Battery Opinion

[James Graham]

Looking for opinions on use of one 8D battery verses two 31 series batteries for one bank. Room and accessibility not an issue. Mainly interested in the house side.
Thanks,
Jim

Topic

[James Graham]

House Battery Load

This not a new build. 30 single engine downeast boat, 330B power.
DC requirement on the house side is:
Furuno TZT2 15L MFD. Fish finder function runs all day. Radar runs when engine runs,
Additional Fishfinder FCV 295, runs all day.
Furuno GP33
VHF radio
Lighting runs when the engine runs for the most part. Three durabrite mini’s, couple of small rigid floods, couple of cabin lights, and running lights. All lighting LED’s.
Bilge pumps also on house side
Bait well runs on separate group 31 circuit.
Engine runs on 2-31 series in parallel.
House is currently 2-31 series in parallel.
I am replacing batteries this winter and was wondering if the house would be better off with a single 8d. Responses seem to indicate I should stick with the doubled up group 31’s
Thanks too all for the info. Still learning
Jim

[Rob Schepis]

Justin Riege wrote:

Sizing a bank for loads is a whole other discussion.

Amen to that! Figured it was worth a little digging into..

[Justin Riege]

Rob, I was assuming James knew 8D was sufficient capacity for him.

1 8D weighs about 130LBS and has about 240AH @ 12 Volt

2 group 31’s in parallel weigh 120 lbs together and have about 210AH @ 12 Volt

Or two – 6 Volt Golf cart batteries in series together weigh 125 lbs and have 230AH @ 12 Volt

Doing some Tony style math, you get about 1.75 Ah per pound @12 volts. or a bit over 20 wh/lb

Sizing a bank for loads is a whole other discussion. Hehe. But might be one worth having.

[Rob Schepis]

James Graham wrote:

Mainly interested in the house side.

The House Bank needs to fit your “use” more importantly than anything else. What kind of DC demands will it need to meet?

What is the House Bank setup now? Or is this is new build?

[Roger Franklin Williams]

On a 40′ Bayliner Avanti 6BTA’s I did this years ago and use 2-31’s for engines and 2-4Ds marine wet cells for house with inverter. As a guy at cummins said one time when i asked do i need seperate 8D’s for each engine he said not if their going to start. Have never had a problem but also installed a cross over switch just in case I need to combine house and engines.

[Justin Riege]

James,

I worked in the solar industry for 10 years, and still do some design and engineering for solar/battery/generator systems.

I have had my hands on just about every type of battery out there. From my kids power wheels to my Dads electric forklift battery.

At the end of the day your capacity is a simple function of how many pounds of lead you have.

Dividing one big battery in to two or four smaller ones is fine, you will have some more connections to make, but the other advantages outweigh the disadvantages in my opinion. Of course you don’t want to overdo this. But within reason its fine. In the solar business we try to limit parallel connections, so we will use 6v or even 2v batteries, to get to our 48v, with most of the connections in series, but on a boat, when you are talking only 2 or 4 parallel connections this isn’t as big of a concern.

To me the most important consideration is what type of battery to get.

AGM = A bit pricey, very hassle free and clean, but life is limited to about 4-8 years depending on abuse. Good CCA can put out a lot of amps relative to size.

Off the shelf Flooded deep cycle batteries like golf cart, group 27, group 31 etc. Cheaper and Can last a long time if they are of the heavy duty variety, due to the thicker plates, but are messy and require watering and cleaning. and can spill. Don’t like producing amps per size as well as AGM’s but if you have a decent size bank this doesn’t really come into play.

Gel Cell= don’t bother.

Dual purpose marine a bit light duty for an offshore boat, but would probably work for a couple years. I use the DPM from costco in my little ski boat, works just fine less than 100 bucks. But I won’t be stuck drifting in the ocean if it fails.

Fancy brand high dollar batteries = overpriced, paying for the label, marginal if any improved life.

Careful with super cheap unbranded batteries off ebay or the like, if you need to depend on them, they could be rejects that didn’t make the cut for quality control for another brand. I like to buy something from a reputable retailer, competitively priced, but something that will have at least a basic quality control behind it. There are dozens and dozens of fine brands, inevitably the failure is due to abuse, or age, not the sticker on the side.

[Tony Athens]

Jim,

I think is was my back ( maybe 25 yrs ago) that made the decision for me about using 2-3 smaller batteries vs an 8D.

But as time went on, I also began to believe that you can better better technology and get it easier & cheaper in multiple group 24, 27, and 31 batteries than in larger batteries..

Tony

[David Hays]

If you are going to pay someone else to install the batteries, I don’t see that it matters. However, if you are going to do it yourself, I can’t imagine anyone choosing an 8D over two group 31s.

[Corey Schmidt]

I think the idea of having “modular” batteries (GP31) that are spec’d well (re: CCA, etc.) combined to serve both save space and weight and still provide enough ummmph, are in my personal opinion, the way to go. However, it really is a personal decision as there are so many ways you can argue the overall pro’s and con’s of both approaches. It’s all about assessing and managing the trade-offs of either selection…

[Author]

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