Fixed up

using UnityEngine;
using System.Collections;

public class accel : MonoBehaviour
{

    public float maxVel = 10;
    public float accelRate = 900f;
    // Update is called once per frame
    void FixedUpdate ()
    {
        Debug.Log ("before accel: " + rigidbody.velocity.magnitude);
        rigidbody.AddForce(transform.forward*accelRate, ForceMode.Acceleration);
        //fix: rigidbody.velocity = rigidbody.velocity + (transform.forward*accelRate) * Time.deltaTime;
        Debug.Log ("after accel: " + rigidbody.velocity.magnitude);

        if (rigidbody.velocity.magnitude > maxVel)
        {
            rigidbody.velocity = rigidbody.velocity.normalized*maxVel;
            Debug.Log ("after cap: " + rigidbody.velocity.magnitude);
        }
    }
}