# Battery Substitution



## DarkLore (Jan 25, 2009)

After sitting on a shelf for the past year or so, I'm finally hooking up a MDFLY audio card. This is not the card currently offered, but rather the card we referred to in an older audio thread.

It's 1:40am and not the time to run up to the local electronics part store. I see that the board calls for 5v. Obviously matching the 5v on a USB. Hmm...I've got a 3AA battery pack, a 4AA pack, and various wall warts that aren't 5v. So...when is it appropriate to slap one of these on to a board...knowing the battery pack doesn't exactly match the requested voltage?

I will wait and run up to the store tommorrow and try to pick up a 5v pack with a regulator. But the question stands. When is it okay to risk using a 3AAA (4.5v), or 4AAA (6v) on a board?

(Note - for this prop, I have a Scary Terry Cowlacious board, intended to be driven using the MDFLY board. I'd prefer to drive them using one or two battery packs.)


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## corey872 (Jan 10, 2010)

I'd think you almost have to look at it on a case by case basis. If the board has some type of built in regulation, then either of those two voltages would probably work OK. Or if the board has no regulation, but is still fairly tolerant of voltage (most chips run on a fairly wide range), then probably OK, too.

One place I've run into trouble was in powering various LED and laser circuits. Some of the really cheap ones actually rely on using a relatively small battery pack and are somewhat automatically current limited because the batteries can only put out so much power. If you try to duplicate the battery rated voltage with a source which can put out full current at that voltage, the LED / laser goes 'POP'.


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## fritz42_male (May 5, 2009)

I'd always go for the lower voltage first. Most boards are happy with 4.5 instead of 5V but often if the boards don't have an onboard regulator, a higher voltage will fry them.

You would think that by now, some enterprising battery manufacturer would have come up with a 5V battery!


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## pshort (May 6, 2008)

Four ni-cad or ni-mh cells come out to 4.8V nominal, which is pretty close.


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## corey872 (Jan 10, 2010)

> You would think that by now, some enterprising battery manufacturer would have come up with a 5V battery!


LOL - Surprisingly, that is a bit harder than you think. The chemistry of the cell dictates the voltage of the battery...alkaline = 1.5, nicd = 1.2, lithium 3.0, nimh = 1.4. etc. If you make 1/2 a cell (or any other size) you still get the same voltage, just a different capacity. None of the common chemistries add up to 5.0V, so that is out. You could possibly invent a new chemistry, but it might be crazy expensive, highly toxic, very low energy density, prone to explosion/fire, or who knows what other problems. If you did discover something, it would only be 5.0V for a split second - ie freshly charged alkaline batteries are usually around 1.7-1.8, and considered 'dead' at ~1.2, so nominal is 1.5.

In the end, it's much easier to make a circuit tolerant of +/- 0.5 volt than make new batteries.


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## fritz42_male (May 5, 2009)

I was thinking more along the lines of some battery manufacturer building a regulator circuit in with the battery array so that a higher voltage array would provide a nice 5v supply. I mean I can buy a 3w stereo amp for $2.50 so a regulator circuit would be pennies by comparison.

Come to that, why not come up with a 4 or 6 aa or aaa battery box with built in 5v ldo regulator?


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## corey872 (Jan 10, 2010)

Those are made:
http://www.bixnet.com/usbbaboxc4aa.html

Though if you're looking to use one for every device, the downside is cost, and some power is lost in the voltage regulation. But if the benefits outweigh those issues, it can be a good solution.


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## HomeyDaClown (Oct 3, 2009)

fritz42_male said:


> I was thinking more along the lines of some battery manufacturer building a regulator circuit in with the battery array so that a higher voltage array would provide a nice 5v supply. I mean I can buy a 3w stereo amp for $2.50 so a regulator circuit would be pennies by comparison.
> 
> Come to that, why not come up with a 4 or 6 aa or aaa battery box with built in 5v ldo regulator?


How about a 2 AA battery box?

http://www.pololu.com/catalog/product/781

A basic buck boost/joule theif circuit and ldo to over-boost the voltage and then regulate it back down.

Lots more here: http://www.bodhilabs.com/

Or feed this guy with anything from 1.5 to 16 volts and get 4 to 25 volts out regulated.

http://www.pololu.com/catalog/product/799


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## fritz42_male (May 5, 2009)

Exactly. Those are (relatively) pricey solutions though; I suppose if the demand was there they would be cheaper. I like this though:

http://www.pololu.com/catalog/product/798


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## HomeyDaClown (Oct 3, 2009)

fritz42_male said:


> Exactly. Those are (relatively) pricey solutions though; I suppose if the demand was there they would be cheaper. I like this though:
> 
> http://www.pololu.com/catalog/product/798


Yep and really that's about the lowest price you'd find. You can't build one from scratch for that price. I built 78 buck boosters for powering white led candles (3.6 volts) from a single AA last November each with a 2n4401 transistor a resistor and a toroid coil and unregulated. You could do the same only add another resistor and a 5 volt zener and get the cost to about $2.50 or even less. But you have to wind the toroid.

On the left is the original yellow led tunning on 2 AAs.








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