Summary of DUAL-LEVEL FLOAT-CUM-BOOST CHARGER WITH PRE-CHARGE FOR 12V – 3 TO 6AH SEALED LEAD ACID (SLA) BATTERIES
This SLA battery charger project is a dual-level float-cum-boost charge controller with pre-charge, designed to optimally charge 12V, 3–6Ah sealed lead-acid batteries. It provides 500 mA output, constant current/constant voltage regulation, pre-charge for deeply discharged batteries, temperature-compensated precision reference, and status pins for monitoring or MCU connection. Built around the Texas Instruments BQ24450 IC, it drives an external pass transistor (NPN or PNP) and sequences charge stages via internal comparators and state machine to maximize battery life and capacity.
Parts used in the SLA Battery charger project:
- BQ24450 IC (Texas Instruments)
- External pass transistor (NPN or PNP)
- Resistor R7
- PRE-CHG pin connection
- CE pin connection (Pin 12)
- Voltage reference V-ref (2.3V) internal to IC
- Supporting resistors and wiring for current sensing and voltage control
- Power supply and battery connection terminals
- Optional microcontroller or monitoring connections to status pins
This SLA Battery charger project is the right choice if you want to obtain maximum battery service life and capacity, along with acceptable recharge time and economy, constant current, constant voltage, and even charge a deeply discharged battery.
The project contains all the necessary circuitry to optimally control the charging of valve-regulated lead-acid batteries. The circuit controls the charging current as well as the charging voltage to safely and efficiently charge the battery, maximizing battery capacity and life. The circuit is configured as a simple dual-voltage float-cum-boost charge controller. It is configured for charging 3 to 6Ah, 12V SLA batteries with dual-level float-cum-boost charger with pre-charge function and provides 500mA current output. The circuit also helps to charge deeply discharged batteries with pre-charge function. In this function Pin 12 (CE) detects the battery voltage, if the voltage at pin12 (CE) is bellow V-ref (2.3V), enable comparator permits a pre-charge current IPRE to flow from the PRE-CHG pin through Resistor R7 into the battery.
The project is built using BQ24450 IC from Texas instruments, which has built-in precision voltage reference that is especially temperature-compensated to track the characteristics of lead-acid cells, and maintains optimum charging voltage over an extended temperature range without using any external components. The ICs low current consumption allows for accurate temperature monitoring by minimizing self-heating effects. The project is configured to charge a 3-5Ah battery with dual-level float-cum-boost charge with pre-charge, but IC can support a wide range of battery capacities and charging currents, limited only by the selection of the external pass transistor. The versatile driver for the external pass transistor supports both NPN and PNP types and provides at least 25mA of base drive. In addition to the voltage- and current-regulating amplifiers, the IC features comparators that monitor the charging voltage and current. These comparators feed into an internal state machine that sequences the charge cycle. Some of these comparator outputs are made available as status signals at external pins of the IC. These status and control pins can be connected to a processor, or they can be connected up in flexible ways for standalone applications.
- What battery types and capacities does this charger support?
It is configured for 12V sealed lead-acid (SLA) batteries of about 3 to 6Ah, though the IC can support a wider range limited by the external pass transistor. - What is the charging current provided by the project?
The project provides a 500 mA current output. - Does the charger handle deeply discharged batteries?
Yes; it includes a pre-charge function that allows a pre-charge current IPRE when CE (Pin 12) detects battery voltage below V-ref (2.3V). - Which IC is used and why?
The project uses the BQ24450 IC from Texas Instruments for its temperature-compensated precision voltage reference, low self-heating, and integrated charge sequencing features. - Can the external pass transistor be NPN or PNP?
Yes; the driver supports both NPN and PNP external pass transistors and provides at least 25 mA base drive. - How does the circuit regulate charging?
The IC contains voltage- and current-regulating amplifiers and comparators that feed an internal state machine to sequence the charge cycle, controlling both current and voltage. - Are there status outputs for monitoring?
Yes; some comparator outputs are available as status signals on external pins for processor connection or standalone use. - Is temperature compensation included?
Yes; the IC has a built-in precision voltage reference that is temperature-compensated to maintain optimum charging voltage over temperature.
