Enabling the balancer

To enable the balancer, proceed as follows:

1. Open a mongo shell onto a mongos instance with sufficient cluster administration rights.
2. Issue the sh.startBalancer() command to enable the balancer. You can supply an optional argument string representing the database and collection if you wish to enable balancing only on a specific collection. The command can be seen in the following code snippet:

sh.startBalancer("DB.COLLECTION");

3. Confirm the balancer state by running the following command. A value of true indicates the balancer is enabled:

sh.getBalancerState();

Here is the output of this sequence from the sharded cluster model:

If you need to enable or disable the balancer from an application, it's best to use a direct database admin command. 
Here is the command to disable the balancer: db.adminCommand( { balancerStop: 1 } ). 
Here is the command to start the balancer: db.adminCommand...