On Wed, 4 Mar 2015 19:20:31 -0800 (PST), Andy K
wrote:

Why mess with weeny batteries ?

Weight.
http://en.wikipedia.org/wiki/Energy_density
Li-Ion will deliver about 0.5 MJ/kg while a lead acide battery will do
only 0.17 MJ/kg. For the same energy, you'll need to carry 3 times
the weight.

I use a 6 volt SLA battery with a circuit that reduces 6 V to 4.5 V.
It powers 2 500 lumen headlights and a 300 lumen tail light.
I charge it about every 3 months.

Let's do the math. You have 2x500 + 300 lumens = 1300 lumens. At 100
lumens/watt, that's 13 watts of power. You didn't specify the size of
your SLA battery or how long you run the light every 3 months. A
common 6v SLA battery is rated at 10 A-hrs. Assuming you don't want
to destroy the battery, you can get about half of that out of the
battery or 5 A-hrs or 6*5 = 30 watt-hrs. At a 13 watt drain, that's
2.3 hrs of operation (assuming 100% conversion efficiency in your
6v-4.5v DC-DC converter and no 5%/month self discharge). Assuming
you ride 5 days per week for 3 months or roughly:
5 days * 12 weeks = 60 days
you get to use the light for:
2.3 hrs / 60 days * 60 min/hr = 2.3 minutes/day operation
That seems unlikely with only a 6v 10 A-hr battery. I would suspect
your battery is larger. Ummm.... how large or how many minutes per
day do you use it?

Incidentally, a 6v 10A-hr SLA weighs about 2.7 kg (6 lbs), which you
get to drag around for 3 months so you have have 2.3 minutes per day
of light.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558