Ideally, all the positive leads from each battery should be the same length and type of cable, ditto for the negatives......then all the positives joined at one point, all the negatives at another - it doesn't look very pretty, but it ensures that each battery bears/gives the same load...........

here's chapter and verse......
http://www.smartgauge.co.uk/batt_con.htmlcomme ci -

-and to answer your original question, attach the charger "after" the equal joining!
