Help Building a battery pack

Some time ago, a friend asked me to rig up something for the same purpose. In my part of the world, the original 7.2V pack is difficult to get, quite expensive and doesn't last very long. OTOH, 6V general-purpose sealed rechargeable batteries are cheap and easily available. So I took two of them and used an LM317 IC to regulate the resultant 12V down to 7.2V.

The camera now receives a constant 7.2V supply even when the 12V battery voltage level varies from full charge to full discharge. He's a semi-pro and doesn't mind the extra weight and bulk.

The linear regulator generates considerable heat when the camera is running, so I used a fairly hefty finned heatsink from my junk box. A switched-mode regulator would be more efficient but more complex.

Try

I have one, and it is great. It would be nicer if it had kelvin sense leads, and the adjust knobs were better quality (or had a digital input, like some of the pricier ones) but it works fine for me, and didn't cost a fortune.

In particular, for your battery job, you can limit both voltage and current. I'm guessing your batteries are liion, so you probably want to limit the voltage appropriately.

Another idea would be to look on EBay; bench supplies come up all the time.

Another idea, you can get a nice charger at frys for $10.

--
Regards,
  Bob Monsen

Man, as a social animal, can no more escape government than the 
individual can escape bondage to his bowels.

Hi, Well, that's the regulator I was looking at Radio Shack/Tandy Adjustable-Voltage Regulator LM317T (see link below). I was expecting to see a dial on it when I went to the store, but it didn't look like it had a dial or switch on it at all, from what I could see shaking it in it's container. The radio shack people knew nothing... So how on earth do I get it to reduce voltage to whatever output voltage I want to set it to?

I've looked at electrical books/diagrams, and I've seen one that uses another component in the layout - but the diagram was an electronics diagram with electronics coding etc, so is confusing to an electronics dummy that just wants in-out lines-components. Thanks

You're talking about an LM317 regulator chip. Here's how you can use it:

Look on the first page for "Typical Applications" - there is a diagram there of the LM317 with two additional parts - a 240 ohm resistor and a 5K potentiometer. The capacitors on the diagram are not always needed - see the notes.

Note that the input voltage has to be at least 3 volts higher than the output voltage. If you want to keep the maximum possible voltage to ~7.2, yet still be able to adjust it lower, use this circuit: (one added 1500 ohm resistor)

-----

+12 ---in|LM317|out---+------> + Vout ----- | Adj [R1] 240 | | +----+---------+ | | | | / | | \\R2 | +--->/5K [R3] 1500 \\ | / | | | Gnd --------+---------+-----> Gnd

Install the LM317 on a heat sink.

Ed

Unfortunately I just need to lower it around 1 volt. I had some NiCD batteries, and got NiMH's to replace. The NiCD used to work fine. The NiMH's seem to charge to a total of around 8 volts or more even though they are two 3.6 volts batteries. Anyway, if the input needs to be 3 volts higher, I'm missing 2 volts. Thanks

I think you may be able to get this to work with just 1 cheap part.

If all you need to do is lower it (your battery pack) about

1 volt, and you want to do that as simply as possible, put a 1N4001 diode in series with the pack and the device, with the banded end of theconnected to the negative side of the battery. That will reduce the voltage about .7 volts. *However*, the pack will soon reduce to the nominal 7.2 volts, and the diode will further reduce that to about 6.5. If your device is "happy" with that, you can use this simple approach. It does not precisely meet your stated requirements (#1 and #2, as it will allow about 7.4 volts output, not 7.2 as you stated, and it might stop the device from working by reducing the voltage to the device to 6.5, even when the battery is not low.) But it is the simplest you can get, and it stands a good chance of working for you.

Here's what is going on that is causing your 7.2 volt pack to provide 8+ volts: a 7.2 volt pack consists of 6 cells whose nominal voltage is 1.2 volts per cell (6 * 1.2 = 7.2). When you charge the batteries, they get charged to a hair over 1.4 volts per cell (as they should be), and the pack voltage is over 8 immediately after the pack is taken off the charger. It soon reduces to 7.2 when used.

Finally - if the single diode approach won't work, use the LM317 circuit already posted. The input voltage will need to be over 10 volts - which you can get by using 3 3.6 volt packs, or a 12 volt DC wall wart supply.

Ed