# 3.7v or 7.4v battery pack option for XM-L light



## dereknz (May 3, 2011)

Hi

New here. I have a question on the advantages and disadvantages if any on using a xm-l bike light kit system with a 3.7v 4x18650 I see on the market as opposed to 7.4v 4x18650 which seem to be more common. thanks, Derek


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## Vancbiker (May 25, 2005)

Depends a lot on what type of drive circuit is used in each type of light. 

I would guess that the 3.7V system is using a simple linear regulator. It's cheap, simple, and not too bad for efficiency when the battery and LED Vf are closely matched. The downside is that the light will draw ~3A from the battery. That places higher demands on the quality of the cable and connectors to minimize losses due to resistance. This light will also gradually dim near the end, maybe about last 20%, of the runtime.

The 7.4V light that I saw disassembled had a switching buck regulator. More efficient than a linear regulator when the difference in the battery and LED is large. It's a bit more expensive and complex than a linear. This will draw ~1.5 A from the battery. This is more easily handled by the cable and connectors. This light will hold its light level until the battery protection level is reached then it shuts down.


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## dereknz (May 3, 2011)

Thanks for the info. Thanks helped a lot


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## dereknz (May 3, 2011)

Another question. When you mention efficiency, is volts forward to the Cree XM-L led 3.33v @ 3A reduced so the light is not as bright as it could be or the battery runtime reduced to the loss or both. Thank you


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## mattthemuppet (Jul 16, 2004)

efficiency here refers to the driver. The more efficient the driver, the less electricity is wasted as heat in the driver, therefore the longer the run time (all other things being equal).

Both drivers will/ should supply the same current and Vf, the only difference is how the battery voltage is converted to LED voltage


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## dereknz (May 3, 2011)

Thanks thats what I thought it was. I asked as in NZ we have a seller who is claiming their xm-l bike light is brighter than others because they use a 4.2v 8.8AH systems rather than the comonly used 8.4v 4.4AH. http://www.trademe.co.nz/Sports/Cycling/Parts-accessories/Lights/auction-374269781.htm


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## znomit (Dec 27, 2007)

dereknz said:


> Thanks thats what I thought it was. I asked as in NZ we have a seller who is claiming their xm-l bike light is brighter than others because they use a 4.2v 8.8AH systems rather than the comonly used 8.4v 4.4AH. http://www.trademe.co.nz/Sports/Cycling/Parts-accessories/Lights/auction-374269781.htm


Oh, now its 9.2Ahr  
http://www.trademe.co.nz/Sports/Cycling/Parts-accessories/Lights/auction-373276391.htm

Theres a bunch of people on TM selling similar lights. They all look for a point of difference. Some give away free sunglasses! The 1200lm probably comes from the nitelights.co.nz exaggerated claim for their product. Most of them have little idea whats inside or any knowledge of things electrical. Theres a few sellers with a good history, stated warranty, and believable specifications. Look around. :thumbsup: 
Or buy from DX direct and save yourself 30$


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## dereknz (May 3, 2011)

Yes I am one of the bunch just who has just started selling them on TM. That's why I have come on this forum to learn more about what I am selling. I Don't give away free sunglasses but try and keep my descriptions honest. Just been on the nitelights web site and that now explains why they show 1000-1200lm on their TM listings. Maybe if nitelights light had a sst50 in it at the 16W instead of a cree.


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## dereknz (May 3, 2011)

Oh yes cyclights is changing the description of the battery packs to the correct 8.8Ahr. Your link above is one not done yet. Had to get him to do it from my listings http://www.trademe.co.nz/Browse/Listing.aspx?id=374237353 as he has blacklisted me when I told him some of the listings had "Lighting Modes: 100% brightness 30% brightness & flashing(2.4Hz)" and the next line "4 interchangeable lenses. Hi/Med/Low/Flash modes"


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## znomit (Dec 27, 2007)

dereknz said:


> Yes I am one of the bunch just who has just started selling them on TM. That's why I have come on this forum to learn more about what I am selling. I Don't give away free sunglasses but try and keep my descriptions honest. Just been on the nitelights web site and that now explains why they show 1000-1200lm on their TM listings. Maybe if nitelights light had a sst50 in it at the 16W instead of a cree.


Have you measured the output drive current of your light? Generally the P7 MS lights were rated at 2.8A for their "900lm" claims but were actually driven at 2.4A. The P7 MS claimed 900lm and output 550 (losses from heat, inflated bin specification, optics, lower drive current).

You could complain to the commerce commission about the 1200lm claims. They have taken an interest in trademe bike lights before. But if they measured your light how would yours do? Are you sure the supplier is using T6 bin. How would you know if he was substituting a lower bin to increase his narrow profit margin?


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## dereknz (May 3, 2011)

Yes I should look at measuring the output drive current of these lights. I’ll need to find a tutorial or guide so I know what I am doing. All I can do is go on is the word of the light manufacture that they are T6 bin, with Cree not marking the bin rating on them. What method is used to test the lumens of a led?


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## Vancbiker (May 25, 2005)

dereknz said:


> Yes I should look at measuring the output drive current of these lights. I'll need to find a tutorial or guide so I know what I am doing. All I can do is go on is the word of the light manufacture that they are T6 bin, with Cree not marking the bin rating on them. What method is used to test the lumens of a led?


IMO, the specs stated by Chinese light manufacturers is not worth the ink/paper or bytes used to state them. As the end user has no means to verify the performance it is easy to claim pretty much anything.

The lumen output of an LED (and other light sources) is measured by an instrument called an integrating sphere. Wikipedia has the details.


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## znomit (Dec 27, 2007)

Vancbiker said:


> IMO, the specs stated by Chinese light manufacturers is not worth the ink/paper or bytes used to state them. As the end user has no means to verify the performance it is easy to claim pretty much anything.


Yep. In NZ we have reasonably strong consumer laws. So if you buy something and then find out that the specifications are misleading you can return the goods. I might not find out until after a winters riding that my 1000lm is actually 650, then decide to return it. So you need to realise that every single light you sell may come back to you in 1-2 years.


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## AE25 (Feb 21, 2011)

just bought one of those TM sunshine lights from seller cyclelights. I was curious to see if the 8.8ah pack lasted twice as long.. but now i see they're 4.2v. DOH! so they wont interchange with the normal MS batteries or the surefire p7 light im already using even if i were to change the plug size grrr. I also asked early on if they had the XML led in these ones since they were only described as a cree T6 back then (xpg for all we know), and they replied they have the T6. fail. then I asked should the 8.8ah version last longer since its double the Amp/hours, (my bad). but they replied with "yes it is 8.8ah" and totally avoided the question. so just shows their knowledge about the product is pretty poor. I dont see any warrantee stated on the auction, so i can imagine they'll magically disappear from trademe if faults start arising from these things. time will tell


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