I also
STRONGLY recommend an LED pack monitor which graphically displays the state of charge of each battery in your pack WHILE UNDER LOAD.
It's particularly important for an older or "mystery" pack to know which battery is the weakest when pulling serious amps, so you can adjust your driving style and trip length to protect it from damage (ie drawing the voltage too low under load - below 5.25v for a 6v lead acid battery, for example).
Otherwise you WILL commit battricide - just a matter of time!
EG:
You can also get commercial versions of this, eg Pak-trakr. (Probably others by now.)