ‘s cell technology will make it the most efficient and ultimately the cheapest solar cell in the world – just as it is with the battery cell. We have the best cell in the world that is also the cheapest cell.
What will the energy density be? And the final cost? It's hard to say at this point. Stay tuned and as soon as we find out you'll know.
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