There is a way you can no need to register again for an indonesia prepaid sim card but continue to use your existing sim card...
Normal validity period of a prepaid sim is 2 months, however you may top up the validity of the sim in singapore one day before expire.
There are two ways that maybe you can do...
First is to buy some prepaid top up card in Batam and keep. Key in the pin of the top up when in singapore. (as advised by one mobile phone shop owner)
Second is buy a prepaid sim top up in Lucky Plaza @Orchard Road in case top up cards all used up (should have sell telkomsel prepaid top-up card)
Correct me if I am wrong
