How do you know when your trolling battery gives up its ghost?

[glassyeyedPaul]

I am a 3rd year boat owner ( 16 foot Tracker Pro Angler) and have the battery recharger on the battery since 11 am yesterday and now it is between 4 and 6... seems so slow to get it fully recharged. So just how do you know how much life does the battery have as far as long term goes?

Also what do you think of those on-board battery rechargers... do they work? Is it worth the cost?
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