In my simplistic view of the world, a battery
is recharged when the resting voltage (and the rest period is at least an hour, more is more accurate) is back to the proper voltage for the battery
Since waiting so long is problematic, I fall
back to my second position which is that the battery is recharged after I've returned a matching number of amp hours to that which was consumed between the charging cycles.
For example, checking the battery after only 10-15 minutes will mislead you as to battery charge state.
Having no spouse that I can confine to the trailer(
), I built a box with a volt and ammeter which I can insert between the tow and trailer so I get a pretty good sense of what the tow is passing to the trailer.
Between the test box and my Linkpro meter (in the trailer), I've learned that for at least my combination (Ford Flex w/tow package) I don't count on travel-time recharging for much.
The picture shows the conditions with the reefer on DC which is about a 10amp load. It explains why my trailer battery is so deeply discharged even after a 5 hour trip if the reefer is on DC.