Charging float

Voltage uniformity does not exist in VRLA batteries unless all cells are fully saturated — and even then, this will change as water is lost and electrolyte is re-distributed from the separator into the plates. As noted previously, the ease of 

Cell number Original voltage (V) Voltage at 30 days (V) Voltage at 78 days (V) Voltage at 106 days (V) 

The picture is somewhat more complicated at the positive electrode. Again, the float voltage must be sufficiently positive to drive the PbS04/Pb02 reaction to completion and maintain it, but this must be balanced against the secondary 

The above data on float charging of VRLA batteries raise several significant points  

There is a gradual drop in capacity, which reaches the 80% cut-off point in only 133 cycles, and at cycle 312 the capacity drops to 50% of rated. The switch to a 2-A/l-h finishing step (no voltage limit) is when a 1-A current taper point is 

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A schematic circuit for an emergency power supply is shown in Fig. 1.5. The load is fed directly from the mains supply (which also float charges the battery), but in the event of mains failure, the load is automatically switched to the inverter by the mains monitoring system. 

Nominal 3.6 V batteries have been designed for direct mounting on printed circuit boards for CMOS and NMOS memory support applications. Such cells are normally float charged from the main microcomputer DC power supply and constitute a form of uninterruptible power supply (UPS). 

It must be emphasized that the most appropriate charging regime is very dependent on the cell system under consideration. Some are tolerant to a considerable amount of overcharging (e.g. nickel-cadmium batteries), while for others, such as zinc-silver oxide and most lithium secondary cells, overcharging can result in permanent damage to the cell. Sealed battery systems require special care float charging should not be used and trickle charge rates should be strictly limited to the manufacturer s recommended values, since otherwise excessive cell temperatures or thermal runaway can result. 

Liquids formed from covalently bonded solids are different from ionic-bond solids in another way as well. Covalently bonded liquids do not contain free-floating charged particles. Instead, they contain tightly-bonded, self-contained, neutral molecules. As a result, a liquid produced from a covalently bonded solid does not conduct electricity well at all. These liquids are good insulators. 

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. 

Under normal float-charging conditions this heat, though excessive, can be easily dissipated. If conditions hinder heat dissipation and/or higher float currents are drawn, however, this process can become a vicious cycle that can result in thermal... 

Vanted lead-acM battary. Haat evolution during float charging at 2.25 V/eall. 

The fundamental problems to be overcome in float charging of VRLA cell strings are the cell-to-cell variability in saturation levels and the gradual decrease in saturation during life due to water loss and consumption in grid corrosion. Both these conditions, combined with the mild charging that attempts to keep all cells just fully... 

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