24 V 8 A Battery Charger for Diesel Generators

Overview

This battery charger is made for diesel generators and was designed in 2016. It charges 24 V lead-acid or nickel-cadmium batteries up to 300 Ah and keeps the battery charged and ready, so the generator can start when it is needed. The charger is a floating type in line with the NFPA20 standard, and it is not a switching design.

How it works

A transformer brings 220 V down to 32 V AC, and a controlled rectifier module converts it to DC. The control section (a microcontroller and a regulation circuit) reads the battery voltage and current, and keeps the output at the required level with pulses that reach the gate of the rectifier through optocouplers. The diagram below shows the power and control path.

The charger always holds the battery at the lowest current that is needed, and raises the current only when it has to. The Boost input turns on fast charging, and the Halt input stops charging. Both are isolated and are activated by 3 to 30 V.

It also has a few protections:

• Battery detection: if the battery is not connected, or another fault occurs, the output voltage is cut.

• When a battery is connected, the voltage and current rise gradually so no spark appears.

• A fault relay (15 A) reports the fault to the panel.

LED indicators

Seven LEDs show the state of the unit. When the power is connected the first LED (Supply) turns on. If no battery is connected, the second LED (Failure) turns on. When a battery is connected, one of the three charge-level LEDs is lit:

Specifications

• Lead-acid / nickel-cadmium battery charger, 24 V, 300 Ah

• Fast charge (Boost) and charge disconnect (Halt)

• Rated current: 8 A

• Rated voltage: 27.5 V; fast-charge voltage: 28.8 V

• Current control: 4 to 12 A

• Voltage regulation: 26 to 29 V

• 15 A fault relay

• Isolated Boost and Halt inputs, activated by 3 to 30 V

• Battery detection, and output cut when there is no battery or a fault

• Gradual rise of voltage and current when a battery is connected (no sparking)

Hardware design

I designed and built the board. It carries an LM317K regulator on a heatsink, a PIC12C508A microcontroller, optocouplers, operational amplifiers, current shunt resistors, fuses and a 10-pin terminal block. The rectifier module is separate and is mounted on a metal plate behind the board, which cools it. The pictures below show the built board, the PCB layout and its 3D view.

Result

A simple and reliable floating charger that keeps a diesel generator battery charged, reports faults with LEDs and a relay, and starts without sparking when a battery is connected.