I have just joined the forums purely to help with techincal issues with the galaxy tab as ive repaired hundereds of them.
Regarding the red cross appearing on the battery sign. It will only appear when the charger is not making contact with the charging port on the tab itself.
in simple words what happens. the charger has copper connections and the charging port on the tab itself has copper connections. Over time what happens is that the connections on the TAB itself move out of their place, even though it might look straight to you but they have moved and when you insert the charger it doesnt make correct connection with the port.
NOW THE SOLUTION
First of all its not the fault of operating system. its a hardware problem. upgrading to ICS or any other ROM is not at fault.
There is only one solution to the PROBLEM. WHAT YOU NEED IS a NEW CHARGER. Then what you also need is a new CHARHING PORT RIBBON. it all comes as one unit. on ebay they sell for about £5 approx.
its a VERY VERY SIMPLE SOLUTION. now what you do is once you have the new charger and the charging port ( new), you will need to open the galaxy tab. AND BELIEVE ME ITS NOT HARD or SCARY. its very very simple. there are videos on you tube tell you how to do it. Once you have opened the, charging port and its ribbon are there openly. you simple unscrew 2 small screw, take the old one out and put the new one in. its very very simple. believe me.
once done put the cover back on and you will have a working tab again. thats the only solution to the problem. its happens because we put the charger in and take it out regularly and it then starts wearing itself out.
if you have any questions you can ask me. if you live in london then i can also help you out more.
thank you ... i am GALXY TAB RAMBO