This Technical Report covers the opportunity charging of lead-acid traction batteries, i.e. the use of free time during a working period to top up the charge and thus extend the working day of a battery whilst avoiding excessive discharge. The object of this report is to lay down rules or recommendations for the use of opportunity- charging of lead-acid traction batteries of vented and valve-regulated types, when the battery manufacturer has not recommended the required operating procedures. When the battery manufacturer's recommendations are available, they take precedence over these rules and guidelines.