Charging VRLA Batteries

8/13/2019 Charging VRLA Batteries

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A VRLA battery(valve-regulated leadacid battery) more commonly known as a sealed batteryis a leadacid

rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the

cells, and vent less gas than flooded lead-acid batteries. The reduced venting is an advantage since they can be usedin confined or poorly ventilated spaces. But sealing cells and preventing access to the electrolyte also has several

considerable disadvantages as discussed below.

VRLA batteries are commonly further classified as:

Absorbed glass mat (AGM) battery Gel battery ("gel cell")

An absorbed glass matbattery has the electrolyte absorbed in a fiber-glass mat separator. Agel cellhas theelectrolyte mixed with silica dust to form an immobilized gel.

While these batteries are often colloquially calledsealedleadacid batteries, they always include a safety pressure

relief valve. As opposed to vented(also calledflooded) batteries, a VRLA cannot spill its electrolyte if it is

inverted. Because AGM VRLA batteries use much less electrolyte (battery acid) than traditional leadacid

batteries, they are sometimes called an "acid-starved" design.

The name "valve regulated" does not wholly describe the technology. These are really "recombinant" batteries,

which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready toevolve on the negative plates, creating water and preventing water loss. The valve is a safety feature in case the rate

of hydrogen evolution becomes dangerously high. In flooded cells, the gases escape before they can recombine, so

water must be periodically added.

Comparison with flooded leadacid cells

VRLA batteries offer several advantages compared with flooded leadacid cells. The battery can be mounted inany position, since the valves only operate on overpressure faults. Since the battery system is designed to be

recombinant and eliminate the emission of gases on overcharge, room ventilation requirements are reduced and no

acid fume is emitted during normal operation. The volume of free electrolyte that could be released on damage to

the case or venting is very small. There is no need (nor possibility) to check the level of electrolyte or to top upwater lost due to electrolysis, reducing inspection and maintenance.

Because of calcium added to its plates to reduce water loss, a sealed battery recharges much more slowly than a

flooded lead acid battery. Compared to flooded batteries, VRLA batteries are more sensitive to high temperature,

and are more vulnerable to thermal run-away during abusive charging. The electrolyte cannot be tested byhydrometer to diagnose improper charging, which can reduce battery life.

AGM automobile batteries are typically about twice the price of flooded-cell batteries in a given BCI size group;

gel batteries as much as five times greater. VRLA batteries:

Are less reliable than flooded lead acid Have longer recharge time than flooded lead-acid. Cannot tolerate overcharging: overcharging leads to premature failure. Have shorter useful life. Discharge significantly less hydrogen gas.

The voltage regulator of the charge system does not measure the relative currents charging the battery and for poweringthe car's loads. The charge system essentially provides a fixed voltage of typically 13.8 to 14.4 V, adjusted to ambient

temperature, unless the alternator is at its current limit. A discharged battery draws a high charge current of typically 20 to

40 A. As the battery becomes charged the charge current typically decreases to 25 amperes. A high load is when multiple


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high-power systems such as ignition, radiator fan, heater blowers, lights and entertainment system are running at the same

time. In older (up to the 1980s) vehicles the battery may discharge unless the engine is running at a higher than idle rpm

and the alternator/generator is delivering enough current to power the load. This is not an issue for modern vehicles where

alternators provide enough current for all loads and a regulator keeps charging voltage in check. In such cars rpm has little

influence on the battery voltage - tests show near normal voltage regardless of the AC / headlights / music / fan / defrosting

/ other electrical loads, even at idle.

Exploding batteries

Any lead-acid battery system when overcharged (>14.34 V) will produce hydrogen gas (gassing voltage) by

electrolysis of water. If the rate of overcharge is small, the vents of each cell allow the dissipation of the gas.

However, on severe overcharge or if ventilation is inadequate, or the battery is faulty, a flammable concentration of

hydrogen may remain in the cell or in the battery enclosure. An internal spark can cause a hydrogen and oxygen

explosion, which will damage the battery and its surroundings and which will disperse acid into the surroundings.

Anyone close to the battery may be injured.

Terminal voltage

The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately

related to the charge of the battery by:

Open circuit voltage Approximate

charge

Relative

acid density12 V 6 V

12.60 V 6.32 V 100% 1.265 g/cm

12.35 V 6.22 V 75% 1.225 g/cm

12.10 V 6.12 V 50% 1.190 g/cm

11.95 V 6.03 V 25% 1.155 g/cm

11.70 V 6.00 V 0% 1.120 g/cm

Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.

The following is common for a six-cell automotive lead-acid battery at room temperature:

Quiescent (open-circuit) voltage at full charge: 12.6 V Fully discharged: 11.8 V Charge with 13.214.4 V Gassing voltage: 14.4 V Continuous-preservation charge with max. 13.2 V After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a

more accurate reading.

All voltages are at 20 C, and must be adjusted -0.022V/C for temperature changes (negative temperaturecoefficient - lower voltage at higher temperature).


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These batteries have a pressure relief valve which will activate when the battery is recharged at high voltage,

typically greater than 2.30 volts per cell. Valve activation allows some of the gas or electrolyte to escape, thus

decreasing the overall capacity of the battery. Rectangular cells may have valves set to operate as low as 1 or 2 psi;

round spiral cells, with metal external containers, can have valves set as high as 40 psi.

The cell covers typically have gas diffusers built into them that allow safe dispersal of any excess hydrogen that

may be formed during overcharge. They are not permanently sealed, but are maintenance free. They can be

oriented in any manner, unlike normal leadacid batteries, which must be kept upright to avoid acid spills and to

keep the plates' orientation vertical. Cells may be operated with the plates horizontal (pancakestyle), which mayimprove cycle life.

VRLA cells may be made of flat plates similar to a conventional flooded leadacid battery, or may be made in aspiral roll form to make cylindrical cells.

At high overcharge currents, electrolysis of water occurs, expelling hydrogen and oxygen gas through the battery's

valves. Care must be taken to prevent short circuits and rapid charging. Constant-voltage charging is the usual,most efficient and fastest charging method for VRLA batteries, although other methods can be used. VRLA

batteries may be continually "float" charged at around 2.35 volts per cell at 25 C (= 14,1 V.) Some designs can befast charged (1 hour) at high rates. Sustained charging at 2.7 V per cell will damage the cells. Constant-current

overcharging at high rates (rates faster than restoring the rated capacity in three hours) will exceed the capacity of

the cell to recombine hydrogen and oxygen.


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Charging VRLA Batteries