Overdriving MR16 Halogen bulbs

[Packhorse]

I am thinking of changing my DIY canister light from SLA to Nicads to reduce its size. NIMH are too expencive and hard to charge IMHO. I can get 5000ma cells but only in packs of 4.
What are the pros and cons of using 12 1.2 volt cells instead of 10?

 

[Packhorse]

I should add that I am thinking of encasing the cells in epoxy or something to protect the cells incase of flooding. Any comments?

 

[JessH]

Overdriving it will decrease bulb life and increase color temperature, light output and heat output. Just how much it will decrease the bulb life depends on a lot of things. I am actually rather curious as to what the life expectancy of the bulbs would be. Keep in mind that when fully charged each cell will actually be more than 1.2v

~Jess

 

[Charlie99]

For any particular lamp, the light output and life depend
upon the voltage at which a lamp is operated. For
instance, as approximations, the light output varies as
the 3.6th power of the voltage and the life varies
inversely as the 12th power of the voltage.

For example, a 10% overvoltage will increase light output 40%, reduce bulb life to 32% of nominal. The power consumed does not increase the 21% one would assume if the resistance stayed constant, but instead only goes up about 16%.

Attachments seem to still be turned off. PM me with an e-mail address and I can send you some GE and Gilway pdf's on this.

Note that, unlike "12V" auto bulbs where the nominal voltage is NOT 12V, the nominal voltage for MR16 bulb really is 12.0V.

 

[miketsp]

Given that many MR16s are rated at 2000hrs life (not all - you need to check) it should be possible to significantly overdrive them for dive light applications.
Many smaller divelights use 4.5V halogen bulbs overdriven at 6V and accept a life of 30 - 40 hrs.

As for epoxying your cells, I personally wouldn't risk this for 2 reasons. Firstly the thermal isolation problem. Cells get hot when being charged and discharged and unless you use an appropriate potting epoxy with good thermal conductivity this heat is going to be retained in the cells. And of course heat cycles imply expansion & contraction cycles with significant stresses due to the potting.
Secondly in any sort of fault situation like an external wiring short or even an internal short within a cell a significant amount of gas can be given off through the cell safety vents. By potting you are creating a bomb casing with potentially disastrous consequences.