This article is from the Misc Bicycles FAQs, by various authors.
Why would you want to build your own lighting system? There are
several reasons. First, you can build a system that will put out more
light than a commercially available one at a fraction of the cost.
Second, there is a tremendous amount of satisfaction one receives from
constructing and using such a system.
In this article I will detail the information needed for you to
build your own battery operated bike lighting system. I will not
attempt to describe specific details on what items you will need,
but I will give you the needed information in order to size and
construct a system from whatever parts you may have available and
will show by example how to engineer one.
What makes up such a system? You will need:
-Some sort of headlight
-12 volt sealed lead acid gel-cell battery, or same voltage as
-Some sort of tail light
-quick disconnect connector
-a means to recharge the battery.
The amount of light you get is dependant on the source of light
and the time per battery charge is dependant both on how much
power the lights consume and how much charge your battery stores.
Here are the basics that you need to apply:
1 - The power consumed by the lights is measured in WATTS and is
easily computed using OHMs Law: P=IxV. Or, the power in WATTS (P)
equals the current in AMPS (I) times the voltage in VOLTS (V).
Knowing any two, allows you to solve for the third.
2 - The charge stored in a battery is measured in AmpHours (AH),
and equates to the numbers of amps per hour that the battery can
deliver, with a de-rating (to be explained in a minute). For
example, a 12V, 7 AH battery can supply 7 Amps for 1 hour before
the battery is depleted. But the rating is somewhat misleading.
The rating is based on a discharge rate that is one tenth the
charge storage of the battery. So, for the 12V, 7AH rating of
the above battery, it really means that the battery can supply
7/10 or 0.7 Amps for 10 hours. If you desire to draw much more
than 1/10 you must derate. The Derating is not linear, however,
to derate, use this simple rule: If you will draw significantly
more than one tenth the charge storage, figure the capacity at 80%
of the rating. So the above battery could supply 7A for 0.8 hours,
or for 48 minutes. The application of this will be shown in the
3 - If the battery is going to be operated AT very cold temperatures,
such as below 50 deg. F, then derate again by 80%. The key here is
the word "AT". If the battery is left on your bike outside, and will
reach these temperature ranges, then this derating is needed. If you
keep the battery inside, except for when you are on the bike, this
derating will most likely not be needed (a large mass will take a while
to cool). So, if you draw lots of amps when the BATTERY is cold, the
above battery would only be capable of supplying 7A for 0.8 x 0.8
hours, or 7A for 38 minutes.
That's it for the basics. Now on to engineering a lighting
system. What is needed is an idea of what you have which will
enable you to calculate what kind of time you will get, or if you
know how much time you need, that will enable you to determine
what battery and/or lights you will need.
EXAMPLE 1: You have a 55 watt, 12 volt head light, a 5 watt tail
light and a 12V 8.5 AH battery. How many minutes of light will
this give you?
SOLUTION: Powers add, so you have 55 + 5 = 60 watts. You know
the power consumption and the voltage, so you can calculate the
current using Ohms law and use that to calculate the time. I =
P/V = 60watts/12volts = 5 amps. This is much greater than the
one tenth rule, so you must derate. The time is equal to
8.5AH/5A x 0.8 = 1.36 hours or 81 minutes of light.
You can expect this amount of light when the battery is fully
charged. I use, and suggest, that you take about 75% of the time
as the amount that you can count on. This should help cover you
for the times that the battery may not be fully charged or if you
have a mechanical problem that adds time onto your trip. For the
above example, this will give you one hour of usable time with
about 20 minutes of margin.
EXAMPLE 2: You have a 21 watt, 6 volt moped head light, a
battery operated flashing tail light, and a 35 minute commute.
What size battery do you need to cover both the ride in and the
ride back, with 20 minutes of margin?
SOLUTION: The current draw is 21watts/6volts = 3.5 amps. This
current must be supplied for 35 + 35 + 20 = 90 minutes or 1.5
hours. The AH rating is 3.5amps x 1.5 hours = 5.25AH. Since
this number is large, you must derate, the needed battery must be
a minimum of 5.25AH/0.8 = 6.5AH.
Now to discuss charging. This is the area that causes the most
confusion. The principle is simple, you must supply current to
the battery opposite in direction that the battery supplies it.
Too little current, and the battery may not get fully charged by
the time it is needed. Too much current, and the battery may
suffer some loss of life if it is charged too long. Your choices
are simple: build one, buy one, or use one you already have. In
all cases connect positive to positive and negative to negative.
To build one, if you have access to an adjustable DC power supply,
set it to 2.25 to 2.3 volts per cell (a cell provides 2 volts, so
a 12V battery has 6 cells, a 6 volt has 3 cells). Another simple
charger to build is to buy a cigarette lighter power connector,
plug it into your lighter socket (hey a use for this thing!) and
use your cars charger to charge while you drive. To buy one,
check the automotive stores for motorcycle battery chargers. If
the charger puts out more than 13.8 volts, do not leave the
charger on for more time than is necessary. You could also check
places like Radio Shack or a good video store for a charger.
Many Camcorders use 12v gel-cells and supply a charger. If you
own such a beast, you are in luck because you can use that
charger for your bike battery, and you can use your bike battery
for additional battery time on your camcorder! You can achieve
this at a fraction of the cost of what the camcorder vendor sells
an additional battery for! As for the length of charging time,
if you use a power supply or charger with a current meter, charge
until the current draw is low. If the charger has a charge light
that goes out after a while, you can leave it connected as long
as you want as the charger automaticly reduces the current so as
to not stress the battery. If you have neither, experiment.
Charge for an hour then see how long it takes for the light to
get real dim, then charge for 2 hours and repeat. Use this to
determine how long is needed to charge the battery for your
Wiring: Wiring is simple. connect positive to the light and use
your frame as a ground return. A quick disconnect connector
allows quick and easy installation and removal of the battery
from the bike and similarly for connecting the charger. A switch
makes it easy to turn the light on while on the bike when it gets
Care and feeding of your battery: Sealed lead acid cells require
no maintenance. The only restrictions are: don't allow the
battery to become completely discharged as this will reduce the
life of the battery. Similarly, don't over charge as stated
Availability of parts: Here are some suggestions:
- Lights: automotive head and fog lights, tractor headlights,
motorcycle and moped headlights. Also look into low voltage
flood lights used in security lights such as those found in
stairwells or for illuminating the sides of your house. Lights
used on vehicles will tend to be built more rugged and therefore
be more reliable. Utility trailer tail lights can be used.
Also, battery operated tail lights made for bikes can be real
- Batteries: Gel-cells can be mail ordered from Digi-key, Thief
River Falls, Minn. Call 1-800-344-4539 and ask for a catalog.
Also, from E. H. Yost & Company, Sauk City, Wisc. 1-608-643-
3194. Call for a catalog. Batteries are available in different
capacities and physical packages. These are detailed in the
catalogs. Also look into motorcycle batteries and batteries for
power fail security lights. You can also check out a local ham
- Wire, switches, and quick connectors: Try Radio Shack.
Hopefully I have presented enough information for you to search
out and build your own lighting system. If you have any
questions, please email me at firstname.lastname@example.org.