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From: Jasen Betts on 9 Dec 2009 05:31
On 2009-12-08, Robert Roland <fake(a)ddress.no> wrote:
> On 07 Dec 2009 15:01:09 GMT, John Doe <jdoe(a)usenetlove.invalid> wrote:
>
>>I just weighed eight 2ah AA NiMH batteries. They weigh less than
>>one third of the next alternative, so they are looking very good.
>
> If weight is very important, you may want to take a look at LiPo
> batteries. They give you specific charge density at least double of
> what you get from even the best NiMH stuff.
>
> They're more expensive, and you need a new charger, so cost might
> become an issue. Radio control plane and helicopter modelers don't
> even consider anything else these days. They are, of course, very
> picky when it comes to weight.

It's energy density is better than nitromethane?
From: John Fields on 9 Dec 2009 11:23
On 07 Dec 2009 15:01:09 GMT, John Doe <jdoe(a)usenetlove.invalid> wrote:

>Jasen Betts <jasen(a)xnet.co.nz> wrote:
>
>> John Doe <jdoe(a)usenetlove.invalid> wrote:
>
>>> I need to power three Cree XLamp XRE LEDs. The standard sealed
>>> lead acid battery is 12 V, but it is heavier than I would like
>>> to carry. I have some spare rechargeable 14.4 V DeWalt tool
>>> batteries.
>>>
>>> The L7812 regulator says it needs at least 14.5 V. will a 14.4
>>> V NiCad tool battery be acceptable for that?
>>
>> what exactly are you doing with with the 12V. - what circuit are
>> you planning to use for the LEDs ?
>>
>> In most cases 14.4 from a rechargable battery pack is a good
>> replacement for 12-14V from from a "12V" lead-acid battery.
>
>I just weighed eight 2ah AA NiMH batteries. They weigh less than
>one third of the next alternative, so they are looking very good.

---
I think something has gone awry here.

According to the data on page 3 and the curve on page 6 of the Cree XR-E
data sheet at:

http://www.cree.com/Products/pdf/XLamp7090XR-E.pdf

the thing is an LED with a Vf of about 3.5V with an If of 700mA through
it, which means that you will _have_ to use some sort of current
limiting.

The most basic is a series limiting resistor: (view in Courier)


.. E1
.. |
.. [R1]
.. |
.. +---E2
.. |
.. [LED]
.. |
.. GND

The value of the resistor can be found from"

E1 - E2
R1 = ---------
If

For a 14.4V supply and with 700mA through the LED, that would be:


14.4V - 3.5V
R1 = -------------- = 15.57 ohms ~ 16 ohms.
0.7A

The power dissipated in the resistor would be:


P = I(E1 - E2) = 0.7A * (14.4V - 3.5V) = 7.63 watts,


which is outrageous, considering that the LED itself would be
dissipating (only???) 2.45 watts!

IMO, the right way to drive these things would be with a constant
current buck regulator, which would keep the current into the LED at
700mA (or whatever current you wanted) as the battery discharged, while
dissipating very little power itself.

It seems unlikely that there isn't something like that out there, and if
anyone would know it'd be Cree, so why not give them a call?

JF
From: John Fields on 9 Dec 2009 11:25
On Sun, 6 Dec 2009 18:53:59 -0500, "Tom Biasi" <tombiasi(a)optonline.net>
wrote:

>
>"Tim Wescott" <tim(a)seemywebsite.com> wrote in message
>news:uL6dneaZrNjegYHWnZ2dnUVZ_vZi4p2d(a)web-ster.com...
>> On Sat, 05 Dec 2009 18:59:49 +0000, John Doe wrote:
>>
>>> I need to power three Cree XLamp XRE LEDs. The standard sealed lead acid
>>> battery is 12 V, but it is heavier than I would like to carry. I have
>>> some spare rechargeable 14.4 V DeWalt tool batteries.
>>>
>>> The L7812 regulator says it needs at least 14.5 V. will a 14.4 V NiCad
>>> tool battery be acceptable for that?
>>>
>>> Thanks.
>>
>> In case you haven't figured it out, DO NOT connect a plain-old LED to a
>> stiff voltage source like a NiCd or lead-acid battery. At least, unless
>> you want a burnt out LED and an unhappy battery. You can kinda sorta get
>> away with it if you have an LED that drops 1.5V and a dry cell battery,
>> but it's still not good practice.
>>
>> LEDs want constant current sources -- which is what the other posters are
>> offering up to you.
>>
>> --
>> www.wescottdesign.com
>
>I was assuming he was using the Cree XLamp used as a replacement bulb
>operating at a nominal 12 VDC. They are used routinely in RVs and routinely
>operate at 14+ volts while the vehicle is running.

---
Not without some sort of current regulation/limiting.

JF
From: Tom Biasi on 9 Dec 2009 15:47

"John Fields" <jfields(a)austininstruments.com> wrote in message
news:8pjvh5pjacvgfgqlusgel88a441a2jo47c(a)4ax.com...
> On Sun, 6 Dec 2009 18:53:59 -0500, "Tom Biasi" <tombiasi(a)optonline.net>

>>
>>I was assuming he was using the Cree XLamp used as a replacement bulb
>>operating at a nominal 12 VDC. They are used routinely in RVs and
>>routinely
>>operate at 14+ volts while the vehicle is running.
>
> ---
> Not without some sort of current regulation/limiting.
>
> JF
The ones I see are replacement bulbs. I gues the regulation is built in.
Tom


From: John Doe on 10 Dec 2009 00:20
Jasen Betts <jasen(a)xnet.co.nz> wrote:

> On 2009-12-08, Robert Roland <fake(a)ddress.no> wrote:
>> On 07 Dec 2009 15:01:09 GMT, John Doe <jdoe(a)usenetlove.invalid>
>> wrote:
>>
>>>I just weighed eight 2ah AA NiMH batteries. They weigh less
>>>than one third of the next alternative, so they are looking
>>>very good.
>>
>> If weight is very important, you may want to take a look at
>> LiPo batteries. They give you specific charge density at least
>> double of what you get from even the best NiMH stuff.
>>
>> They're more expensive, and you need a new charger, so cost
>> might become an issue. Radio control plane and helicopter
>> modelers don't even consider anything else these days. They
>> are, of course, very picky when it comes to weight.
>
> It's energy density is better than nitromethane?

At least here, we have already passed the point of diminishing
returns...




--
thanks to the replies
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