Standby-float charge

The majority of VRLA batteries produced today are used in standby applications to provide a reliable source of power in the event of failure of the mains supply. Discharges are infrequent and the batteries are maintained by float charging at a preset voltage. For example, European practice is to use parallel strings (usually 48 V) across a 54.5 V supply (2.27 V per cell). Adequately designed new cells, after conditioning and free of impurities, have a float current of < 1 mA per Ah at 20°C. 

Sealed lead-acid batteries are in both cylindrical and prismatic shapes. The cyclindrical ones (usually designed as SLA batteries) have excellent high-rate characteristics. Other than in portable devices, sealed batteries can be used in standby applications, e.g. telephone exchange stations, were they are kept in float charge. In this case too, oxygen recombination is possible. 

Yuasa claim that, due to the use of heavy-duty grids. 1000 cycles are obtained for their batteries and a normal life of 4-5 years is expected in float charge on standby applications (based on normal charge voltage of 2.28-2.30 V/cell at 20°C). 

When the battery approaches the state of full charge, the charging reaction diminishes, and finally the secondary reactions are the only ones that remain when the battery is overcharged or kept at a comparatively low float voltage in standby operation. 

In some applications such as emergency and standby, it is necessary to keep the battery in a high state of charge. A convenient way is to connect the battery in parallel with the ordinary current source and the load and to float the battery at 1.40 to 1.45 V per cell. The floating may be combined with a supplementary charge at fixed intervals or after each discharge. 

Batteries in standby service can be maintained in a fully charged condition by a float or trickle charge similar to pocket plate batteries. The float voltage for vented sintered-plate batteries is 1.36 to 1.38 V per cell. 

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