NiCd battery electrolyte should be changed when the specific gravity drops to 1.17 or when carbonate content reaches 75 g/l.
Lifetime of valve-regulated lead acid batteries depends on initial specification and operating conditions, i.e. temperature and float voltage. Typical replacement intervals are between 5 y and 10 y. A six-monthly (on line) cell resistance check may be considered for suspect batteries.
Battery capacity measurements are made every 2 years with the rectifier 'off' and with a suitable load connected to prove that the system voltage/time parameters remain acceptable. During this test defective cells can be readily located by measurement of cell voltages