A 6V Lead-Acid Battery Charger Using the LM555

How do you power a device that must stay on through an event that may cause a power outage? Battery backup of course. But that answer leads to whole new level of complications. There must be a circuit in place to allow power to be drawn from the battery or the power supply. The proper battery technology should be chosen. You need a another circuit to care for the battery, allowing for very long term reliability. Long term? Years.

6V Lead-Acid Battery Charger
A 6V lead-acid battery charger based on the venerable LM555.
A good answer for the battery technology is sealed lead-acid. A properly used lead-acid battery should last a decade or more while providing power to operate for over a day. Lead-acid may not be a good choice for a portable device where weight and size are the primary considerations. But for a stationary application this venerable technology is a good choice.

What about the charge circuit? A simple charger that can fully charge the battery, but not overcharge the battery is needed. In the case of lead-acid charging is usually accomplished by charging to a chosen voltage before shutting off or lower the voltage to a safe lower value called a “float charge”.

It is in the choice of that voltage that things get complicated. The charge voltage must be above around 2.15V per cell to charge the battery, higher voltages will allow the cell to be charged more quickly. As the voltage and current is increased more care must be taken as to not damage the battery.

Temperature (°C):

Valid from -20 to 50°C only

Battery Gassing Voltage (v):

Often the consideration is in what is called the gassing voltage, the voltage at which point the battery will start to generate gas and lose electrolyte. Gassing can be dealt with in flooded batteries by simply replacing the lost water. Sealed lead-acid batteries contain mechanisms for recycling the released gas, but these have limited effectiveness.

Thus many chargers are designed to keep the charging voltage below the gassing voltage. This is made more complicated as the gassing voltage changes with temperature, a higher temperature will lower the gassing voltage. Either the battery temperature must be measured and the voltage compensated, or some assumptions made. Often we simply choose to keep the voltage a bit lower and live with a slower charging cycle.

For this design I chose a 6 volt battery. As the powered device requires 5 volts, this allows the use of a low-dropout regulator and minimal inefficiencies. The assumption is also made that the battery should stay at 20°C or less based on where this will be installed. Thus I chose to limit the voltage to 2.3V per cell to stay below the gassing voltage of 2.4V per cell at 20°C.

The schematic for the 6V lead-acid battery charger
The schematic for the 6V lead-acid battery charger
The circuit uses common components found in any well stocked parts bin, nothing exotic here. The ubiquitous LM555 timer provides for the needed control logic. The equally common LM317 is used as a current regulator, and the LM78L05 is used for a voltage reference.

The charger is based on Kenneth Finnegan’s LM555 battery charger with a few key changes. The original design was setup for a 12 volt battery, this could not be used with a 6 volt battery without some substantial changes. The LM78L05 regulator draws power from the input of the charger, not the battery, as the 78L05 needs at least two volts headroom to maintain regulation. The resistors in the voltage dividers were re-scaled for the proper 6 volt charge points.

Keneth’s original circuit uses a level shifter from the LM555 output on pin 3 to shut down the charge current. This extra transistor can be eliminated by using the built in discharge transistor in the LM555 found on pin 7. Pulling down on this line disables the LM317.

Otherwise the circuit functions the same. Placing a 2.7 ohm resistor on the output of the LM317 as shown sets the charge current at a little under half an amp. This was chosen as the charger needed to share a one amp power supply with the remainder of the device.

The potentiometers are adjusted so that the charger shuts off when the battery voltage rises above 7.00 volts and activates when it falls below 6.00 volts. The seven volt upper charge point is chosen to keep the battery below the gassing voltage, which is about 2.4 volts/cell at 20°C, or about 7.20 volts.

A few options are possible here to adapt the charger to the application. A high value resistor could be placed from input power to battery plus to provide a trickle current charge. Re-scaling the output resistors would allow charging of a battery with a different voltage, anything from 4 to 20 volts should be possible. The circuit is suitable for lead-acid batteries only, it does not have the needed accuracy for lithium-ion chemistries.

The reminder here is that the venerable LM555 timer does far more than timing. With it’s high and low comparators, flip-flop, and output stages a creative designer can use it to solve so many problems. For an IC first created nearly five decades ago this simple little device remains a basic building block for any savvy designer.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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