Sir Bikesalot

This is a question for the halogen afficionados out there. I have a Turbocat Trailguide helmet light (4.8V 15W MR11) that I would like to modify to take a Li-Ion battery. Motivation here is to reduce the weight and bulk of the battery which I carry in my pocket. Currently, it uses a 5 cell 4000mAh NiMH battery at 12 oz.

In changing the battery, I also have the opportunity to go to the more readily available 6V or 12V MR11 bulbs (over-volted of course), of which I plan to use the 20W variety. Checking out batteryspace.com, I see a 7.2V 4000mAh li-ion for $27 weighing in at 7 oz. In comparison, a 14.8V 2000mAh li-ion (equivalent capacity) costs $30, weighs 6.9 oz. The 14.8V charger is also slighter more expensive, but cost differences will not be a deciding factor here obviously.

So, what are the pros and cons of 6V versus 12V systems? I have a few items to get the list started.

1) Higher voltage means lower current at the same power draw (power = current x voltage), so less resistive losses through wiring. If memories of freshman physics serve, this is true...however Turbocat uses 18 gauge wiring, which is pretty darn thick so losses would probably be minimal. Still, +1 for 12V.

2) More cells, more chances for failure? Smaller fire if the battery explodes? Kinda reaching here, but...+1 for 6V.

Ok that's all I have...need more inputs please!

Edit: BTW, I'll have a Turbocat TG NiMH battery pack + charger for sale soon (asking $50, normally $120). PM me if interested

oldmtngoat

Per Jetlites website:

6v vs. 13v

Both the 6v and 13v system use the same internal battery cells. The 6v system is comprised of 5 cells and the 13v system contains 11 cells. The key advantage of the extra voltage is run time. By doubling the voltage and maintaining the same relative wattage of output you can more than double your run time. The 13v lamps run hotter. This makes them more efficient, increasing run time, and the higher temperature is produces more light output. The 13v 12w lamp shoots further down range and has a slightly wider beam than the 6v 15w lamp. The downside to the 13v system is the battery is twice as heavy because it has twice and many cells.

A general rule of thumb to go by when trying to decide between a 6v and 13v systems is:

Buy a 6v when size and weight is your main buying motivation.
Buy 13v systems when run time and brightness is your main buying motivation.

cyccommute 

Yes, higher voltage does use lower current. However there's no free lunch. Higher voltage needs more cells. A 7.2V 20W halogen system will pull 2.8 A. A 14.4V 20W halogen system will pull 1.4A. But you need to carry twice as many cells to get the power for the 14.4V system. But if you want the same run time, you'll need to carry twice as many batteries for the 7.2V system so there's not advantage there.

Since both lights are overvolted by 20%, there's no advantage to either there.

If you were to use the same battery capacity for each system, for example a 3.3 Ah battery, the 7.2V system would have an advantage for a short commute...about an hour...over the 14.4V system in terms of weight. I use one of the 7.2V systems in the fall and spring when I don't need as much burn time. When I need a longer burn time...full winter riding...I switch over to the 14.4V system. My batteries are easy to use in either mode.


I don't know about this one. Smaller battery means higher current draw which stress the battery more which is more likely to cause cell failure. Li-ion don't like high current draw. Ni chemistry batteries don't really care. Pulling 2.8A from a 4Ah battery isn't that high of a draw but pulling 1.4A from the same battery is easier on it.

The other issue is availability. 6V MR-11 bulbs aren't as readily available as 12V. They aren't impossible to find but their just not as easy as the 12V.

Sir Bikesalot

Good point. I think that swings things in favor of 12v.