Global class copyWarmAccounts implements Database.Batchable<Sobject>
{
    Global Database.QueryLocator start(Database.BatchableContext bContext)
    {
        string query = 'Select name, id, rating from account';
        return database.getQueryLocator(query);
    }
    Global void Execute(Database.BatchableContext bContext, list<Account> batch)
    {
        list<warm_accounts__c> lstwac = new list<warm_accounts__c>();
        for(Account acc : batch)
        {
                  if(acc.rating == 'warm')
                {
                           Warm_accounts__c wac = new Warm_accounts__c();
                        wac.name = acc.name;
                        wac.Rating__c = acc.Rating;
                        wac.Id = acc.Id;
                        lstwac.add(wac);
                 }
         }
        insert lstwac;
    }
    Global void finish(Database.BatchableContext bContext)
    {
        //do nothing
    }
}