3.Monitoring a battery’s state of charge. ‘The battery monitor’
3.1. The different ways of measuring a battery’s state of charge
3.1.1. Specific gravity (SG) of the electrolyte
As explained in sect. 2.2.1, the electrolyte of a lead-acid battery consists of a mixture of water and sulphuric acid. When fully charged, the active material in the negative plates is pure sponge lead; in the positive plates it is lead oxide. The concentration of sulphuric acid in the electrolyte (and consequently the SG) is then high. During discharging the sulphuric acid from the electrolyte reacts with the active material in the positive and negative plates forming lead sulphate and water. This reduces the sulphuric acid concentration and consequently the SG of the electrolyte.
During discharging, the depth of discharge (DoD) of the battery can be tracked quite well by using a hydrometer to monitor the SG of the electrolyte. The SG will decrease as shown in the following table:
Depth of discharge (%)
Specific gravity
Battery voltage
0
Between 1,265 and 1,285
12.65 +
25 50 75
1,225 1,190 1,155 1,120
12.45 12.24 12.06 11.89
100
During charging the reverse process takes place and sulphuric acid forms once again. Because sulphuric acid is heavier than water, in batteries with liquid electrolyte (this does not apply for gel and AGM batteries) it settles downwards, so that the acid concentration increases at the bottom of the battery. However, above the plates the acid concentration in the liquid does not increase until the gassing level is reached!
Some useful information about electrolyte:
- Stratification Only once the gassing voltage (2.39 V per cell, or 14.34 V for a12 V battery at 20°C) is reached will the electrolyte slowly become well mixed again by the gas bubbles. The time needed depends on the construction of the battery and on the amount of gassing. The amount of gassing in turn depends on the charge voltage, on the amount of antimony doping and age of the battery. Batteries with relatively high antimony doping (2.5 % or more) in general do gas sufficiently during the absorption charge for the electrolyte to become homogeneous again. Modern low antimony batteries (1.6 % or less antimony content) however gas so little that a normal charge cycle is not sufficient. It then takes weeks of float charging (with very little gassing) before the electrolyte is well mixed again. As a result flooded batteries, after having been fully charged, may nevertheless show a low hydrometer reading ! Note: Vibration and motion in a boat or vehicle will in general adequately mix electrolyte. Temperature correction for hydrometer readings: SG varies inversely with temperature. For every 14°C of temperature increase above 20°C, the hydrometer reading will decrease with 0.01. So a reading of 1.27 at 34°C is equivalent to a reading of 1.28 at 20°C. Specific gravity variations per region: The SG values as mentioned in the table above are typical for a moderate climate. In hot climates SG is reduced as shown in the table below in order to diminish the effect of temperature on service life of a battery - -
Fully charged SG, moderate climate: Fully charged SG, sub tropical climate: Fully charged SG, tropical climate:
1.265 - 1.285 1.250 - 1.265 1.235 - 1.250
20
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