Recommended Battery Replacement

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Dan Corcoran
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Joined: Sun Jan 29, 2017 4:32 pm

Recommended Battery Replacement

Post by Dan Corcoran »

I have 3 large Lifeline AGM batteries, GPL-31T's, that are end of life. Charging system includes what is standard for all of us, the smart regulator, high output alternator, and 2-bank shore charger.

What is the preferred choices for replacing batteries? 3 large batteries? 2 large batteries, 1 starter & 1 house? 2 large house batteries and a small starter battery?

What are the preferred battery chemistry? Firefly, Mastervolt Odyssey TPPL, Lifelife 31T? Lithium Ion anyone?

Has anyone fitted the inch longer Firefly battery into a 109? How did you change the mounting?

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Re: Recommended Battery Replacement

Post by Mvallus »

Hi Dan,

Replaced our batteries a year ago. We do a variety of overnight races and find having 3 group 31 batteries to last very long (2 house; 1 start). They are so good that we probably could get away with just two. I am uncertain as to whether downgrading to group 27 would be beneficial. What I can say is Trojan AGMs batteries are great. Make sure you set up your charging system correctly for proper chemistry. This is addressed in an earlier post. ... 244-1.html

Phoenix 251
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Re: Recommended Battery Replacement

Post by TSweather »

Have two of these that came with the boat that are still holding charge while thankfully not providing anything near as sketchy as their namesake. It's like a child seat named kid killer.

Will likely switch to AGM for my next batteries.

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Joined: Tue Jan 28, 2014 12:14 pm

Re: Recommended Battery Replacement

Post by cercole »

Hi Dan,
I was able to fit two Firefly batteries in the stock location. I am running one regular Lifeline AGM as a reserve and two Fireflys as the house bank. The Fireflys will fit if you put them in the aft two locations. Then I changed the positive wiring to retain the 1 BOTH 2 positions. The Fireflys have a lot of capacity. So far we are very happy with their performance. Excuse the blurry pic.
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Dan Corcoran
Posts: 192
Joined: Sun Jan 29, 2017 4:32 pm

Re: Recommended Battery Replacement

Post by Dan Corcoran »

That is a great solution. When I had measured for the firefly I saw that it was going to be a close fit.

Something for me to share, I rewired everything so that the starter battery & starter are isolated from the house & house load, so that starting the engine doesn't spike the electronics (probably not a problem with 2 firefly batteries !)

To do this I
1. Ran a new 9 foot red electrical cable from the engine compartment to carry the alternator output direct to the house batteries, by connecting it to the same post on the electrical switch as the house battery. No matter where someone turns the battery switch, the alternator output goes to the house battery (protecting the alternator diodes.)
2. I disconnected and taped the wire in the engine compartment that bridges the starter and the alternator (Be sure to first disconnect batteries and shore power before doing this, there are no fuses to stop an arc if you accidently touch engine)
3. I moved the Starter wire from the common lug to the #1 battery lug. Now the starter battery and starter are connected together. Unfortunately the Off position on the switch has no effect of cutting off the engine key. In the future I will install a BlueSea e-series switch (4 lugs) and then that will allow me to have an off position that also cuts off the starter.
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Re: Recommended Battery Replacement

Post by jkalucki »

I've been using West Marine (really East Penn) Group 31 AGM for about a year and they've been fine for a 2 day race / cruise.

Note that the house batteries wired in parallel as pictured above are done incorrectly. Putting the boat leads to one battery and then piggy-backing two off to the second battery will cause a small, but critical, voltage drop to the second battery. The charger will slightly and then eventually continuously over-charge the first battery, damaging it. (This happened to me: charger started running constantly, first battery was bulging, quite warm and held no charge, second was fine.) It's best to put one boat lead to each battery to keep the voltage levels identical. There are detailed treatises on this topic online.
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Vento Solare
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Re: Recommended Battery Replacement

Post by Vento Solare »

The previous owner had problems boiling batteries. Mine are hooked up the same way as your picture when I run all 3 batteries. Normally I only keep two on the boat (Group 24 AGM start battery and Group 31 AGM house battery with 2nd Group 31 AGM in parallel when cruising). I set the charger and voltage regulator properly and installed a temperature sensor on the center battery (see this old post). I've never had a problem and am getting good battery life.
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