Senin, 23 Maret 2009

Reaksi Kimia Pada Battery Lead Acid


Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% w/w (6 Molar) sulfuric acid (H2SO4). In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. Due to the freezing-point depression of water, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze.
The chemical reactions are (charged to discharged):
Anode (oxidation):

Cathode (reduction):

Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with excessive charging voltages will generate oxygen and hydrogen gas by electrolysis of water, forming an explosive mix. The acid electrolyte is also corrosive.
Practical cells are usually not made with pure lead but have small amounts of antimony, tin, or calcium alloyed in the plate material.
These are general voltage ranges for six-cell lead-acid batteries:
Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V (2.10-2.13V per cell)
Open-circuit at full discharge: 11.8 V to 12.0 V
Loaded at full discharge: 10.5 V.
Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5 V for AGM (absorbed glass mat) and 13.8 V for flooded cells
1. All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.
2. Float voltage recommendations vary, according to the manufacturer's recommendation.
3. Precise (±0.05 V) float voltage is critical to longevity; too low (sulfation) is almost as bad as too high (corrosion and electrolyte loss)
Typical (daily) charging: 14.2 V to 14.5 V (depending on manufacturer's recommendation)
Equalization charging (for flooded lead acids): 15 V for no more than 2 hours. Battery temperature must be monitored.
Gassing threshold: 14.4 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.
Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries. It is relatively simple to determine the state of charge by merely measuring the specific gravity (S.G.) of the electrolyte, the S.G. falling as the battery discharges. Some battery designs have a simple hydrometer built in using coloured floating balls of differing density. When used in diesel-electric submarines, the S.G. was regularly measured and written on a blackboard in the control room to apprise the commander as to how much underwater endurance the boat had remaining.

Gel battery

A gel battery (also known as a "gel cell") is a rechargeable valve regulated lead-acid battery with a gelified electrolyte. Unlike a traditional wet-cell lead-acid battery, these batteries do not need to be kept upright (though they cannot be charged inverted). In addition, gel batteries virtually eliminate the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration. These batteries are often colloquially referred to as sealed lead-acid (SLA) batteries due to their non-leaking containers, but they are not completely sealed; the valve regulation system allows for gas to be expelled. Chemically they are the same as wet (non sealed) batteries except that the antimony in the lead plates is replaced by calcium. This preserves the mechanical characteristics but renders the construction far less prone to gassing. The battery type is often referred to as a Lead-Calcium battery.
At high 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. Charging with a constant voltage (called the float charge voltage; 2.26 V per cell for a lead-acid chemistry) can cause a rapid initial current, so therefore it is suggested to begin with a constant current, using constant voltage only for the final portion of the charging. However, the float charge voltage should not be exceeded by much for typical usage, so the switch between the two modes typically occurs when the float voltage is needed to sustain the charging current through the battery's internal resistance (as per Ohm's Law). The easiest way to implement this is to use a constant voltage device with a current limiter.