how to choose a battery for a bike light

[ksbikecommuter]

I have been looking for some information to continue the
development of the bicycle light that I built last fall.
Having not found it I will ask on here.

The requirement
An inexpensive light for a two mile commute in residential
areas with lots of trees and street lights at corners, lots of
shadows and potholes, the occasional cat or was it a skunk.

Progress so far.
A light using a MR16 medium spot which I am satisfied with.
The need to improve the battery pack.
First experiment was with 8 cheap AA Alkaline batteries. Light
a little dim, Voltage with no load about 12 which dropped to
10 within a few seconds and by the end of the ride was down to 9. I surmised the batteries could not keep up with the
amperage needs of the light.
Second experiment was with 8 cheap D Alkaline batteries. Light bright, Voltage dropped to 11 after 15 seconds and on down to 10.5 by the end of the ride. I decided to see how long this pack would last, after 3 month of commuting it was getting dim and needed to be replaced. Capacity for my use seemed acceptable.

My questions.
These are cheap batteries, would quality Alkaline batteries
have significantly less voltage drop?
Would rechargeable AA or D batteries provide good performance with regard to voltage drop?
A SLA battery would probably be the cheapest solution but not
necessary the most instructive.
On the question of overvolting the light how does one figure
the voltage? In other words does one use the no load voltage
or the voltage under load, if you say you want to overvoltage
to 13.2 do you go for 14.4 which may give a voltage of about
13.2 under load? How much voltage drop do you see when the
battery is of adequate size?

Thank you for your help
Ian

 

[TigerhawkT3]

If D cells are lasting you three months, that's probably fine. Quality alks might provide a slight improvement, but you'll see the biggest benefit with NiMHs. An SLA would probably be the simplest solution. However, the rechargeable methods will most likely need attention much more often than once every three months.

Can you give some details on your light?

If you want to stay with D alks but decrease the load, try hooking up a second 8-cell battery in parallel with the primary one (make sure they're all the same age, brand, etc.). Since the lighter load will enable the cells to provide higher total capacity, you'll get more than double the runtime. It might be too large and heavy, though.

 

[ksbikecommuter]

Thank You for your reply.


I will describe the light shortly.

Back to the top with my question.

I am particularly interested in answers about the overvolting question.

One reason I am hesitant on going with SLA's is my experience in years past with camcorder batteries. My experience and that of several friends is that camcorder batteries (used in camcorders) only last several recharge cycles (maybe 6 to 10).


Thank you

Ian

 

[TigerhawkT3]

If by the "overvolting question," you mean how to know whether the recommended voltage is no-load or under load... it varies. Many manufacturers give the "nominal" voltage. For example, auto accessories are labeled as "12V," but car batteries vary from around 10V (cranking) to 14-15V (no load). If you're talking about an incan, those can take various amounts of voltage over the manufacturer's specs. Some bulbs, with huge rated lifetimes, can be tremendously overdriven, while others are much less tolerant of overdriving. If you're talking about an LED driver, the manufacturer's max is probably the no-load value.

SLAs have to be properly taken care of. Cars use SLA batteries, and you know how long those last. Your friends probably had bad SLA experiences because camcorders aren't usually used that often, and are normally discharged as far as possible between charges. Both of these factors are bad for SLAs. Ideally, you shouldn't fully discharge them, and they need to be recharged at least once every month or two.

If you give more specifics about your light, we could more easily recommend an appropriate battery solution.