Raymarine Raystar 125 battery replacement
If you have Raymarine gear like C or E Classic series and your GPS takes a long time to get a fix, then you likely need to replace the GPS unit's internal battery. This is a very easy job. Don't fall for Raymarine's line that you need to buy a new RS130 unit along with converter etc.
First, confirm you don't have any other problems, like a totally dead GPS unit. On top of the GPS (what a lot of people think is the antenna) is a tri-color LED. In the manual you can see what the different blinks mean. Slow blink yellow means no fix. Blinking green means it has a fix. Slow or fast blinking green is to signify different connection, nmea or seatalk, can't remember which. In either case, if your MFD shows no fix and the GPS LED shows no fix, then after a period of time it gets a fix and both the MFD and the LED on the GPS agree, then it is working. I don't know what red LED means but it's probably not good.
gTo replace the battery you need to unmount the GPS, again, what most people think is the antenna, but I can assure you it is the GPS. You can get manuals on Raymarine's site but I wasn't able to find the manual for the RS125. Mine is pole mounted so I just unscrewed the two Phillips screws that are accessible through the pole mount. Then I disconnected the cable.
Bring the GPS inside somewhere with decent light and a work surface where you won't lose little screws. Get a small phillips screwdriver and a multimeter if you have one. On the bottom of the unit there is a foam gasket. It has a fairly heavy plastic base on the foam. DO NOT PEEL AWAY JUST THE FOAM - peel away the foam ALONG WITH the heavy (clear) plastic. It should come off the GPS including the adhesive leaving the GPS not sticky.
There are 6 Phillips head screws holding the GPS together. Take out those screws and the GPS should come apart. The battery is in a slot on the board. You can check it with your multimeter. Mine was flat 0V. Replace with a new CR2032 positive side up as shown by the + sign on the battery slot.
Put back together carefully. Note, there is an o-ring that keeps the unit sealed. I had trouble with the o-ring slipping under the board. You don't want that. I had to put a tiny bit of teflon grease on the inside of the GPS cover where it contacts the o-ring to allow it to slip together easily. Make sure the board is all the way down and the o-ring is not trapped under the board at all, even a little will make it impossible to put back together correctly. Also note there are alignment marks on the top cover and the base - a small arrow on the base and a dot on the cover. This is so the LED lens lines up with the LED.
Hold everything together and put the 6 screws back in. The base should be flush inside the cover. If not, the o-ring is probably out of position.