Home > Bicycles, Engineering > Midnight MTB

Midnight MTB

It has been a few years since I’ve tried night mountain biking. According SnowBikers, I am on step 3 of the night riding process. To GTFM for this post, I cheaped out by building a DIY light instead of spending $100 for a decent one, and paid for it with a really poor ability to see in the dark. My battery (brand new, undercharged) was dead within 30 minutes, and the faint yellow glow that it emitted was easily drowned out by the Waxing Gibbous perched in the clear sky. Fortunately, those 30 minutes on Skullbuster were a lot of fun.

Following my headlamp failure, I did what any good engineer would do: came home, trickle charged the battery to about 14V, and then started a discharge test. Here are the results:

My discharge curve looks very typical for a NiMH-based battery pack. Steep initial voltage drop, voltage plateau across most of the discharge capacity, then another steep drop at the end. There’s a pretty good writeup on NiMH on Wikipedia. From this test, it appears that I had not quite reached a full charge before I began discharging. At 12V, a 10W lamp should be drawing about 0.83A. From a 2000mAh (or 2Ah) battery, I should get ~2 hours of discharge. However, I hit the second voltage drop on the discharge curve before 1.5 hours. Some thoughts on this: I’m charging at 0.5A (about C/4 for a 2.0Ah battery), so I’m likely out of the range for the most appropriate manual charge. Additionally, I have now discharged the battery past the lower discharge inflection point twice, so that might be lowering the actual capacity. This most likely explanation is that I have not yet fully charged the battery. At a 66% charging efficiency, the 0.5A charge should take about 6 hours. I’ll try to be a little more patient before my next discharge test 😉

For you spec-hungry readers, here is what I’m working with:

MR16 Lamp: 12V, 10W, narrow flood pattern (from Harrington Lights – $2.60)

12V NiMH battery pack: n=10 cells, AA-size, 2000 mAh (from Batteries Plus – but comparable prices at Battery Space on NiMH and LiFePO4)

Tamiya connectors (for 9.6V RC Car battery): $4

And for you historians, here’s my 5-year-old description of this same project.

Overall conclusion from this experience: I wasted a bunch of time building this project and still don’t have a really good light. Next time, just buy a MagicShine from GeoManGear.

Categories: Bicycles, Engineering
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