My MTG900 says, "Charge" on the screen...I got it last Christmas, does this mean I need a new battery?

stick the mtg900 in a window sill for a day and that should charge it completely. If that does not work that means you will need a new capacitor. It might be easier at that point to buy a new or pre owned on ebay . they go for like 60 dollars or so.
even though they are solar they still need a battery to store the electricity needed to run the watch. A capacitor will need to be replaced from every 5 to 10 years.

I am assuming you are reading the battery charge correctly. If so you need a new rechargable battery. Special battery so it is a little expensive. Make sure the person knows it is a "rechargable battery" you want. A regular battery will not work properly or recharge.

I have a watch which is solar type and the instructions say that several hours of sunlight will charge the battery but lamps need much longer (1 -2 days!), my watch now is left in a drawer.

If you haven't got instructions for your watch, to check what Casio recommend, you can get them on the net.

You can get instructions from the Casio web site.

Go to the casio web site "casio.com".

Select "support" and the "manuals",
watches;

Accept terms

Select "watch manual archive"

Read the number of the back of your watch and enter it into the search box

CAPACITOR IN YOUR WATCH IS DEAD. VISIT YOUR NEAREST WATCH REPAIR SHO[PN AND ASK FOR CAPACITOR REPLACEMENT

If you go to "world.casio.com" you will see the alternative languages available. Unfortunately Portuguese is not listed.

All you need is to find the nearest watch repair shop. Ask them to replace capacitor (in case if your watch is light powered). If it's run on battery - replace by new one. It is very hard to guess what's the MTG900 is. Next time you are here, plz give:Make, model, ALL the writings on case back and every detail you know.

Thanks all! My G-Shock was only in need of charging in direct sunlight as opposed to indoor lighting.

The watch needs a new battery.

There are two three-bar displays on this watch.
The three in the top left will always show only one bar (or possibly no bars).  The top left indicates the level of charge in the battery and not the atomic receive function.
The three in the bottom right are related to receiving.  The atomic receive is controlled from settings.  With the time displayed, hold down the ADJUST button for several seconds.  Once the timezone name (e.g. LAX) starts flashing at the top, release the ADJUST button and press MODE once to get to the DST control. Press MODE again to get to the Atomic receive control.  Use the receive button to switch ON->OFF->ON... Press ADJUST twice to exit settings mode.