Given positive integer $n (n \geq 2)$, find the largest positive integer $\lambda$ satisfying :
For $n$ bags, if every bag contains some balls whose weights are all integer powers of $2$ (the weights of balls in a bag may not be distinct), and the total weights of balls in every bag are equal, then there exists a weight among these balls such that the total number of balls with this weight is at least $\lambda$.
1. **Claim**: The largest positive integer \(\lambda\) satisfying the given conditions is \(\lambda = \left\lfloor \frac{n}{2} \right\rfloor + 1\).

2. **Lower Bound Establishment**:
    - Any integer \(m\) can be expressed as a sum of powers of 2, derived from its binary representation. For example, \(7 = 2^2 + 2^1 + 2^0\) can be rewritten as \(2^1 + 2^1 + 2^1 + 2^0\) by splitting \(2^2\) into \(2^1 + 2^1\).
    - This implies that we can assume, without loss of generality, that the total weight of balls in each bag is a power of 2. If not, we can adjust the weights by splitting the powers of 2 in the binary representation of the total weight.

3. **Assumption**:
    - Suppose the total weight of balls in each bag is \(2^k\). Then, we need to show that \(\lambda \geq \left\lfloor \frac{n}{2} \right\rfloor + 1\).

4. **Inequality Derivation**:
    - Consider the sum of weights in each bag: \(\lambda (2^k + 2^{k-1} + \cdots + 2^0) \geq n \cdot 2^k\).
    - Simplifying the left-hand side, we get:
      \[
      \lambda (2^k + 2^{k-1} + \cdots + 2^0) = \lambda (2^{k+1} - 1)
      \]
    - Therefore, the inequality becomes:
      \[
      \lambda (2^{k+1} - 1) \geq n \cdot 2^k
      \]
    - Solving for \(\lambda\):
      \[
      \lambda \geq \frac{n \cdot 2^k}{2^{k+1} - 1}
      \]
    - Since \(2^{k+1} - 1 < 2^{k+1}\), we have:
      \[
      \lambda > \frac{n \cdot 2^k}{2^{k+1}} = \frac{n}{2}
      \]
    - Thus, \(\lambda \geq \left\lfloor \frac{n}{2} \right\rfloor + 1\).

5. **Construction of Example**:
    - To show that \(\lambda = \left\lfloor \frac{n}{2} \right\rfloor + 1\) can be achieved, consider \(2^k \leq n < 2^{k+1}\) and let \(\lambda' = \left\lfloor \frac{n}{2} \right\rfloor + 1\).
    - Write \(2^k\) \(\lambda'\) times, \(2^{k-1}\) \(\lambda'\) times, ..., \(2^0\) \(\lambda'\) times.
    - This ensures that the total weight in each bag is balanced and the number of balls with each weight is at least \(\lambda'\).

6. **Verification**:
    - For \(2^i \lambda' > n\) for \(i \in \mathbb{N}\) and \(\lambda' (2^0 + 2^1 + \cdots + 2^k) = \lambda' (2^{k+1} - 1) \geq \frac{n+1}{2} (2^{k+1} - 1) \geq n \cdot 2^k\).
    - This follows since \(\frac{n+1}{2n} = \frac{1}{2} + \frac{1}{2n} \geq \frac{1}{2} + \frac{1}{2(2^{k+1}-1)} = \frac{2^k}{2^{k+1}-1}\).

\(\blacksquare\)

The final answer is \( \boxed{ \lambda = \left\lfloor \frac{n}{2} \right\rfloor + 1 } \).