Battery and Powering

Part of Telephony

How telephone systems are powered — from local batteries in early phones to centralized exchange power.

Why This Matters

Every telephone system needs a stable, reliable power source. The voice currents produced by a microphone are too weak to travel any useful distance; they need amplification from a battery-powered circuit. The design of the power system — where batteries sit, how they are maintained, what voltage they provide, and how they handle multiple simultaneous calls — fundamentally shapes the entire telephone network architecture.

Early telephone systems placed batteries locally in each telephone instrument. This made installation simple but maintenance burdensome: every subscriber had to maintain their own battery. The shift to central battery systems — where a single large battery bank at the exchange powers all instruments on the exchange — was one of the most important administrative and technical advances in telephony. It concentrated maintenance at one location and dramatically improved call quality.

For anyone rebuilding telephone service, understanding both battery architectures lets you choose the right design for your scale. A two-farm intercom uses local batteries efficiently. A ten-building community telephone network benefits strongly from central battery design.

Local Battery Systems

In a local battery (LB) system, each telephone set contains its own battery — typically a 1.5V to 3V dry cell or wet cell. When the user speaks, their local battery drives the microphone current. The battery stays at the instrument location and the user or their household is responsible for keeping it charged or replacing it.

The primary advantage is simplicity: no special wiring infrastructure is needed beyond a single pair of conductors connecting each instrument to the exchange. The exchange itself does not need to power the subscriber lines.

The disadvantages are significant. Batteries at subscriber locations go dead, are neglected, or are used for other purposes. Call quality varies depending on battery state. During emergencies or heavy-use periods, batteries may fail precisely when communication is most needed. Maintenance requires visiting every subscriber location rather than servicing one central point.

For a local battery system, the telephone instrument typically uses a magneto generator (a hand-cranked or automatic bell-ringing generator) for signaling and alerting, and a 1.5-3V battery for microphone power. The circuit includes a local battery disconnect when the handset is on the hook to prevent battery drain during standby.

Central Battery Systems

Central battery (CB) systems revolutionized telephone engineering. A single large battery bank at the exchange — typically 24V or 48V — powers all subscriber instruments simultaneously through the subscriber line conductors. The instruments themselves contain no power source.

The 48V standard that emerged in the early twentieth century remains the global standard for POTS (Plain Old Telephone Service) even today. The 48V (negative with respect to ground in most systems) provides enough voltage to drive carbon microphones over long subscriber lines while remaining below the 50V threshold considered hazardous for most people.

At the exchange, the battery bank consists of lead-acid cells in series. A charging system (originally engine-driven generators, later rectifiers from the AC power grid) maintains the battery continuously charged. The battery acts as both primary power source and filter — its large capacitance absorbs transient loads and keeps the supply voltage stable.

Each subscriber loop passes current from the exchange battery through the subscriber’s microphone and out through the other conductor. The exchange detects loop current to determine whether a subscriber has lifted their handset. When you lift the handset, you complete the circuit and current flows, signaling the exchange to connect a dial tone or operator.

Voltage and Current Requirements

Carbon microphones require 15-100 mA of bias current to operate correctly. Too little current and the carbon granules pack loosely, producing poor sensitivity. Too much current heats the microphone and degrades it. The telephone instrument includes a resistor (or the line resistance itself for long lines) that sets the operating current to the appropriate range.

Typical subscriber loop resistance is 200 to 1,800 ohms including both conductors. At 48V with 1,000 ohms of loop resistance, the microphone receives about 48 mA — well within the correct operating range. For very long loops where resistance exceeds 1,500 ohms, the microphone may receive insufficient current. Long-range solutions include line amplifiers, loop extenders, or using lower-resistance wire.

The ringer circuit presents a different power requirement. Electromechanical ringers are driven by 90V AC at 20 Hz in the North American standard, or 75V AC at 25 Hz in some European systems. The exchange generates this ringing voltage from the DC battery through an oscillator circuit. Modern electronic ringers operate on much lower voltages and can ring from the 48V DC supply directly.

Battery Bank Design

A practical central battery system for a community telephone network should provide stable voltage under varying load. Load increases when many phones are off-hook simultaneously; load decreases during quiet hours.

Calculate your battery capacity by estimating peak simultaneous calls. Each active call draws approximately 25-50 mA. A community of 20 telephone users might have 5 simultaneous calls at peak, drawing 125-250 mA total. A 200 Ah battery bank at 48V (24 lead-acid 2V cells in series) provides 200,000 mAh — enough for 800 hours at this peak load before depletion. In practice, charging keeps the battery maintained and depletion never occurs under normal operation.

Lead-acid cells require specific gravity maintenance. Flooded lead-acid cells need electrolyte level checked monthly and distilled water added as needed. The cells should be in a ventilated enclosure because charging produces hydrogen gas. Sealed lead-acid (gel or AGM) cells require no maintenance but cost more and are less tolerant of overcharging.

Charging voltage for a 48V (24-cell) lead-acid bank is typically 54-56V (2.25-2.35V per cell) for float charging. A simple constant-voltage rectifier circuit can supply this from an AC generator or solar panel array, maintaining the battery in ready condition continuously.