Security deposits are a good way to mitigate risk, but may hinder your ability to get a rental. Too high of a security deposit may not be competitive to other suppliers. If you would like to require a security deposit, KWIPPED recommends an amount high enough to cover the cost of repairing common issues when using the equipment. Security deposit that are more than one month rent tend to be less than competitive.
Regardless if you require a security deposits, renters will be responsible for and damage or loss of the equipment.