Generation 1 base station (no ethernet ports) blue light out but alarm works. Where is the rechargeable battery that may need replacing?

I see there have been blue light questions posed but no direct answer to explain it. I don't want to reset my base station then have to redefine all of my devices. I would rather check if the rechargeable battery is still good. I have seen three responses on how to reset the base station. Press the reset button for one second, press the reset button quickly but hold #5 button and press the reset button long. I would appreciate an explanation for each of these methods. What is the difference between them? I would appreciate someone answering why the blue light goes off? What causes this? I have unplugged the base station, counted to five, then replugged it in. No change. I moved the power cord and straightened its wire in case the copper inside was starting to break. No change. I took off the base from its wall plate but see no door to open to check the rechargeable battery. Please someone knowledgeable help me troubleshoot this! Understanding why the blue light goes out should help solve this.