Lighting/battery question

[Gringo]

OK so what I have is a T6 LED cycle light, USB powered (5V) and a whole bunch of Lipo's.
1S & 2S aswell as several 18650 cells and both a step up 3.7v to 5v 1A board and a step down 2S to 5V 3A device.
Today while tinkering with bits of wire and batteries I tried the light on every combination of cell & device and found the light brightness appeared constant from 8v down to 3.4v when it started to dim.
Also the current draw also remaind around 1A whatever the voltage.
To me it makes more sense to make a battery pack with all cells in parallel (3.7v nom). This would charge as one cell with no balancing and as the current draw is constant the higher ampage of say a 1S2P would last twice as long as the same cells in a 2S1P.
I'm sure there's folks out there who know more about elastic trickery than I and if it was as simple as it seams, why are battrey packs made up in 2S format not 1S.

Any views or comments welcome.

 

[Alan Quay]

Sounds like your lamp has a voltage regulator inside it. This will cut the voltage down to (from your test results) 3.4v, or a little bit less. Its not the most efficient way to do things, but its cheap and simple.

So yes, running from 1s would be about right.

 

[Deleted member 4366]

A 1S battery starts at 4.2v when fully charged, and goes down to 3v when nearly empty, so the light will go dim before the battery is flat, which is why they use 2S I guess, where the minimum voltage is 6v.

I just ordered one of these to try. You can make up a 1S pack to power your phone or tablet. It steps up to 5v and can output up to 2A, so could be OK for a single T6 XML emitter. You also get both over-charge and over-discharge protection:

http://www.ebay.co.uk/itm/111358747234?_trksid=p2060778.m2749.l2649&ssPageName=STRK:MEBIDX:IT

 

[Gringo]

Usefull bit of kit D8veh. I'd need a smart phone to get the best out of it.

I'm stocked up with RC stuff, lipos & chargers etc. In the RC world we don't like our lipos to drop as low as 3V, we tend to set the LVC (low voltage cutoff) around 3.3V. It may be different for li-ion cells but I've more lipo's than li-ion's
Here's my thinking,
Given 2 x 2Amp lipo's I could make either a 2S1P 8.4v @ 2Amps or a 1S2P 4.2V @ 4Amps.
The 2S1P fully discharged gives 2Amps = 2 hours of light.
On the other hand the 1S2P would not fully discharge before the light fades but even at 75% that's still 3Amps = 3 hours of good light and no sudden drop in light.

 

[Alan Quay]

...but....

If you make a 2s pack, run it through a dc-dc converter (say 90%. Efficiency) down to 3.5v, you will get 4 hours 20 minutes.

 

[mfj197]

Exactly as d8veh says. You do find flashlights running off 1S but they do get dim before the end of the discharge cycle. So 2S is the minimum to maintain output throughout the cell discharge, and your light has what is called a "buck" driver that drops the voltage (and increases the current). It's as efficient as any other type of driver really.

Incidentally the XM-L LED actually lasts quite well on a depleting cell as it has a very low drive voltage - typical forward voltage is 3.35V at the full 3A. Earlier LEDs which the original Magicshine lights used (upon which all of these cheap 2S lights are based) had a rather higher drive voltage.

Michael

 

[KirstinS]

A 1S battery starts at 4.2v when fully charged, and goes down to 3v when nearly empty, so the light will go dim before the battery is flat, which is why they use 2S I guess, where the minimum voltage is 6v.

I just ordered one of these to try. You can make up a 1S pack to power your phone or tablet. It steps up to 5v and can output up to 2A, so could be OK for a single T6 XML emitter. You also get both over-charge and over-discharge protection:

http://www.ebay.co.uk/itm/111358747234?_trksid=p2060778.m2749.l2649&ssPageName=STRK:MEBIDX:IT

Apologies if this is slightly off track but I've ordered one of these to play with

http://pages.ebay.com/link/?nav=item.view&alt=web&id=111436466115

It takes 2 to 6s, useful if you have multiple packs in this range (like I do).

 

[Alan Quay]

.... your light has what is called a "buck" driver that drops the voltage (and increases the current).....

I had assumed not, since the current remained constant during the OP's tests. This would indicate a voltage regulator.

 

[Deleted member 4366]

Apologies if this is slightly off track but I've ordered one of these to play with

http://pages.ebay.com/link/?nav=item.view&alt=web&id=111436466115

It takes 2 to 6s, useful if you have multiple packs in this range (like I do).

No need to apologise. That looks quite useful too. It can give up to 3A.

 

[Kinninvie]

Apologies if this is slightly off track but I've ordered one of these to play with

http://pages.ebay.com/link/?nav=item.view&alt=web&id=111436466115

It takes 2 to 6s, useful if you have multiple packs in this range (like I do).

I have ordered one of those.Handy to power my smartphone on long Strava runs.

 

[Gringo]

As we're buying things to play with, Iv've just brought an adjustable step up module.
http://www.ebay.co.uk/itm/281503744261

 

[mfj197]

I had assumed not, since the current remained constant during the OP's tests. This would indicate a voltage regulator.

Mike, forgive me - I'd missed that bit of the OP's post, and the bit where he'd said it was a USB light...!

If the current drawn from the test batteries didn't change whatever voltage was applied then the light has a linear driver - not stepping up or stepping down the voltage but simply limiting the current by dumping excess voltage (and power) as heat. As Mike says, not the most efficient - drive voltage of an XM-L at 1A is 3V so 2V is being dropped by the driver. LED power 3W, driver is dumping 2W.

Gringo, with your current setup you are right - the most effective approach would be 1S2P rather than 2S1P. It would be rather more efficient than driving from USB or 5V as you are dropping less voltage across the driver, and therefore wasting less power. Running off 2S you'd have to be dropping over 5V over the driver on freshly charged cells, which would therefore be kicking out over 5W of heat. It won't last long like that.

Michael

 

[Gringo]

Cheers Michael, that sort of what I was thinking in a roundabout way.
I've ordered an adjustable step up device.
When it arrives I'll tweek the voltage to the lowest possible output before any noticeable loss of brightness, this should compliment a 1S lipo allowing me to use it to its full capacity.
I hope it's not sent via city link

 

[KirstinS]

No need to apologise. That looks quite useful too. It can give up to 3A.

Well it arrived and works a treat. I've tested with 2,3 and 4s and it will happily charge an iPad and an iPhone ( getting near 3a so that but was correct)