5839 states that the interval between service visits should not exceed 6 months (two visits minimum),meaning that you can do 50% each visit over a 12 month period giving you 100% annually (most of my contracts are still 25%/quarterly visits).However,vented battery systems should be quarterly.
The interval can be increased if it is felt necessary following an assesment (or by customer request) on a case by case basis.
Interestingly,I have just received a copy of the new IS 3218:2009 where the service schedule has been kept at quarterly (as per 1988 5839).
Excerpt from 45.3 of 2002/A2:2008 -
"The period between successive inspection and servicing visits should be based upon a risk assessment, taking into account the type of system installed, the environment in which it operates and other factors that may affect the long term operation of the system. The recommended period between successive inspection and servicing visits should not exceed six months. If this recommendation is not implemented, it should be
considered that the system is no longer compliant with this part of BS 5839".
and from 45.4 -
"NOTE 1 The work described may be carried out over the course of two or more service visits during each twelve month period."
The above relates to the testing of all devices etc. annually
