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How often should a battery be equalized? It all depends on type and usage. For batteries with high antimony doping, the best way to find out is to check SG after a normal charge: - If all cells are equal and at 1.28, there is no need to equalize - If all cells are between 1.24 and 1.28, it would be good to equalize when convenient, but there is no urgency - If the SG of some cells is less than 1.24, an equalization is recommended. - If all cells are below 1.24, the battery is undercharged and the absorption time or voltage should be increased. On VLRA batteries and low antimony flooded batteries the SG cannot be measured, respectively the reading will be unreliable. The easiest way to check if they are really charged to the full 100 % is to monitor the charge current during the absorption charge. The charge current should steadily decrease and then stabilise: a sign that the chemical transformation of the active mass has been completed and that the main remaining chemical activity is gassing (decomposition of water into oxygen and hydrogen).

4.4. Temperature compensation

As has already been mentioned in sect. 2.5.9, temperature is of importance when charging batteries. The gassing voltage and consequently the optimum absorption and float voltages are inversely proportional to temperature.

This means that in case of a fixed charging voltage a cold battery will be insufficiently charged and a hot battery will be overcharged.

Both effects are very harmful. Deviations of more than 1 % of the correct (temperature dependent) float voltage can result in a considerable reduction of service life (according to some studies up to 30 % when the battery is float charged for long periods of time), particularly if the voltage is too low and the battery does not reach or stay at 100 % charge, so that the plates start to sulphate. On the other hand over-voltage can lead to overheating, and an overheated battery can suffer “ thermal runaway ”. Because the gassing voltage decreases with increasing temperature, the absorption and float charge current will increase when the battery heats up, and the battery becomes even hotter, etc. Thermal runaway quickly results in destruction of the battery (the excessive gassing pushes the active mass out of the plates), and there can be a risk of explosion due to internal short-circuits and high quantities of oxygen and hydrogen gas coming out of the battery. Although manufacturers’ recommendations differ to some extent, a temperature compensation of - 4 mV / °C per cell is a generally accepted average. This means – 24 mV / °C for a 12 V battery and – 48 mV / °C for a 24 V battery. Where the manufacturer specifies an absorption voltage of for example 28.2 V at 20°C, then at 30°C the absorption voltage must be reduced to 27.7 V. This is a difference of 0.5 V that certainly cannot be neglected. When in addition to an ambient temperature of 30°C, the internal temperature of the battery rises another 10°C, which is quite normal during charging, the absorption voltage must be reduced to 27.2 V. Without temperature compensation the charge voltage would have been 28.2 V which would quickly destroy a gel or AGM bank worth some ten thousand dollars! The charging voltage, as quoted by European battery manufacturers, applies at 20°C battery temperature and may be kept constant as long as the temperature of the battery remains reasonably constant (15°C to 25°C).

What the above means is that temperature compensation is important , and must be implemented, especially on large, expensive house batteries, and when a high rate of charge current is used.

All charging voltages mentioned in this and in other chapters are subject to temperature compensation.

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