I have replaced my 8 year old Osram xenon bulbs with 5500k Morimoto ones. They are brighter but not as bright as I thought they would be. As they light up and don't flicker the ballasts must be doing there job.

Would a battery starting to fail cause a drop in brightness?

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5500k will be less bright than OEM 4300k bulbs.

They have the same luminosity as Osram Xenarc Night Breaker Unlimited

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As I understand it here is more to light ouput than just lumens.

 

The colour temperature is measured in Kelvin (K)

The brightness is measured in lumens

The intensity is measured in Candela

 

It is very unusual for a 5500K bulb to have higher lumens than 4300K...

 

4300K - Lumen output: 3200 usable lumens

5000K - Lumen output: 3000 usable lumens

6000K - Lumen output: 2800 usable lumens

8000K - Lumen output: 2500 usable lumens

 

This site is quite useful...

 

http://www.mobilehid.com/3000k.htm