Electronics Forum / Components / July 2008

If you want to use 1200 mA for 6 hours, your battery capacity has to
be at least 7200 mA-H - but you should never fully discharge a
rechargeable battery, so you need at least a 10 A-H battery - I'd
probably use a small sealed Lead-acid battery. (but I haven't looked
at the capacity/price of the current Lithium batteries.)

Whoof.  That's a flashlight-grade high-power LED!

Well, the LED certainly can work for that long (if you have a good
enough heatsink on it).  The battery and solar panel, on the other
hand...  well, getting as much power as you need is going to be
nontrivial.

You're looking at around 5 watts of power just for the LED... that's
30 watt-hours per day.

Given that the battery-charging process isn't 100% efficient, and the
regulation of current for driving the LED isn't 100% efficient, you
should probably budget 50 watt-hours per day from the solar panels if
you want to be able what you want.

These days, a typical consumer-type solar panel capable of delivering
4-5 watts of power (e.g. 400 mA at 12 volts) costs anywhere from $40
(flea-market bargain price) to over $100, and has close to 2 square
feet of panel area.  These panels are fairly well suited to charging
12-volt lead-acid cells.

That's probably about the largest panel that it would make sense to
place on the back of a bike (e.g. above the rear mud-guard).  And, in
order to get 6 working hours of light out of a monster LED like the
one you're considering, you're going to have to have that panel in
direct sun for the full day.

You're going to need something reasonably sophisticated to handle this
arrangement, I think.

If you're going to need more than an ampere at 2 volts (nominal), the
issue of choosing the proper battery type and regulator are quite
significant.  I'd expect that some form of switching buck regulator
will probably be most efficient, as you can draw power from a
higher-voltage battery (such as a LiION or a lead-acid gel cell) and
transform it down to a higher current at a lower voltage.

Lithium batteries are tricky to charge... they *require* a charge
control IC circuit which is correctly configured for the battery
chemistry and size.  Doing it wrongly can and will damage the battery,
with consequences up to and including a catastrophic fire.

Although they're heavier, a good modern NiMH battery pack might be a
bettery choice - they're less tricky to charge.  

Lead-acid are the heaviest, and (I believe) have the lowest amount of
energy storage per pound, and many types won't live very long if you
deep-discharge them regularly... but they're probably the easiest to
charge-control.

You might find that you need to use a split arrangement... have a
solar panel and charge controller at home, the LED on the bike, and
two battery packs - one installed on the bike (for use) and one
connected to the charger at home.