Quote:
Originally Posted by Dave R
My guess is that you are slowly running them down. I don't know what size batteries you have, but lets say they are a pair of 100AH group 27s. If you run them down to 60% state of charge by hanging out at anchor with the fridge and stereo on for a few hours, you have to make up 80 amp hours of charging to bring them up to 100%. Lets say your OEM Mercruiser alternator produces 70 amps max (probably 50 amps at cruising speed). The fridge and stereo probably stay on while you are underway, so there's at least 10 amps used, the engine probably absorbs another 10 amps to run, now you have 30 amps to spare for charging. If everything was perfect and batteries charged at 100% efficiency all the way to 100% state of charge, you would need to cruise for 2 hours and 40 minutes to recharge the batteries. However, the batteries will not charge at 100% efficiency and after 2 hours and 40 minutes, they won't even be to 90% charged. Realistically, who's really going to drive around on plane for nearly 3 hours after a day spent anchored? A bigger alternator would help and AGM batteries are a better choice then flooded lead acid (they charge more efficiently than flooded lead acid) but the best solution with what you have now is something you already know, you need to charge them while the boat is parked.
Another solution would be to use a Lithium Iron Phosphate (LFP) battery for house (everything but engine starting and running) loads. Install a Blue Sea Systems ML ACR that is connected to prioritize the starting battery first and a single 100AH LFP and you will have vastly improved your situation. One big caveat though is that the LFP cannot reside in the engine space, they cannot handle the heat, so it will need to be moved somewhere cooler. LFPs recharge at about 99% efficiency from dead to 100% SOC and they are not bothered by being stored partially charged. Another caveat is that the LFP should never be used to start the engine, so you lose a some redundancy but if you never run house loads from the start battery, it'll always be fresh.
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In my case, we run no accessories at the dock or sand bar, I actually turn the battery switch to off every time the engine is off and we are docked/anchored anywhere.
Turns out one of the 2 batterys has recovered sufficiently to start the engine. The other is 100% dead ZERO voltage. So I replaced the dead one with a Excide Marine 31 series battery from Home Depot at a truly bargain price of $139.00 and was thrilled to get it for that. I shopped around and mostly got prices from about ~$180.00 to well over over $1000.00 at Bass Pro for their top of the line model!
Interestingly the dead battery is 1 year older than the boat, not sure if it got swapped out at the dealer/marina by mistake or Formula installed a leftover in it. The other one is one year newer than the boat,,, Both were the same maker so now I have gauge as to what to expect out of the other AGM. Assuming Home Depot still sells the Excide Marine Batterys for a similar price next year, I will probably swap out the other AGM and then have the comfort of knowing I have several good years ahead and with a season in between them, hopefully they wont both need replacing at the same time. Not even so much about the cost, the group 31 batteries are just a beast of a battery to haul in and out of the engine compartment and down the docks to the boat! ;-)