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optimizer
Joined: 05 Dec 2005
Posts: 3
Location: right here on the screen
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 Posted:
Mon Dec 05, 2005 5:43 am Post subject:
Help Building a battery pack |
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Hi,
I built a 7.2 V battery pack using two rechargeable 3.6V dc batteries. They eventually died and I purchased some different new 3.6 V replacements. Unfortunately, when these are charged they pump out a total of 8+ volts and shut down my device.
I'm an electronics dummy other than the basics. I thought I was going to be able to buy a 0 to 32 volt regulator from the Shack and just turn the switch (or adjust it) on it to 7.2 Volts, but upon seeing one in the store, there is no switch/adjuster, just a block and 3 metal pins.
Basically, I want something that I can just attach to the battery pack output wire that will ensure that 1/ no more than 7.2 V DC gets output, 2/ it allows voltage less than 7.2 Volts (it doesn't stop the circuit if the battery is getting low), 3/ Is safe - I can just attach it into the wire and no real electrics shock protection (it's not going to build up a huge charge or anything), "just cover with electrical tape" type of project.
I suppose whatever is suggested may cause heat, need heat to be released. Let me know/advice if that might be an issue.
It's just for a small video camera - so doesn't use much juice.
Thanks for doing the forum. Hope I get some good help, and hopefully use it for some other projects.
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Guest
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| Posted:
Tue Dec 06, 2005 1:32 am Post subject:
Re: Help Building a battery pack |
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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. |
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optimizer
Joined: 05 Dec 2005
Posts: 3
Location: right here on the screen
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Tue Dec 06, 2005 5:16 am Post subject:
Thats the regulator I was looking at - how do I use it |
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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
http://www.radioshack.com/product/index.jsp?productId=2062601&cp=2032058&f=Taxonomy%2FRSK%2F2032058&categoryId=2032058&kwCatId=2032058&kw=regulator&parentPage=search
[/url]
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Bob Monsen
Guest
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| Posted:
Tue Dec 06, 2005 9:02 am Post subject:
Re: Help Building a battery pack |
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On Mon, 05 Dec 2005 03:34:55 -0600, optimizer wrote:
<his lust for an adjustable supply exposed>
Try www.mpja.com: http://tinyurl.com/733hn
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. |
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ehsjr
Guest
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| Posted:
Tue Dec 06, 2005 9:35 am Post subject:
Re: Help Building a battery pack |
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optimizer wrote:
| Quote: | Hi,
I built a 7.2 V battery pack using two rechargeable 3.6V dc batteries.
They eventually died and I purchased some different new 3.6 V
replacements. Unfortunately, when these are charged they pump out a
total of 8+ volts and shut down my device.
I'm an electronics dummy other than the basics. I thought I was going
to be able to buy a 0 to 32 volt regulator from the Shack and just
turn the switch (or adjust it) on it to 7.2 Volts, but upon seeing
one in the store, there is no switch/adjuster, just a block and 3
metal pins.
|
You're talking about an LM317 regulator chip. Here's how you can
use it:
http://cache.national.com/ds/LM/LM117.pdf
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
| Quote: |
Basically, I want something that I can just attach to the battery pack
output wire that will ensure that 1/ no more than 7.2 V DC gets
output, 2/ it allows voltage less than 7.2 Volts (it doesn't stop the
circuit if the battery is getting low), 3/ Is safe - I can just attach
it into the wire and no real electrics shock protection (it's not
going to build up a huge charge or anything), "just cover with
electrical tape" type of project.
I suppose whatever is suggested may cause heat, need heat to be
released. Let me know/advice if that might be an issue.
It's just for a small video camera - so doesn't use much juice.
Thanks for doing the forum. Hope I get some good help, and hopefully
use it for some other projects.
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optimizer
Joined: 05 Dec 2005
Posts: 3
Location: right here on the screen
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| Posted:
Tue Dec 06, 2005 8:58 pm Post subject:
but I just need to lower 1 volt |
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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 |
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ehsjr
Guest
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| Posted:
Thu Dec 08, 2005 1:35 am Post subject:
Re: but I just need to lower 1 volt |
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optimizer wrote:
| Quote: | 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 |
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