Battery Basics (Part 2): Battery Back-Up Power

Circuit Cellar columnist George Novacek has been “burned” more than once by manufacturers of battery-powered devices that include chargers poorly suited for the batteries required.

To get the full life out of a rechargeable battery, the charger must be “specifically designed” for the battery type, Novacek says in his October issue column “Battery Basics (Part 2): Battery Back-Up Power.”

The charger schematic diagram is shown.

The charger schematic diagram is shown.

When cordless phone, lawn mower, or power tool batteries die before their time, it gets expensive.

“The replacement cost of battery packs in my equipment represents about 50% of the equipment’s purchase price,” Novacek says. “Consequently, I would have expected the supplied or built-in chargers to be optimized for the batteries. Unfortunately, this is just wishful thinking. I have spent a fortune on replacement power packs for several of my battery-operated devices.”

Novacek’s column discusses what causes short battery life, how to purchase the right charger, and tips on building your own charger.

“As an engineer, if I can’t buy what I need or have been burned by commercial,off-the-shelf equipment, I design my own solution to the problem,” he says. “Building a good battery charger is easy these days because many ICs are specifically designed for battery chargers.

“The plethora of available ICs—together with exhaustive application notes—enables you to design a charger with minimal external parts for almost any battery type. The resulting circuits are often small enough to fit inside enclosures of your original, poorly designed chargers.”

You can also build a good charger out of parts found in your component box, Novacek says. In fact, he did just that when he needed a reliable floating backup power source for the fans of his home’s gas-fired Franklin stove. “I selected two 12-V/7-Ah sealed lead-acid batteries connected in parallel. They are widely available, reasonably priced, and have adequate capacity for my needs. I also built the charger with components I had lying around my workbench.”

Photo 1: The charger is built on a small piece of a perforated board. An ample heatsink is needed during constant current mode. The six-pin header on the right side is used to in-circuit program the Atmel ATtiny85 microcontroller.

Here is a small portion of his description the charger design.

Figure 1 shows the charger schematic diagram… Series regulator U1, a Texas Instruments LM117 adjustable regulator, works as a current source with R4 determining the 1.5-A current. When the battery voltage reaches 14.5 to 14.7 V, MOSFET Q1 is turned on, changing U1 to a constant voltage source. Potentiometer R1 trims the voltage to 13.6 V, which is crucial for long battery life.

“U2 is a 5-V regulator that feeds the microcontroller U3, an Atmel ATtiny85, which monitors the charger’s status and switches between the operating modes. Zener diodes D7 and D8 and signal diodes D9, D10, and D11 together with JFET Q3 serve as a level shifter to monitor the battery voltage. They subtract 10.5 V from the battery voltage to place it within the 5-V input range of the ATtiny’s ADC.JFET Q3 ensures constant current through the diodes and the voltage drop across the diodes is essentially independent of the battery voltage.”

Photo 1 shows the charger construction. “The header at the right-hand edge is for the ATtiny85’s in-circuit programmer,” he says. “Also, notice that the ATtiny85 inputs are protected by Schottky diodes and 10-kΩ series resistors to prevent its input pins’ excursion beyond 5 V.”

Novacek’s column goes on to explain how you can change a few components of the charger to make it work with different lead-acid battery types.

For those details and others, check out Novacek’s full column in the October issue.


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Battery Basics (Part 2): Battery Back-Up Power

by Circuit Cellar Staff time to read: 2 min