My 8 month old Guest onboard charger going bad and killing a battery cell or a cell on an 8 month old Interstate deep cycle trolling battery going bad on its own?

Need a little more info! Have you added water to that battery? And if so , did you use distilled water or tap water? The chlorine in tap water along with the natural minerals will kill the cells in a battery. The best way to find out what is going on is to take the battery, perferrably to a interstate dealer, and have them test the cells, which they can do individually.
If it does not end up being the battery I would check all connections first, then take a voltmeter and check the output on each lead from your charger. I am trying to put this a simple as possible because I understand that not everyone is mechanically inclined. Wiring is one of my specialties and finding short circuits. Hope this helps and good luck! Need anything else PM me!!

Thanks jcartwheel! I took the battery to Autozone since they are right around the corner from me and it tested as a bad cell. I opened it up and the water was a tad low. I added some distilled water to it and it didn't help.

I took the battery back to the boat dealer since it was under warranty and got a new trolling battery. Works just fine now.

Hopefully this one will last a bit longer.

Glad that worked out for you!! If you ever need help with any wiring issues let me know.

Once a cell goes bad there's nothing you can do to make it better.

Letting them get low on water prevents full charging which shortens their useful life.

Don't know how a charger can single out one cell for damage.

Possible the battery was low on water or not kept charged on the shelf or just defective. my own experience with interstates has been real good.