Dear Richard,
Thanks for your inquiry.
We are sorry for our late reply as our technical team are day off on the New Year’ Day holiday.
The voltage of car battery will change. It will be possible among 10.5-14V, sometimes higher or lower than 12V, then a boost converter is needed when lower than 12V, and need buck converter when higher than 12V. So you need a automatically boost buck converter.
please view the item B00SM3UALQ or B00JKFNEQ8 on Amazon, hope these will help you.
If you need constant current voltage regulator , please view the item: B00C9TKHC6 ( 090409)
More questions, please feel free to write back to us.
Best regards