as docs saids in 

https://www.odoo.com/documentation/8.0/reference/orm.html

class openerp.fields.Float(string=None, digits=None, **kwargs) 
The precision digits are given by the attribute
        Parameters digits -- a pair (total, decimal), or a function taking a database cursor and returning a pair (total, decimal)

Your definition could be changed to this:

exchange_rate = fields.Float('res.currency.rate', 'Exchange Rate', related='currency_id.rate', readonly=True, digits=(16,2))

or you could use a managed decimal precision already defined like 

import openerp.addons.decimal_precision as dp
...
exchange_rate = fields.Float('res.currency.rate', 'Exchange Rate', related='currency_id.rate', readonly=True, digits=dp.get_precision('Account') )