UNO + datalogging module (DS1307) - RTC issue

Hi,

i bought arduino Uno and datalogging module (SD card and RTC(DS1307) in one module). According documentation my module should comunicate with Arduino Uno using SPI (D10,D11,D12,D13) - SD card and I2C (A4,A5) - RTC.
I am able to write and read to/from SD card but i am not able to write/read RTC. I have read a few topics regarding DS1307 issues and i tried to fix it. Here is what i found/tried:

HW
I measured voltages on my DS1307 pins.

Ubat = 3,03V, Ucc = 4,45V

In DS1307 datasheet is written that it is not possible to read/write if Ucc < 1,25Ubat. In my case Ucc limit should be __1,253,03V = 3,79V__. I guess i should be able to read/write RTC because 4,45V > 3,79V.

On the other hand there are written recommended operation conditions and supply voltage (Ucc) should be at least 4,5V. And 4,45V < 4,5V. BTW power LED on datalogging module shines red.

Datashett DS1307

Do you think that my problem is low Ucc?
If yes, what should i do? How can i increase supply voltage on ready-to-use module?

SW

I tried to read/ write to RTC using this code (i tried enable/disable oscilator - write to RTC but result of all registers was 0 = i was not able to write to RTC). I found the code somewhere on this forum:

// DS1307 I2C Read and Write Timekeeper Registers 0x00-0X07

#include "Wire.h"
#define DS1307_I2C_ADDRESS 0x68
#define startRegister 0x00
#define endRegister 0x07
//data storage registers 0X08-0X3F

void setup()
{
  Wire.begin();
  Serial.begin(9600);
  Serial.print("Register");
  Serial.print("\t");
  Serial.println("Bit Values");
  Serial.println();

 // writeNVRAM(0x00, B00000000);//enable oscillator and set seconds to 0
 //writeNVRAM(0x00, B10000000);//disable oscillator set CH bit

  for (int a = startRegister; a <= endRegister; a++)
  {
    byte b = readNVRAM(a);
    Serial.print("0X");
    if (a < 16)
      Serial.print("0");
    Serial.print(a, HEX);
    Serial.print("\t");
    Serial.print("\t");


    for (int i = 7; i >= 0; i-- )
    {
      Serial.print((b >> i) & 0X01);//shift and select first bit, no speed advantage
    }
    Serial.println();
  }
}

void writeNVRAM(byte location, byte data)
// writes data to DS1307 NVRAM location
{
  Wire.beginTransmission(DS1307_I2C_ADDRESS);
  Wire.write(location);
  Wire.write(data);
  Wire.endTransmission();
}

byte readNVRAM(byte location)//// reads data from DS1307 NVRAM location
{
  Wire.beginTransmission(DS1307_I2C_ADDRESS);
  Wire.write(location);
  Wire.endTransmission();
  Wire.requestFrom(DS1307_I2C_ADDRESS, 1);
  Wire.read();
}

void loop() {
}

Can be my problem I2C?
How can i test that I2C communication between UNO and datalogging module works?
How can i know that commands Wire.Write or Wire.Read works? Is possible to get some error code in case that Wire.Write or Wire.Read fails?

Thanks for answers and recommendations