You absolutely never ever have to disconnect them, If you only have one good charger, charge each one , one at a time and leave them be, Its just not worth over thinking this, The charger will never put in more than what a new fully charged batt has, Meaning , If its ok for 4 batts to be hooked up fully charged, It is absolutely no different to charge em each one at a time until full, As U do it the other batts will pick up some of the over flow charge, All good, but they will never be fully charged unless you charge each one, And YES that is true with your trucks charger, Its like filling a pool The last batt is always starved a bit, and sulphation develops faster in the last one , and that is always the one that fails first, So to keep this from happening, Always... at least once a year rotate and fully charge each batt, You can do it in the truck or pull them and charge them and rotate, Its up to you , one way is NOT better than the other,
))))))))) Good luck ... If you have 2 chargers charge 2 of the batts and when the charger cuts off charge the other 2, If you have 3 chargers and 4 batts , Charge 3 of them and when they are done charge the last one, Here is a trick, If you have 6 - 2 amp chargers you can hook them all up to one batt for and fast and fuller charge than a trickle charger can do alone, ( simple math , 6- 2 amp chargers are 12 volts
) - as the charger's feel resistance they will shut down one at a time, Oh yes they will
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