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Rojony Khatun
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Last updated: September 16, 2025

12V Auto Cut-Off Battery Charger Circuit & Installation Guide

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1K Fixed Resistor 1/4W

1K Fixed Resistor 1/4W

A 1,000-ohm resistor rated for 0.25 watts

10K Fixed Resistor

10K Variable Resistor

Adjustable resistor with 10,000 ohm resistance

3V LED 5mm

3V LED 5mm

3V, 5mm round light-emitting diode

TIP120 Transistor

TIP120 Transistor

NPN Darlington power transistor with high current gain

2 Way Switch

5-pin Relay 12V DC

Electromechanical relay with 5 pins, operates on 12V DC

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auto cut-off 12v Battery Charger

Learn a 12V auto cut-off battery charger circuit that charges safely and stops at full voltage. Includes schematic, parts, installation, and testing steps.

automatic battery charger 12v circuit

A 12 V auto cutoff charger uses a comparator/reference to monitor battery voltage and use a MOSFET or relay to disconnect the charger when a battery reaches a set voltage, and may then float the battery at a low voltage.

charger with voltage cutoff 12v

A 12V auto cut-off battery charger is a charging circuit that provides a current flow to a 12V lead-acid or SLA battery and switches and cuts off to float mode when the battery reaches the predetermined full-charge voltage (usually 13.6–14.4V depending on the battery type). This is normally built with a voltage-sensing comparator (LM324/LM358), an adjustable reference (zener or precision reference), a switching element (MOSFET or relay) to prevent charging, an indicator, and a current-limiting resistor or constant-current stage. Newer models introduce temperature compensation and soft start to increase the life of the battery. This charger can avoid excessive charge, gassing, and minimize maintenance- it can be used with car batteries, solar battery banks, UPS, and small off-grid systems. The ability to select a proper transformer/adapter, heat dissipation, and protection with fusing is the warranty for safe and reliable functioning.

Work / Installation (Inputs → Outputs)

Input: AC mains → transformer/SMPS → rectifier & filter → current limit stage → comparator control → switching device → battery.

  • AC mains (or DC adapter) is stepped to ~15–18V AC (or supplied by suitable SMPS).
  • Rectify (bridge) and filter to obtain DC (~18–20V unloaded).
  • A current-limiter (resistor, or better: constant current regulator / CC stage) controls charge current (e.g., 0.1C–0.3C).
  • A voltage-sensing comparator monitors battery voltage; reference set to cutoff (e.g., 14.4V for lead-acid).
  • When measured voltage ≥ cutoff, comparator drives MOSFET/relay to disconnect charge or switch to float-mode regulator (~13.6V).
  • Indicators (LEDs) show CHARGING / FULL; optional reverse polarity protection and fuse on input.

Output: Battery receives safe, limited-current charge and is automatically disconnected or held at float voltage when full.

Testing & Final Adjustments

Check no-load voltages. Before connecting a battery, check rectified DC no-load output voltages: measure rectified DC, and make the comparator reference adjustable. Install the fit input fuse and utilize a bench power supply/current-limited source during initial tests. Connect a discharged battery and measure the current of charge: This must be within the specifications (e.g.,1 3 A at charger rating). Watch the increasing voltage of the battery. When it goes to the cutoff setpoint (adjustable pot), be sure that the MOSFET/relay closes and the charging current is reduced to almost zero or floats. Characteristics of thermal behavior of MOSFET/heatsink with prolonged charging. Test reverse polarity and short-circuit. To be reliable in the long term, test at varying temperatures (or add a temp sensor) and vary the cutoff of battery chemistry: bulk charge =~14.4V, float = 13.6 -13.8V. And lastly, check LED indicators and safety cutouts (fuse, thermal limits). Label the charger and settings of the document.

Frequently Asked Questions - auto cut-off 12v Battery Charger:

What is a 12V auto cut-off charger?

A charger that automatically stops charging when a 12V battery reaches a preset full voltage.

What cutoff voltage should I use?

For lead-acid batteries typically 14.2–14.6V for bulk charge and 13.6–13.8V for float.

Can I charge a car battery with it?

Yes, if the charger current rating and cutoff settings match the battery type.

What components are used for auto cutoff?

Comparator (LM358/LM324), reference (zener/precision), MOSFET or relay, current limit stage, and filters.

Is float charging supported?

Good designs switch to float mode to maintain battery at a safe lower voltage after full charge.

How to set charge current?

Choose current by design (resistor or CC stage) usually 0.1C–0.3C for lead-acid; use current limiting/regulation for accuracy.

Do I need temperature compensation?

Recommended: leads acid batteries need lower cutoffs at higher temperatures—temp sensor improves longevity.

What safety features are essential?

Input fuse, reverse polarity protection, thermal management, and overcurrent protection.

Can I use an SMPS instead of a transformer?

Yes, using a regulated SMPS (≥ charger power) improves efficiency and reduces heat.

Is a relay or MOSFET better for cutoff?

MOSFETs offer silent, fast switching and lower loss; relays provide galvanic isolation but wear mechanically.

Rojony Khatun

Rojony Khatun

CalculatorToolsProject

Rojony Khatun is the creator of a free online tools website designed to simplify everyday and engineering-related calculations for students, professionals, and hobbyists. The platform offers a wide variety of basic and advanced calculators across key categories, including Electrical Calculators, Electronic Calculators, and Time & Date Calculators, helping users solve problems quickly and accurately.