Long term use amd more to weekend projects.Thank you

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When I plug it in the light is on and steady and never changes, so how do I know when it is fully charged? It is the slide type battery for a multi-tool. I bought it used. I also have a 12 volt and the same thing happens w/ it. They both seem to charge but I hate to leave it on too long, I hear that is not good for it.

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