It would be wise to remove the batteries, if that's an option.

Thank you. Remove the batteries, then how to charge ?

Some devices run plugged in to charge without a battery and some do not.

I have a phone that I turned into a clock with an app I wrote. I've had it plugged in continuously for the last 5+ years and I've never had a problem. I bought it on a whim for 20 bux and had no use for it so I turned it into a clock soon after I bought it. I've never used it as a phone. Every so often, when the clock time falls out of sync, I connect it to my wifi network to sync but other than that it stays connected. I have it on airplane mode since the phone starts to request that I activate it otherwise.