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Old 15-05-2020, 07:20   #1
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Join Date: May 2020
Boat: Pearson 30
Posts: 3
Firefly (and standard AGM) alternator requirements

Hey all, I have a relatively quick question about charging voltages.

Currently in my boat I’m running 2 starter batteries as a house bank, obviously not ideal, they came with the boat and I’m looking to replace them. Though they are in good shape and presently fine for my power needs (no fridge, no freeze, no AC, mostly 12vdc to USB charging and occasionally running a 500w 110v inverter to charge a laptop and lithium power tool batteries), I know that they will have a low lifespan used for deep cycle/charging. I have currently 1x100w 2x50w solar running through a Victron 75/15 bluesolar mppt controller. I have another 50w panel to add when I rewire some stuff, and will likely be trying to move to a 4x100w setup in the near future, as I have ready access to solar panels basically for free and would prefer the larger panels over more smaller ones. Anyway.

I’m looking to replace with two group 31 agm/sla batteries, and have been searching through previous threads, but haven’t found a straightforward answer to this:

I have an A4 motor with a stock-adjacent (read: nothing fancy, and I don’t know what the model is but I’m sure it’s not specialized) alternator. I don’t use my engine to charge, but I do motor sometimes obviously, so I want to be sure I am not going to damage the batteries I install. When the motor is running, my voltage readout off the load out on my charge controller is consistently maxed at 14.4v. I don’t have the Bluetooth dongle for the victron yet, so the charge profile automatically set to the default LA settings, I believe 14.4 bulk/absorb, 13.8 float. The starter batts settle at 12.8/9, and I have yet to go below 12.5v with my normal (monitored) use.

When I install the new AGM batteries, will I have to look at regulating my alternator output and/or tweaking the charge profile of the mppt controller? I had assumed for regular AGM I would be ok with my current charge setup as-is, but now I’ve read about the firefly batteries (I’m considering them, but haven’t decided yet) that 14.4v may be too high for regular bulk charging, and I’ve seen others mention that they regulate their alternator or have installed a special alternator even when using AGM batteries like Lifeline or Northstar.

I would hate to spend the money on batteries of any sort without taking proper precautions to ensure that I’m treating them as well as possible to maximize cycle life. A word on changing my setup: I know my current usage suggests that I am a very light power user and don’t need to look for the absolute maximum capacity and performance; I am intentionally limiting power usage right now, and am not cruising at the moment. I would like to design and outfit my setup to ensure that when cruising for long periods I will confident in it, as well as to be able to charge tools, electronics, etc, without concern for putting myself in a situation where my capacity is overwhelmed by my usage needs.

Edit: Forgot to mention I did see on the firefly specs page that they list max charging voltage as 14.4v, so intuitively I would think I’d be ok, but all the talk of alternators and regulating has got me second guessing myself. I am also not super confident in my knowledge of electrical systems and batteries so I figured I would turn to those here who clearly have experience that vastly surpasses my own.

Kevin Spacebag is offline   Reply With Quote

agm, alternator, men

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