I am using the metadata to store info about the block and so far I resets to 0 after i hit 15, heres my code

 

package kore.korecraft.mod;

import java.util.Random;

import net.minecraft.block.Block;
import net.minecraft.block.material.Material;
import net.minecraft.creativetab.CreativeTabs;
import net.minecraft.entity.EntityLiving;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.item.Item;
import net.minecraft.util.AxisAlignedBB;
import net.minecraft.util.MovingObjectPosition;
import net.minecraft.util.Vec3;
import net.minecraft.world.IBlockAccess;
import net.minecraft.world.World;
import cpw.mods.fml.common.Side;
import cpw.mods.fml.common.asm.SideOnly;

public class BlockExpBank extends Block
{
private int maxExpStorage = 24;

    protected BlockExpBank(int par1, Material par2Material)
    {
        super(par1, par2Material);
        this.blockIndexInTexture = 0;
        this.setCreativeTab(CreativeTabs.tabBlock);
        this.setHardness(6F);
        this.setBlockName("Experience Bank");
    }

    public void onBlockClicked(World par1World, int par2, int par3, int par4, EntityPlayer par5EntityPlayer) {
    	if(par1World.getBlockMetadata(par2, par3, par4)>=1) {
    		par5EntityPlayer.addExperienceLevel(1);
    		par1World.setBlockMetadata(par2, par3, par4, par1World.getBlockMetadata(par2, par3, par4)-1);
    		tick(par1World.getBlockMetadata(par2, par3, par4));
    	}
    }

    public boolean onBlockActivated(World par1World, int par2, int par3, int par4, EntityPlayer par5EntityPlayer, int par6, float par7, float par8, float par9)
    {
        if(par5EntityPlayer.experienceLevel>1&&par1World.getBlockMetadata(par2, par3, par4)<this.maxExpStorage){
        	par5EntityPlayer.addExperienceLevel(-1);
        	par1World.setBlockMetadata(par2, par3, par4, par1World.getBlockMetadata(par2, par3, par4)+1);
        	tick(par1World.getBlockMetadata(par2, par3, par4));
        	return true;
        }
        return false;
    }
    
    public void tick(int metadata) {
    	double maxExpScaled = maxExpStorage/6;
    	if(metadata>=maxExpScaled*5) {
    		this.blockIndexInTexture=5;
    	}else
    		if(metadata>=maxExpScaled*4) {
    			this.blockIndexInTexture=4;
        	}else
        		if(metadata>=maxExpScaled*3) {
        			this.blockIndexInTexture=3;
            	}else
            		if(metadata>=maxExpScaled*2) {
            			this.blockIndexInTexture=2;
                	}else
                		if(metadata>=maxExpScaled) {
                			this.blockIndexInTexture=1;
                    	}else{
                    		this.blockIndexInTexture=0;
                    	}
    }
    
    public void onBlockDestroyedByPlayer(World par1World, int par2, int par3, int par4, int par5) {
    	if(par5>0) {}
    }
    
    public int idDropped() {
    	return 1500;
    }
    
    public String getTextureFile() {
    	return Korecraft.blockTexture;
    }
}

 

If I am correct metadata can't go above 15, because it is stored in 4 bits.

Make a Tile Entity of your block and store it in another tag. (Like a furnace does with burning time)