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My old 2008 Macbook’s power adapter appeared to die recently and since I was not sure if it was the adapter or the battery causing problems, I ordered a new generic battery along with a new power adaptor just in case.
After I received the new power adapter I discovered that the old adapter was indeed the problem, so I continued to use the original battery.
My question is, can I leave the new Lithium-Ion generic battery I bought in its sealed packaging until I need it? Will it degrade if left unused or should I swap them out once a week to keep the new one alive?
Li-ions self-discharge, although very slowly. If you leave it sitting around for a very long time (probably many months), it could discharge enough so that its “electronic fuse” will open. Once that happens, it is nothing more than a paperweight.
It probably came to you with a charge in the 40-60 percent range since that is the most stable charge level for storage.
If you are not going to use it, I would check it every month or so. If it drops below 20 percent, then charge it to bring it back up to around 60 percent or so before storing it again.
If it were me, I would switch to the new battery and keep the old one as a “just in case” spare. The new one should give your laptop a much longer running time.