Looking for recommendations for a deep cycle house battery. Boat came new with (4) Optima D 31M blue top. House battery always fails. Have replaced under warranty but time to buy a new one.

House battery is also a blue top ?

Did you know they will discharge below what a charger will charge, thus making it appear the battery has failed ? Simply, connect another battery to it until it reaches over 11 volts, then the charger should be able to take over.

Lifeline, Odyssey, Trojan

Your charger is set to AGM correct?

Yes, And I have an AGM charger with reconditioning mode..

Gel cell

I have a ProMarine charger that I installed on my boat to keep the batteries up. I had discharged both AGM batteries totally dead and it recharged them both like new.