There are patches for them that let you use the L+R+ some other buttons to modify how much "sunlight" is let in.

I believe links to patches are allowed, so here you go

 

Ah, I modified my post above. It depends on which ROM release you are using.

edit: wait a minute, I think that site only has the (E) version of Boktai 2. The Boktai 1 patches look like something else.

 

I came across a .zip with refined solar sensor patches for all three Boktai (1-E, 2-E, 3-J version) games some time ago (I actually forgot where I got this).

To quote the readme:

Warning: Spoilers inside!

PATCHES:
The patches in this archive are solar sensor patches. They allow you
to play the Boktai or Bokura no Taiyou games without a solar sensor,
so that they can be played on a flash card or emulator that doesn't
support it. The retail cartridges have a solar sensor built in,
which affects many things in the game. Sunlight is crucial to
playing the games; without sunlight, you cannot recharge your energy
and thus cannot use a large part of your weaponry. Some puzzles are
also impossible to do without sunlight.

There were solar sensor patches for all three games already, but
some of them (the patch for the third game in particular) had some
faults like inconvenient button activators or a continuous
increase/decrease of sunlight instead of one bar at a time. I took
those patches and improved on them a little in order to make them
a bit easier to use in combination with in-game functions.

You can download it here: http://gba.dellicious.de/patches/boktai_solar_sensor_patches.zip

edit: seems like I was too slow

 

If you're emulating with VBA, there's an option you can set in the settings to choose a light level. I'm thinking it's for the solar sensor of Boktai games, but I'm not exactly sure.