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Charge Shuffler

Chris Greacen

©1994 Chris Greacen

Do you want to run 12 Volt appliances off a 24 Volt battery pack? You've got several options. The easiest but least elegant solution to the problem is to tap your battery pack in the "middle". The problem with this is it eventually leads to battery imbalance; the cells powering the appliance end up with lower voltages than their neighbors. For small appliances, you can use a linear regulator like the LM 317, or LM 7812 to "burn off" the excess voltage as heat. Functional but inefficient. Or you could use the more efficient buck regulator Homebrew circuit in HP#37, but still you're limited to about three Amperes. The elegant solution is to tap your battery in the middle, but use a circuit that shuffles the charge from the high pack to the low pack as necessary to keep them at equal voltages.

That's what this circuit does — and it will deliver up to three amperes to the lower 12 Volt battery. This means you can power appliances continuously which draw up to six amperes. And you can use 12 Volt appliances which draw even more than this, provided the appliance is turned off enough each day for the charge shuffler to "catch up".

The Vanner Voltmaster™ is a commercial circuit which does this same job, but shuffles more current (5, 10 or 25 Amperes). See Things that Work! HP#33, p 84. The Voltmaster also shuffles current both ways, whereas this circuit requires you to put the load on the 0-12 Volt pack. But this circuit does have a lower idle current — 1 mA compared with 17 mA or so for the Vanner 60-50A.

How it works

How does the circuit shuffle charge from the top pack to the lower one? It's easiest to think first of a buck regulator charging a 12 Volt battery from a separate 24 Volt battery. (for the basics of buck regulators see HP #37, page 40). Pulses of current from the 24 Volt battery flow through the inductor and into the 12 Volt battery. As each pulse dies, the inductor's decaying magnetic field draws yet more current up from ground through the diode. If the buck regulator and batteries were 100% efficient, the 12 Volt pack would get twice the current drawn from the 24 Volt pack.

Now imagine using the 24 Volts to charge the lower half of itself! The current flows in the somewhat bizarre path shown below.

Meet the Parts

The heart of the circuit is the LM2576 buck regulator chip. This, in combination with the inductor, diode and filter capacitors, changes higher DC voltage, low current into lower DC voltage, higher current. See homebrews in either of the last two issues for more on how this DC to DC converter works.

The two resistors R (use identical resistors, the value can be anywhere between 100 kQ and 250 kQ) and the 20 kQ potentiometer form a voltage divider, splitting the 24 Volts in half. This voltage is compared with the voltage of the lower 12 Volt pack in the LM339 quad

comparator. When the voltage of the "lower 12" (pin 7) is lower than half voltage of the 24 Volt battery (pin 6), then pin 1 of the LM339 goes low, turning on the LM2576. Otherwise, the pull-up resistor (R can also be 100 kfl to 250 kfl) from pin 5 of the LM2576 to 24 Volt keeps the LM2576 turned off.

Efficiency and Quiescent Draw

When the 24 Volt pack was giving 150 mA to the buck regulator, the regulator was delivering 250 mA to the lower 12 Volts of the pack. This works out to 83% efficiency (250 mA/(2 x 150 mA)). Expect this to lower to around 75% at maximum load.

When it's not shuffling current the circuit uses very little power. I measured 1010 ^A, or 1.01 mA. This low standby power criterion is why the circuit was built with the LM339 chip. Using the common LM331 comparator, or the LM723, would have cost more than five times as much quiescent power. The low power LP331 or LP339 could have been used, but it is less readily available than the LM339.

Building and Adjusting the Circuit

Even through the circuit switches power at radio frequencies, it works fine on a prototyping breadboard, using alligator clips to the batteries. Try to keep wire leads short. For a more permanent construction, I used the ugly but cheap ham radio "bug" technique: use a piece of unetched PC board and solder all grounded pins of the chips directly to it. Bend up the other pins and solder the connecting components. The low parts count of this circuit makes this less messy than you might imagine. The technique provides a great ground plane, and it's cheap and quick.

Connect the ground wire to the battery first, then the 24 Volt wire, and finally the 12 Volt wire. To disconnect, pull the 12 Volt wire first. You'll probably need to adjust the potentiometer to get the battery voltages equal. This is easiest with two voltmeters, one across each of the 12 Volts. But it's possible to use one: attach one lead of the voltmeter to the +12 Volt, and the other alternately to 24 Volt or 0 Volt.

Radio Frequency Interference

Since the switcher operates at 52 kHz, I wouldn't recommend using it to power radio equipment which operates near this band. But I noticed little interference on an FM (~100 MHz) radio. If you notice interference, there's always the option of temporarily disconnecting the charge shuffler when you want to use the power from the lower 12 Volts. Use a switch on the wire to the +12 Volt to do this. When you reconnect, the charge shuffler will work to bring the voltages back to equal.

Access

Chris Greacen, Rt. 1 Box 2335B, Lopez, WA 98261 • 206-468-2838 (Mondays).

Parts: DigiKey now sells the LM2576T-ADJ — for $14.16. But they don't have any in stock and they don't know when they will have them. Call 1-800-DIGIKEY. I'll order a bunch of these and sell them to you for $15 each — but I warn you, depending on how many folks are interested, these too may be on back order. Schottky diodes (1N5822 3 Amps, 40 PIV) are available from All Electronics • 800-826-5432. Hosfelt Electronics, 2700 Sunset Boulevard, Steubenville, OH 43952 • 614-264-6464 has a 160 ^H inductor for a buck, part number 18-123. All the other parts are widely available. I'll send you all the other parts (not including the LM2576), but including a piece of unetched circuit board, fuses and holders, instructions, and a box for $20. Allow six weeks for delivery.

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Things that Work!

Things that Work! tested by Home Power

Things that Work! tested by Home Power

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Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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