Hydrometers are obsolete. Pro battery tester do not use them anymore. W/ todays electronic testers they are no longer needed. You can't use them on AGM, and w/ experience you don't need them on lead acid. All you need is a good smart 3 stage converter/charger to keep your batteries in good condition.
But if you lack the experience of "knowing your batteries" and how they operate, then do whatever works for you. That's why they still make hydrometers, for those that don't know any other way.
It always just slays me how no one ever has a battery problem, until they get talked into buying a hydrometer. That should tell them something, but doesn't.
So in other words, if its lead acid, there is a problem, but if its an AGM there isn't. But keep listening to the "battery god", he'll keep you buyin hydros and battery cookers, and babysittin your batts, while I'm campin
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Rich
'01 31' Rexall Vision, Generac 5.5k, 1000 watt Honda, PD 9245 conv, 300 watts Solar, 150 watt inv, 2 Cos 6v batts, ammeters, led voltmeters all over the place, KD/sat, 2 Oly Cat heaters w/ ox, and towing a 2012 Liberty, Lowe bass boat, or a Kawi Mule.