SEALED LEAD-ACID BATTERIES （4）

Published:2011/8/8 21:17:00 Author:Phyllis From:SeekIC

In practice

Most battery manufacturers recommend charging using a constant voltage and current limiting, with temperature compensation. A simple charging circuit that satisfies these recommendations is shown in Figure 7. An LM317 is used as the voltage regulator with a current limit of 1.5 A. To ensure that the charge voltage is maintained at the level set by PI, the voltage at the input to the LM317 must be kept at least 3 V higher than that at the output. The input voltage can be provided from a (plug-in) mains adaptor (unregulated is fine) or from a 14 V boat power supply. The latter option is only suitable if the battery to be charged contains no more than four cells.




The charge voltage is set using PI to the optimal value for the number of cells, battery type (see manufacturer’s data) and type of charging (normal or trickle). The voltage at the output of the charger should be checked using a digital multimeter with the battery not connected. The range of voltages offered by PI can be altered if necessary by changing R2 to suit the number of cells in the battery.

If charging is always to be carried out at room temperature, the temperature compensation diodes between R2 and PI (shown dotted) can be dispensed with. Otherwise it is necessary to find out what temperature coefficient (TC) the manufacturer recommends for compensation. A TC value in the range from -2 to -5 mV/:C can be realised using our circuit as follows.

Inserting as many 1N4148 diodes in series between R2 and PI as there are cells in the battery will give a TC of -2 mV/:C: for a 12V battery, that means six diodes. If the number of diodes is doubled to 12, the TC value will be -4 mV/:C; with nine diodes, the value will be -3 rnV/:C. As you can see, the TC can easily be adjusted by varying the number of diodes. In order to correct for the voltage drop across the diodes, the resistance of R2 must be decreased by about 120 fi for each diode added.

Gel and AGM-lead batteries

Conventional lead-acid batteries have a few decisive disadvantages: they are filled with highly acid and therefore hazardous liquids which may spill into the environment not just when the battery case is cracked but also when far from positioned upright. Also, these batteries are marked by a high degree of self-discharging and a limited number of charging cycles. This may be tolerable for a vehicle starter battery, but if the battery is to power a vehicle supply system (and be subject to complete charging cycles including deep discharging) then it will protest by developing amounts of sulphate.

With the above in mind, it is not surprising to see lead-acid batteries with fixed electrolytes used in such applications as back-up PSUs and solar systems. These variants are sealed, maintenance-free and will not easily spill their contents. Sulphation can hardly occur because the free sulphuric acids are ’captured’ by the internal gel mass. Self-discharging is noticeably smaller, although charging instructions must be observed closely. Unfortunately sealed lead-acid (SLA) batteries exhibit considerably lower energy densities than their liquid acid counterparts, which makes them generally unsuitable for use as vehicle starter batteries.




Apart from the well-known lead-gel technology there is another method to fix electrolyte in a lead-acid battery: AGM. AGM stands for ’absorbent glass mat’; a separating layer pressed between the electrodes. The construction of an AGM battery is shown diagrammatically in the illustration (source: Hawker). The capillary action of the mat causes the electrolyte to be fully absorbed, preventing it from moving around freely. AGM batteries typically supply higher currents than gel types and are also cheaper. On the down side, they are marked by some excess electrolyte as well as worse heat dissipation — at high temperatures, AGM batteries are susceptible to drying out.

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