You will get a lot of variation in the battery voltage between full charge (over 20V) and near flat, around 16.5 - 17v.
The best option is a little buck regulator module, as Dandak suggested.
Get one with an output covering 12V and a current rating of 3A or more (as the listed ratings are usually the short-term just before failure ones, not continuous use).
This is a very common type available from numerous places; they seem to be pretty god, I've not had a problem with one so far:
(Load regulation: 0 0.5%. IN-IN + input Positive Input Negative. Could not boost). Output current 3A (max). Output ripple <30mV. Positive output OUT + OUT-output negative. Power Supplies. YOU MIGHT ALSO LIKEVIEW ALL OUR PRODUCTS.
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Adjust the output voltage before connecting the LEDs.