Help with a battery pack design

Ok so I have a wireless device the plugs into the wall with no option to make it completely wireless (darn..guess I have to do it).

What I know is that the power supply outputs is DC 5V/2.5A, switching type. Also the tech specs say "Power Communication 4.5W (900mA x 5v)" which I don't know what they mean by power communication.

I'm thinking of going to Radio Shack and getting some AAA, AA packs and testing the output and then just putting some diodes on until I get the desired output.

Can anyone help and elevate some of the guess work with some tips for me? Should I be working for an output of 5v/2.5a from the battery pack?

Thanks for any information. Ryan

Reply to
Ryan
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I don't have a plug-pack. I have the power supply for the device which states Output 5V @ 2.5A. But I also look at the tech specs online which stated "Power Communication 4.5W (900mA x 5v)" which I have know clue what they mean by this?

Do it mean the device can operate at 5v @ 900mA? Can I test this any way?

Reply to
Ryan

4 AA? How long would they last? And how can I calculate how long so I can type in differet numbers for different batteries?

Thats what I though as well but its not. It might be a typo in the doc though.

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Reply to
Ryan

You need to supply 5V at 900ma to power the device. A set of 4 2400mah NiMH will give you 4.8volts for less than 3 hours.

--
Luhan Monat: luhanis(at)yahoo(dot)com
http://members.cox.net/berniekm
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Reply to
Luhan Monat

So if I go with something like 6v @ 2900mah with a 5v zener diode would I be ok?

Reply to
Ryan

The problem is I don't have a battery pack. I only have the power supply that plugs into the wall. I'm looking to make one from parts. I just don't know enough of the engineering to do it correctly the first time. (Trying not to destroy my device)

Thanks for the replys. Ryan

Reply to
Ryan

I agree, but I'm trying to learn as I go. I got 4 NmH AAs to work with, no I just need to get battery life to last longer. I guess the question I'm asking what does the 5.1v zener diode do? Does the diode only allow the 5.1v to pass through? And if so how much tolerance would it have, 6v ok 20v bad.

5v @ 10000mah would be ok, right, it would just mean the battery power would last longer?

Thanks for the patients and help.

Ryan

Reply to
Ryan
4 Batteries in series will probably make it work but the batteries will not last long enough to be useful.

I think the label may say power consumption not communication.

--
Dan Hollands
1120 S Creek Dr
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Reply to
Dan Hollands

So, I used 4 NiMhs that tested at 5.1v fully charged. The device ran for about 25min, I added a simple toggle switch so I could kill the use of the batteries. I turn the device on and off a couple of time to change settings. I'm recharging now to do a constant run to get a better time. When I took them out they tested at 4.6v. So indeed you are right when the voltage drop it wasn't able to run the device.

My question was more geared to learn the function of a diode. So I get now that it takes (wastes) current but if I passed 20v to a 5.1v diode would to still function or would it burn?

So this brings up the question what can I do to make the life last longer now. Different batteries but I don't know of any that run a 5v that provide more mAh, thus the need to bring the voltage down.

Reply to
Ryan

So I'm understanding the flaw in using a diode now...enless I use something that gives me a larging increase in mAhs its a waste.

Thats a good question. First test using Luhan proposal gave me about

25min of run time (testing again after recharge complete). I would like a lot more then I'm going to get for the size I want but I think a good goal would be 2-3hrs.
Reply to
Ryan

It's probably a convenient inexpensive 'off the shelf' unit that made sense to package with the device.

It can cost *more* to make a 'special' with a lower rating !

That sounds like the actual power consumption.

It doesn't matter if the power supply is capable of delivering a bit more.

That's what it sounds like to me.

Graham

Reply to
Pooh Bear

NOOOOOOOOOOOO !

I think you need to do electricity 101 first !

Graham

Reply to
Pooh Bear

Chinese English I suspect ! I've seen far worse. ;-)

Graham

Reply to
Pooh Bear

4 NiMhs or indeed 4 Nicads will give around 4.8 ~ 4.9 V terminal voltage ( close enought to 5 V ) when fully charged - which is clearly enough to make your device operate. Trouble is - when discharging - the voltage will gradually drop.

This may or may not - depending on the design of your device - prove to be problematic or not.

If you put a zener in parallel with the batteries - it'll simply exhaust them faster. It'll take current ( charge ) that that could be usefully used by the device.

There's no chance of 20V so why are you worrying ?

The more mAh you have - the longer your battery pack will last. As long as you don't put any zeners in there !

Pls report back !

Graham

Reply to
Pooh Bear

All batteries have a capacity in mAh (milliAmphours). This is marked on the battery and you can read it in the specifications for a certain battery.

A big battery like you will need can have a capacity of 4000 mAh. Which means it can deliver 4000 mAmps for 1 hour, or 1 Amp for 4 hours. Or 2 Amps for 8 hours. You need 2.5 Amps so it will run for 6 hours, or so.

You need recharchable batteries, of the biggest sort you can afford, 4 of them in series will probably work.

If your device really needs carefully regulated 5Volt DC you may need to use a few more batteries and build a 5Volt 2.5 Amp regulator.

If this battery needs to work 24/7 you may want to consider a 12Volt car battery, and a 5Volt regulator, because you need really big batteries if this is going to run without being recharched for days and weeks.

2.5Amps is a lot of current to take out continously.

A typical car battery can have 70Ah capacity. That is 70 Amps for 1 hour, or 1 Amp for 70 hours. 2 Amps for 35 hours. 2.5 Amps for...?

--
 Roger J.
Reply to
Roger Johansson

No. A zener is the wrong choice for you, unless you have one that is rated at over 5 watts at 5 volts. Don't know where you'll find a zener like that. You would not get a huge increase in run time, even if you could get the 2900 mah cells and a proper zener. With a 6 volts supply, and a zener shunt regulator, you have to waste about a watt of power.

The solution Luhan proposed - 4.8 volts using NiMh cells rated at 2400 mAh - would work, and wastes no power. You might be able to use a 6 volt battery with a 1N540x diode in series to drop the voltage to about 5.4 (wasting a bit over 1/2 watt). Or you could use a battery with a high efficiency (up to 95% efficient) DC-DC converter. But, if you need more run time than Luhan's solution, your battery capacity *must* be increased, no matter what configuration you use.

So - how much run time do you need?

Ed

Reply to
ehsjr

Does your plug-pack supply 5V DC @ 2.5A, and your wireless device require 5V (DC?) @ 900mA ?

If so then you can use this plug-pack for this device.

Ken

Reply to
Ken Taylor

Will you people please read the original post? Ryan wants to make a battery pack so that he doesn't _have_ to plug the damn thing in.

The existing power supply has the _ability_ to provide _up to_ 2.5 Amps. The device itself only needs 900 mA, which is still a lot. Luhan Monat got it right - a set of four 2400 mAH NIMH cells will power the thing for 2400/900 hours.

Do they make D sized NIMHs?

I'm thinking, a 6 volt golf cart battery and a silicon diode, if Ryan wants it to transmit for a full day before recharging.

Thanks, Rich

Reply to
Rich Grise

Good! You're getting it! :-) :-)

A diode usually conducts current in only one direction. In the reverse direction, an ordinary diode has a "reverse breakdown" rating - this is the voltage (remember, voltage is pressure, or force - current is flow) at which the diode will conduct in the reverse direction.

Zener diodes are almost the same as ordinary diodes, except that their reverse breakdown voltage is calibrated. But the thing is, once breakdown is reached, there is nothing to limit the current. It goes from infinite resistance to almost zero resistance. A 5.1V zener across a 6V battery will conduct as much current as the battery can provide until the battery gets discharged to a terminal voltage of 5.1V, which is that Zener's "switch point", if you will. A big, beefy 5 watt Zener could probably survive this, but this is NOT what you want to do.

An ordinaly silicon diode, like a 1N4004, has a forward voltage drop of typically, .7V. This _does_ throw away some of your battery capacity as heat, but unfortunately, some frat boy decided that 5V was a neat number to design equipment to run off of, and apparently in his ivory tower, they never noticed that there's no such thing as a 5V battery. Maybe it was a conspiracy.

But that's neither here nor there. I digress.

The point I'm getting at is, if you use 6V worth of batteries, and put a silicon diode, like a 1N4004 (actually, any of 1N4001-1N4007 will work, as will almost any silicon diode) in series, in the forward direction, that will give you 5.3 volts, which will probably run your equipment. For a while.

You really need to read something about the difference between voltage and current. Voltage is a difference of potential. It's almost exactly equivalent to pressure, like water pressure in a pipe. Electromotive force. Current is "how much is flowing (past some point)". You don't "pass" a voltage. You "apply" a voltage, and current "passes". Check some of these:

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I answered in another branch here - if you want it to last more than an hour or two, you're going to need a BIG HONKING BATTERY. :-) I suggested a golf cart battery, and was wondering if there are D sized NimHs.

Ditto!

Good Luck! Rich

Reply to
Rich Grise

...

Damn!

I've just done a quick google on '"D size" NIMH' and somebody's getting robbed or something. I have a AA NimH in my hand, and it's got "2000 mAh" emblazoned on its side bigger than its logo. But - wait, here's one:

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Most of the other links were in the 2000-2500 mAh range, which is a lot less than I'd expect from a D cell. Do they just put an AA in a D case, and sell them? That's a ripoff, in my book.

Does anybody remember "The Secret of NIMH?"

Thanks! Rich

Reply to
Rich Grise

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