Battery Dilemma

[Hippogriff]

OK,

Rather surprisingly, my little boost converter arrived from Hong Kong. It only took a couple of days.

I've unwrapped it and tested it in a variety of scenarios and I find the results rather interesting. For all four tests I used my bench PSU and my existing circuit and LEDs - the only change I've made (to test better) is set the PIC to control the LEDs in a simple way - 2 seconds all on, 2 seconds all off, repeatedly.

PSU directly to the circuit at 9v - the circuit drew 0.15a when it was set to white; although it wasn't white, it was kind of a bright pink.
PSU directly to the circuit at 12v - the circuit drew 0.55a when it was set to white.
Interposing the boost converter between PSU and circuit with the boost at 12v and the PSU at 9v - the circuit drew 1.01a - it was bright white.
Interposing the boost converter between PSU and circuit with the boost at 12v and the PSU at 12v - the circuit drew 0.73a - still bright white.

Interesting, I thought.

 

[Blueteeth]

Oh wow, sounds like its not completely efficient...

With your LED's 'drawing' 0.55A @12V = 6.6W. With an input of 1.01A @ 9V = 9W. Efficiency = 6.6/9 = 73% - thats actually quite good given the input-output difference and current draw.

With your output voltage the same as the input voltage, the boost may have trouble keeping the output voltage at 12V, because essentially its no longer switchin the internal MOSFET, duty cycle = 0%. Tis is probably why the efficiency with 12V and 12V out isn't great. (75%).

I would work out the best current for your LED's to run at. Looking at those modules I'm assuming they have a built in current limiting resistor, so the power voltage determines the current in the LED's, and therefore the brightness. The '12V' standard might mean the LED's are running at a fairly high current which is where their brightness specs are taken from. 9V - if its enough to even overcome the forward voltage of the LED's (to actually turn them on..) the voltage left across the current limiting resistor is tiny, and so the current is much much less.

Did you try it with an input of 4.5 - 6V? It could be that the boost chip used (like so many converters) are optimsied to run at a certain duty cycle - in terms of power conversion, this means it'll be most efficient when theres a certain input voltage to output voltage ratio. Eg. 6V in, 12V out. Generally though, the smaller the difference between input and output, the more efficient - but the boost converter itself will always need some power to do its thang.

APologies if it seems like I pushed you into getting that little boost converter and it didn't live up to expectations. They *are* handy for other projects though I bought several step down converters (8-40V in, 2-30V out @ 4A) and after a months they all found homes in battery chargers and car USB scokets.

I say, use your bench PSU to evaluate your LED modules, their current draw, with a certain voltage, and check the brightness. You might be able to change the in-built resistor to reduce how much power this disipates. Its all a comprimise of brightness, power, efficiency (larger resistor value = more power wasted) and complexity. 6W does sound like a lot, but that depends on how many LED's you're driving, and isn't totally ridiculous for battery powered apps with rechargables.

Blueteeth.

Ps. I suspect the reason you got 'pink' instead of 'white' at a lower voltage was the fact that red LED's have a much lower forward vltage than green or blue (1.7-1.9V for red vs 2.8-3.2 for green, and up to 3.6V for blue). So with a fixed voltage, the current in the green and blue dies was so low, they barely lit when compared to the red

 

[audioguru]

Can the boost converter provide an output of 12V when its input is only 6V? Then you can use 4 AA alkaline cells or 5 AA Ni-MH cells.
When the AA cells drop to only 4.0V does the boost converter still light the white LEDs?

 

[Hippogriff]

APologies if it seems like I pushed you into getting that little boost converter and it didn't live up to expectations.

Absolutely not, it was only £5.75 and I'm really pleased a) I know how to achieve this and b) that this can be achieved. That's a small price to pay to expand my investigations. I've appreciated all the input.

However, it has helped me arrive at one decision - my most desired solution is to get the 8 x AAs into the hat for 12v and long timetime. If I simply can't do that 'cos of physical space, I'll go with the 6 x AAs - but then I have the choice of whether to use the boost converter or settle for 9v.

As audioguru also asks... it's probably worth a test with the bench PSU at 6v and the boost converter at 12v. I'll get onto that...

 

[Hippogriff]

Can the boost converter provide an output of 12V when its input is only 6V? Then you can use 4 AA alkaline cells or 5 AA Ni-MH cells.
When the AA cells drop to only 4.0V does the boost converter still light the white LEDs?

Yes. When the bench PSU is set to 6v and the boost converter is at 12v, the draw is 1.62a when the LEDs are set to full white.

If I ran constant white, 4 x AA batteries, the draw being 1.62a - what would my lifetime be? Less than an hour I think?

Obviously, it would not be constantly full white - or, indeed, ever full white - as I removed white from my list of random colours, but I would be cycling all over the place and I suppose that an average draw could be easily half or less what I'm seeing with full white. Therefore the lifetime could double?

Energizer Ultimate Lithium AA are apparently 2,900mAh - that would be over an hour at full white.

 

[audioguru]

I think your AA battery holder will melt with the 1.6A of current. It will also probably melt with 800mA of current.

 

[Hippogriff]

Best stay away from anything but 8 x AA and white then.

 

[Hippogriff]

Well... success!

I have bolted the circuit and two x 4 x AA battery holders into the hat and it all works! It's all rather snug, but it works and it's not stupidly heavy either. The LEDs are really bright and I'm very happy with the outcome. I might even add a switch so I don't have to keep unplugging the PP3 connector to the battery holder.

**broken link removed**

As I say, I've bolted them in so nothing moves around when I'm dancing!

I showed my girlfriend. She said "nice way to ruin a hat"... but I think she was secretly impressed and she had probably meant to say "improve".

 

[Blueteeth]

I showed my girlfriend. She said "nice way to ruin a hat"... but I think she was secretly impressed and she had probably meant to say "improve".

Haha! I take it she's not like those of us who act like 7-year olds when we see things light up in pretty colours? The number of times I've had to justify my projects to partners with the line '...because its a laser!'... or 'these go to 11!'

Congrats man, if the weight does become an issue at some point, then all the plugging of 'use lithium' will come into play as an 'upgrade'. First and foremost though, seems like you've met a good comprimise between weight, size, and batt life - which is something engineers battle with constantly. Well done!

Edit: just thought, not trying to take ove ryour project, but a 'cool' way of turning it on/off would be a touch/proxmimity sensor under the material in the rim of the hat. If you have a spare I/O on the PIC, a single touch on/off switch only uses a bit of code and a couple of passives - and can power down the LED's as well as send the PIC into low power mode using uA, perhaps even nA. Might be cool to just 'tip your hat' touching it at a certain part of the rim

 

[Hippogriff]

Ta!

Excuse the video camera, it can't handle stuff like this very well...

I could get better sized bolts, of course, (and countersunk ones to boot) but I couldn't be bothered waiting for my things to arrive from eBay. These are 12mm, 8mm would be better.

 

[Blueteeth]

Sweet! now al you need is a LED dancing stick!

 

[Hippogriff]

I added a switch to the outside front of the hat... so that people can turn me on and off at their will.

 

[audioguru]

I added a switch to the outside front of the hat... so that people can turn me on and off at their will.

Pretty girls turn me on whenever they ..... never mind.

 

[Hippogriff]

I bought 8 x AA Energizer Ultimate Lithium from Maplin today for £9. As instructed to expect, they are a lot lighter.

 

[Hippogriff]

Having spent all this time and effort get 12v of batteries, so that my LEDs are bright (and knowing that I'm not happy with the brightness provided by 9v) I have now been wondering if there is an easy way for me to reduce the brightness of the LEDs, slightly, if I thought they were so bright they'd give anyone talking to me a headache. I was thinking - is it possible for me to remove 1 of the AAs from 1 of the 2 4 x AA holders so that I then have 7 x AAs providing me with 10.5v?

My ADP667 won't care if it's 12v, 10.5v or 9v.

If so, I'm guessing I need to be careful which one?