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Old 04-29-2019, 05:47 AM   #1
City: San Diego
Country: US
Join Date: Apr 2019
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House batteries, isolator necessary?

Hi everyone!

I have three sealed lead acid non AGM group 31 batteries as a starter bank. 2 weeks old. Bought two group 31 Duracell AGM batteries from Sam's club yesterday for $179 each. 20hr 105ah each. I have a Reliable Power 3000W Pure Sine Wave Inverter that I'd like to hook up for a mini fridge and an electric kettle (1500w, few mins each morning). That's pretty much it.

Can I hook the 2 new AGM batteries in parallel with my starting bank without an isolator? I start my motor every 8 to 12 hours and move around. I'll run the motor when the kettle is on if need be for now. Alternator is 140a.

I did buy an Installbay 500a isolator and a Wirthco 140a smart battery isolator a few weeks ago in preparation for this. Wondering if I still need them.

Thanks! Item links below.

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Old 04-29-2019, 08:53 AM   #2
tiltrider1's Avatar
City: Seattle
Country: USA
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
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Mixing AGM and lead acid batteries is frowned upon but nothing catastrophic will happen. Not using the isolator is risking a no start situation one morning. I have not looked at your capacity vs consumption so I have no comment on how long you can go between charges.

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Old 04-29-2019, 11:46 AM   #3
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City: Litchfield, Ct/Punta Gorda, Fl
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Wow, the only reason I can see for three group 31 starting batteries is twin engines and a genset, although a group 31 is overkill for a genset.

As noted above if the new house batteries are wired in parallel with the starting batteries, all of them may be drawn down too low to start the engine the next morning.

Almost all boats are delivered with a 1/2/all/off battery switch. Usually you connect the house battery to one input (1) and the starting battery to the other (2). This way you can switch the start battery out of the system to protect it from discharging.

A set and forget approach is a combiner or ACR. Requires a bit of rewiring to do it right.

I frown on isolators because their diodes have a 1/2 volt drop which means the alternator won't charge as effectively. Combiners/ACRs are much better.

I am with tiltrider1 in that mixing AGMs and flooded cells isn't the best way, but their charging characteristics are close enough that nothing bad will happen if you do.


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Old 04-29-2019, 06:49 PM   #4
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City: Seneca Lake NY
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Vessel Model: MS 34 HT Trawler
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As usual I'm of same thinking as David.
Here is a link re: various isolator types / technologies. I found it a helpful start when I was reworking the charging systems on my Mainship.
Here's what I had & replaced it with... see charging system mods... not the only way as there are a lot of good options but diode isolators aren't the best as David points out.

Those AGM Gp 31 aren't a true deep cycle batty... if its rated w/ CCA / MCA it's a dual purpose / compromise at best.
Any chance you could sell the GP 31 FLA's, move the current AGM GP 31 to start and use AGM Golf Cart Battys as house - they are true deep cycle and much better for house use.
Many boaters / RVers could use Gp 31's as start battys.
More re Deep Cycle
2008 MS 34 HT Trawler
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