electrical problem

[lolo33]

for history of this thread, go to "what did you do to your truck today"

resume :
1-

2 month ago, battery low ; controlled it and alternator produced 18Ah when engine ran ; so I changed the regulator (LMC Truck)

2-

put my new regulator, and changed battery for a new one

3-

after 3 little trips, battery low ; I don't drive with lights during day

4-

yesterday, charged my battery (13V), started engine, and ... 13
V

question :
is it a problem with alternator ? regulator ? what kind of test can I do now ?

many thanks to you
Laurent