Your LEDs might be rated with a typical forward voltage of 2.2V but each LED might be different. Some might be only 2.0V and others might be 2.5V.
If you connect 8 LEDs in series and each has a forward voltage of 2.5V then their total voltage requirement of 20V would make them fairly dim if the current limiting resistor value was calucated for a voltage of 2.2V per LED (17.6V total).
If the LEDs are all 2.0V then they all might burn out.

So maybe you should measure them all, group them and connect an appropriate resistor value to each group.

 

Thanks a lot for everyone's help!

A couple final questions:

*Is it okay to solder wire directly to a battery?
*What is the best way to connect two 12v batteries together for a 24v supply?
*About how long would a single 12v battery last for 50 or so LEDs for this project?

 

Many batteries have plastic insulation inside that is destroyed by soldering. Use a battery connector.

You cannot see the datasheet for a battery with no part number. There are huge batteries that will last for months and there are tiny batteries that will last for a couple of minutes.

 

Many batteries have plastic insulation inside that is destroyed by soldering. Use a battery connector.

You cannot see the datasheet for a battery with no part number. There are huge batteries that will last for months and there are tiny batteries that will last for a couple of minutes.

How about this 12v battery? It's what I have.

 

Did you look at the datasheet for your tiny 12V alkaline battery?
It can supply only 0.48mA for 81 hours when its voltage has dropped to 8.8V.

It is designed to supply high current pulses (of tens of mA), but not continuously.

If you connect your 120 LEDs in series groups of 4 with a current-limiting resistor for each group and have the current in each group 20mA then the total current is 600mA and the tiny battery might last a few seconds.
If you connect two tiny batteries in series to make 24V and the LEDs in groups of 8 then the current will be 300mA and they will last a few more seconds.

Three 9V alkaline batteries in series might last one hour with the LEDs slowly dimming the entire time.

 

Did you look at the datasheet for your tiny 12V alkaline battery?
It can supply only 0.48mA for 81 hours when its voltage has dropped to 8.8V.

It is designed to supply high current pulses (of tens of mA), but not continuously.

If you connect your 120 LEDs in series groups of 4 with a current-limiting resistor for each group and have the current in each group 20mA then the total current is 600mA and the tiny battery might last a few seconds.
If you connect two tiny batteries in series to make 24V and the LEDs in groups of 8 then the current will be 300mA and they will last a few more seconds.

Three 9V alkaline batteries in series might last one hour with the LEDs slowly dimming the entire time.

Then what would be a good suggestion for something that would last at least a few hours? It needs to be portable and at least somewhat practical.

Could I possibly connect nine 9v batteries together for an 81v supply and have it last several hours?

 

You could use 24 LEDs in series with a current-limiting resistor as one string and make 5 strings for a total of 120 LEDs. If you power them with nine 9V batteries in series then the batteries will last for about 40 minutes with the LEDs dimming slowly all the time. At 40 minutes the LEDs will be so dim that they are hard to see.

 

You can guess a battery of certain charge in Ah to be able to deliver 1/10 that much current for 10 hours. For example a 6 Ah battery will deliver 0.6A for 10 hours.

You really need to see the most efficient way of lighting the sword. Take a look at how commercial products are done, and try to copy the idea. You really want to use the least amount of current possible.

Take a read at the battery information here, it could be useful to select the battery type: http://www.batteryuniversity.com/

 

Ah, I think I see now.


I've decided to compromise and double the distance space between LEDs, so only 60 will be used.

About how long would battery life last if the power supply is three typical 9v batteries hooked up in parallel instead of in a series?

 

I wouldn't wire batteries in parallel, they won't ever have the same voltage. Better to connect them in series.

Anyways, 9v batteries aren't known for their charge capacity: http://data.energizer.com/PDFs/522.pdf

As you can see, to get 10 hours out of one of those, you can use 50mA at most, with the battery voltage lowering to about 6v.

Let's suppose that you wire them to try to use as much as the 9x3=27v available. With 2.2Vf of every led, that would be 27/2.2=12 leds, but then, the leds would turn off when the voltage lowers to less than 12 * 2.2 = 26.4v, which is too fast. If you want the leds to remain on when they're running at 6v, you can use at most 18v/2.2v=8 leds.

For 8 leds, the total Vf is 2.2v x 8 = 17.6v, so with the batteries fully charged, you need a resistance of (27 - 17.6v) / 10mA = 940 ohms. That would deliver only (18 - 17.6v) / 940 ohms = 425 uA when the batteries are at 6v each, which would be way too dim to see them. You could deliver more current at 9v per battery, but they will last less time.

Anyways, at 8 leds per "string", you need 60 / 8 = 7 or 8 strings for the 60 leds. At 10mA per string, that's 70 to 80 mA.

There are several kind of batteries out there. But for 12 hours of operation and this small, the battery needs to have a lot of energy density.

 

If this is to be trusted, you could get that amount of hours from D batteries at 300mA: http://data.energizer.com/PDFs/E95.pdf

You could use 4 batteries to get 6v, that can power 30 "strings" of 2 leds and a resistor each, with 10mA per string that's 300mA.

If you use rechargeables, you need to check if the voltage rating and duration at 300mA is enough.

Why do you need so many hours, anyways? Is someone supposed to be using this for 12 hours straight?

 

I wouldn't wire batteries in parallel, they won't ever have the same voltage. Better to connect them in series.

Anyways, 9v batteries aren't known for their charge capacity: http://data.energizer.com/PDFs/522.pdf

As you can see, to get 10 hours out of one of those, you can use 50mA at most, with the battery voltage lowering to about 6v.

Let's suppose that you wire them to try to use as much as the 9x3=27v available. With 2.2Vf of every led, that would be 27/2.2=12 leds, but then, the leds would turn off when the voltage lowers to less than 12 * 2.2 = 26.4v, which is too fast. If you want the leds to remain on when they're running at 6v, you can use at most 18v/2.2v=8 leds.

For 8 leds, the total Vf is 2.2v x 8 = 17.6v, so with the batteries fully charged, you need a resistance of (27 - 17.6v) / 10mA = 940 ohms. That would deliver only (18 - 17.6v) / 940 ohms = 425 uA when the batteries are at 6v each, which would be way too dim to see them. You could deliver more current at 9v per battery, but they will last less time.

Anyways, at 8 leds per "string", you need 60 / 8 = 7 or 8 strings for the 60 leds. At 10mA per string, that's 70 to 80 mA.

There are several kind of batteries out there. But for 12 hours of operation and this small, the battery needs to have a lot of energy density.

Well gosh, you just shot down every idea I could think of! Is it really so impossible to conventionally power more than a few LEDs for any real length of time?

If this is to be trusted, you could get that amount of hours from D batteries at 300mA: http://data.energizer.com/PDFs/E95.pdf

You could use 4 batteries to get 6v, that can power 30 "strings" of 2 leds and a resistor each, with 10mA per string that's 300mA.

If you use rechargeables, you need to check if the voltage rating and duration at 300mA is enough.

Why do you need so many hours, anyways? Is someone supposed to be using this for 12 hours straight?

The prop is for an event which lasts all day/night so it has to last a while. The battery power doesn't have to last 12 hours continually but I'm thinking it should at least last a few hours before having to change batteries to remain practical.

If the LEDs were only needed to be powered for 3 hours before having to change batteries, would there be a more practical sollution?

 

Lithium 9V batteries seem pretty good, but they are pricey.
http://data.energizer.com/PDFs/la522.pdf
Lithium AAs would be better if you could squeeze 6 or more in there. I use them in my camera and get 1000+ shots compared to 500 shots from 2500mAh NiMH AAs or about 100 shots from alkalines (they are really bad at very high drain).

 

You should also experiment to see how bright is OK for your needs. You may only need 10ma for the brightness you want.

 

I have never seen the new Energizer Lithium 9V battery.
I made a comparison with their 9V alkaline battery:

 

If the LEDs were only needed to be powered for 3 hours before having to change batteries, would there be a more practical sollution?

The 4 D Cells should be able to do it, even at 12 hours straight (if not, you can blame energizer ). Did you read my post?

If this is to be trusted, you could get that amount of hours from D batteries at 300mA: http://data.energizer.com/PDFs/E95.pdf

You could use 4 batteries to get 6v, that can power 30 "strings" of 2 leds and a resistor each, with 10mA per string that's 300mA.

What do you mean with more practical? Smaller? Rechargeable? Both?

 

The 4 D Cells should be able to do it, even at 12 hours straight (if not, you can blame energizer ). Did you read my post?


What do you mean with more practical? Smaller? Rechargeable? Both?

Didn't you say though that it could only be done with the D batteries at 10ma? I need 20ma for the LEDs.

By "practical" I just meant something that could feasibly be carried or embedded into a prop. It sounds like the only way to power 50 LEDs for a few hours is to plug it into a wall. Which I certainly hope is not the case!

 

If the room is not lighted then a small battery can dimly light 50 LEDs for a few hours.
Each LED would need only 1mA or less.

But if the room is lighted and you want the LEDs to be bright at 20mA each then you need 20 little batteries, one huge battery or plug them into a wall.

 

Can a batterypack in a hippack or fanny-pack hold the batteries and you simply run some wire to the sword? What would help. You can make a bandoleer with batteries to get the Ah required for the project. You could also use a fanny-pack with a sealed lead-acid battery in it that would also have enough umph to handle the job.

I think using an off-sword power supply is a good idea. Is there any part of the costume that can be used as a battery hide?

 

If the room is not lighted then a small battery can dimly light 50 LEDs for a few hours.
Each LED would need only 1mA or less.

But if the room is lighted and you want the LEDs to be bright at 20mA each then you need 20 little batteries, one huge battery or plug them into a wall.

They need to be 20ma. 1ma would be far too dim.

I don't mind using "little batteries", but what should I use exactly?