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House Battery-AGM

Upgraded our house battery bank to three Oasis Firefly G31 AGM batteries.

Update: I don't recommend upgrading to Firefly batteries. They moved their manufacturing to India and quality control was lost. The end result was an overwhelming amount of warranty claims. All three of mine failed after 18 months and is why I ended up going to LiFePo4.

However, for AGM batteries that come with the NW edition, I did draw out a schematic for a NW edition to add a battery monitor and balance the house bank to increase longevity (and help run the inverter for the microwave and cofee maker).

RT Electrical Overview-NW+ Edition.drawio
Download PDF • 263KB

Feel free to reach out to me directly with any questions or assistnace installing.


It only took us 3 months to murder our house battery bank on our brand new boat!  I didn't want to just replace them.  I would have just murdered them again. I had to do something different.

My lessons learned are: 

  1. Install a battery monitor.  This would have let me know the actual state of my batteries and would have kept me from murdering them.

  2. Upgrade solar.  This would have replenished the batteries during the day with sunshine and further prevented murder.

My requirements for the battery upgrade were as follows:

  • Remove all unnecessary cables from the + and - battery posts.

  • Make future battery replacements easy.

  • Relocate the house battery bank from the port lazarette to centerline (balance the boat).

  • Configure the house battery bank for equal charge/discharge out of all batteries.

  • Increase capacity.

If you increase battery capacity, you must also increase the size of your battery charger.  ABYC standards recommend a minimum 10% charge rate.  I went with Oasis Firefly G31 (AGM) batteries.  They are as close to lithium as one can get, without actually going lithium. 

Firefly batteries can be safely discharged down to 20% state of charge and they can do that about 1,100 cycles.   They can actually go to 0%, and can do that about 600 cycles and recover each time.  They do require a minimum charge rate of 20% (charge rate of 25 amps per G31 battery).  So I had to add a second battery charger (70 amps) to compliment the 20 amp battery charger that came with the boat.  There wasn't enough space available to simply upgrade the existing charger.  I run both battery chargers for 90 amps total of battery charging.

This was by far the most complex, most expensive, and time consuming project I've done to the boat. It is all electrical work, 120volt AC wiring and 12volt DC wiring.

The pair of OEM Factory 110amp-hour AGM batteries provide 220Ah total, with 99 amp-hours usable.  The three Firefly G31 batteries provide 348amp-hours total, with 225amp-hours usable.  

I purchased the following components

The total was about $4,400.

Labor was about 3 days total.

June 12, 2021

Last update: 9/3/2023

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