Open circuit voltage of a fully charged lead acid 2.1V - 2.13V per cell or 12.8V. If you pull it straight off the charger it could read higher but as long as you wait a few minutes, that is it. The car need to charge the battery, so alternator output is higher, nominally 13.8V, although 14.4v being common wouldn't surprise me. So, I concede, that if it is to be charged from a running car, other things may need to be considered. If no other charge regulation is employed, current should be limited to C/10 anyway. C being the battery rating at 1 hour. Hence for a 1000mAH battery, unregulated charging should take place at 100mA.