We are looking at a backup battery charging circuit in a 24-volt (nominal) fire alarm system. It could also be a 24-volt (nominal) NAC booster power supply or a 24-volt auxiliary power supply. Because two 12-volt batteries are much less expensive than one 24-volt battery, the power supply uses two 12-volt (nominal) batteries as a backup for the 24-volt system.
A Circuit is a Circle
In order for electricity to flow in a circuit, the electrons have to flow in a circle. The electrons flow:
- Out of the negative terminal of the power supply
- Through a wire
- Through the load (like a resistor or light bulb)
- Through the other wire
- Back into the power supply on the positive terminal
- Then through the power supply to the positive terminal
That is the complete circle. The circuit has to be a circle because the electrons are reused over and over.
Backup Batteries are a Power Supply
In a 24-volt (nominal) power supply, there are two 12-volt backup batteries wired in series.
They normally aren't wired this way, but when the backup batteries are wired in parallel, so that the negative terminals on both batteries are wired together and the positive terminals are wired together, the two batteries are still just 12 volts. Because they are wired together, however, they together can provide twice the power of a single battery.
The Battery Charger is a Power Supply
Another way of looking at a battery is the battery is a power supply. It supplies power.
But just like a battery, the battery charger is also a power supply.
From an outside-the-panel point of view, normally, the battery charger is wired in parallel with the battery: the negative terminal of the charger is wired to the negative terminal of the batteries, the positive terminal of the charger is wired to the positive terminal of the batteries.
Because the voltages are nominal, when the batteries are fully charged, the batteries measure about 27.5 volts, and the charger measures about 27.5 volts. Because both the battery charger and the batteries are at 27.5 volts, there's no current flowing between them.
Another way of looking at the circuit is the battery charger is pushing 27.5 volts towards the battery, and the battery is pushing back 27.5 volts toward the charger. The push voltage and the push-back voltage are equal, so no current flows.
Note: Some manufacturers have circuitry inside the charger that prevents proper measurement of the charger's voltage when there's no battery connected to it. EST is one such manufacturer. Without a 24-volt battery connected to the charging circuit, the charger only provides about 1.6 volts. Don't bother measuring the battery charger voltage without a battery in the circuit.
Reversing a Battery
Remember, a circuit is a circle. Normally, the battery charger and the batteries are wired in parallel because the negative terminals are connected together, and the positive terminals are wired together.
However, when the batteries are reverse-wired, the negative terminal of the battery is connected to the positive terminal of the charger, and the positive terminal of the battery is connected to the negative terminal of the charger.
When reversed, the batteries are wired in series with the battery charger.
Because the charger and the batteries are wired in series, the battery charger pushes the current (at 27.5 volts), and the batteries also push the current (at 27.5 volts). Since both the charger and the batteries are wired in series, the voltages add up, and there's 55 volts of push.
With 55 volts of push, there's lots of extra current. The extra current causes the fuse to blow.