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capacity of the battery greatly reduced as the lead sulphate insulates the plates from
having contact with the electrolyte.

For radio use, lead-acid batteries make a very cheap alternative to an expensive power
supply. They are cheap to buy and can be trickle charged with an inexpensive charger.

Lead-acid batteries are also available with gelled electrolyte. Commonly called gel cells,
these may be mounted in any position if sealed, but some vented types are position
sensitive.

Lead-acid batteries with liquid electrolyte usually fall into one of three classes:
1. Conventional, with filling holes and vents to permit the addition of distilled
water lost from evaporation or during high-rate charge or discharge.
2. Maintenance-free from which gas may escape, but water cannot be added.
3. Sealed.

SPECIFIC GRAVITY

Specific gravity is the weight of a substance compared to the weight of water. A
hydrometer (Figure 16-3) can be used to draw some electrolyte from a lead-acid cell
into a reservoir tube; a weighted float can be used to measure the specific gravity of the
electrolyte. This reading gives a very good indication of the state of charge of a cell. In
a fully charged cell, the specific gravity is in the vicinity of 1.280 (which means the
density of the electrolyte is 1.280 times that of water). When the specific gravity is down
to about 1.150, the cell is fully discharged. Water has a specific gravity of 1.0, so at
an electrolyte level of 1.15, means we almost have water for an electrolyte.

Figure 16-3 The hydrometer tests specific gravity
As you can imagine, when a lead-acid cell discharges, its electrolyte becomes
weaker in sulphuric acid and approaches water. As a lead-acid cell discharges, the
specific gravity or density of its electrolyte decreases (it turns towards water).
WARNING. Lead-acid cells produce hydrogen and oxygen when being charged,
these are highly flammable gases in combination with air. The bubbles in a liquid
electrolyte are hydrogen.
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