SEALED LEAD-ACID BATTERIES （1）

Published:2011/8/8 1:12:00 Author:Phyllis From:SeekIC

SLAs still going strong

Lead-acid batteries still form an essential part of the on-board power systems in cars, motorcycles, boats and caravans. In its new form, featuring a fixed electrolyte and a sealed enclosure, this veteran battery technology (invented in 1850) is now more versatile than ever.

Sealed batteries with a fixed electrolyte have been in production for more than forty years. The electrolyte (dilute sulphuric acid) is fixed either using silica gel (lead-gel, or ’dryfit’ battery) or using a glass fibre mat (AGM or ’absorbed glass mat’ technology). This allows the oxygen produced at the positive electrode when the battery is overcharged to diffuse to the negative plate where it can recombine into water. This cycle of recombination makes the battery emit practically nothing in the way of gases, and so it can be built into a sealed enclosure. To prevent rupture in the event of the pressure getting too high, the cells (as in almost every sealed battery) are equipped with self-closing safety valves. The combination of fixed electrolyte and a sealed enclosure allows operation in any orientation (although AGM batteries must not be charged upside-down). The most important advantages of sealed lead-acid batteries, besides their low price, are their low self discharge, simple charging method, and long life, especially when continuously charged.

Although the basic principles of operation of the sealed lead-acid battery have not changed, steady development over the years has led to significantly better battery characteristics:
- Improved behaviour when overcharged
- Relatively low sensitivity to deep discharge
- Improved life
- Higher specific capacity
- Improved cycle life
- Quicker charging
- Improved current output

Manufacturers such as Yuasa, Panasonic and Sonnenschein/Exide offer various ranges of batteries optimised for one (or more) different applications. So, for example, there are batteries available optimised for long life designed to last for 20 years and 1500 cycles. The most important application areas today are emergency power supplies of various types (for example for alarm systems, IT equipment, lighting, and medical devices), electric vehicles (such as golf buggies and wheelchairs), electric starter motors, on-board power supplies in boats, caravans and gliders, and, last but not least, solar power systems that are independent of the mains.

Voltage/ current and capacity




The state of charge of a lead-acid battery can be determined by measuring its no-load voltage. Figure 1 shows typical values for sealed 6 V and 12 V batteries. The figure also indicates the accuracy that can be expected using this measurement: the voltage varies from around 2.2 V for a fully-charged cell to 1.9 V for a discharged cell. These values are valid at room temperature (20 ~C). If the battery has recently been charged or discharged, the values are respectively higher and lower. The usable capacity C of a sealed lead-acid battery, measured in amp-hours (Ah), depends to a great extent on the discharge current. The lower the discharge current, the lower the losses during discharge, and so the higher the usable capacity. For this reason, the discharge current is generally quoted in CA, giving the current as a fraction of the nominal capacity C. For example, a capacity C specified at C/10 (or 0.1 CA) for a 1 Ah battery means that the capacity given is valid for a discharge current of 0.1 A (100 mA). In this case the capacity may also be written as C]Q (capacity for 10-hour discharge). Nominal capacities of lead-acid batteries quoted according to the German DIN standard are given for a 20-hour discharge (i.e., at 0.05 CA) and with a final discharge voltage of 1.75 V per cell.

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