[quote name='Pijaibros']The battery that comes in the Dreamcast originally, is actually rechargable. I am able to get the old battery working again, simply by plugging the system in, turning it on for a few hours and leaving it to recharge the battery. If that doesn't work it's probably fried and I suggest you keep reading.
There is an internal battery that gives power to the memory in the Dreamcast. When the battery runs out, the DC forgets the date when you turn off.
It's the same type of battery that goes in the VMU. To replace it, open up your DC, look at the board at the front where the controllers plug in and you will se a battery. Get it out with a flathead screwdriver (may be a bit hard as it is sometimes glued on) and replace with a new one. Make sure the connections are still the right way round - you want the positive side facing away from the controller ports.[/quote]
A note on replacing that battery.
First of all the battery is a bit different from the VMU one. The VMU one is not a recharble so make sure you get one that recharges.
Second and most important is the fact that the battery is soldered in. And trust me, it is a mother

er to take out. I did everything to get it out, even heating the metal with a soldering iron did not work. I eventually pried it apart and one of the connectors that goes into the PCB got disconneced and I had to solder it in, which is tough to do with since the old solder does sucks and there is not much to stick to, even with fresh solder.
Also when I replaced it with a standard battery, the same as the VMU one, it really only works when it is plugged in. As soon as I unplug it it stops. And it is really finky. Some times even when it is plugged in it does not work.
So if you do decided to do this I suggest you first find a recharble 3v battery that fits there and figure out a good way to remove the old one. Also since the old battery was soldered in you have to find a way to keep the new battery in. I used tape but it does not work well. I would solder it but it is hard to be fast and precise when you can't keep a battery hot for long.