Battery Sizing

[sail sfbay]

Boat has ten (10) 6 volt 220 Ah flooded house batteries that are nearing end-of-life and plan to replace them with 6 volt for low weight (66 lbs each) and AGM for ease of maintenance. I only want to install the minimum number of batteries that can support 9 hour trips between ports for recharging to minimize cost. Calculated power consumption from highest to lowest power consumers for autopilot, sounder, chart plotter, radar (standby), refrigerator (50% load), VHF (standby), engine instruments and bilge pumps (0.2% demand) operating 9 hours per day is 144 Ah. West Marine suggests sizing the battery bank for a minimum of 3 times the daily energy consumption (see link below), which indicates sizing the battery bank for 431 Ah. That would indicate minimum number of batteries are two pairs of 6 volt 220 Ah batteries wired in parallel with the pairs wired in series producing 12 volts and a capacity of 440 amp hours. I plan to install a battery monitor once I take possession of the boat I plan to install a monitor to measure power consumption on its 10 hour trip to her new marina. Does that make sense or is there a better battery sizing approach?

I also investigated the top five performing AGM battery in post #24 of the thread below and was told by the Firefly, Odyssey and Northstar manufacturers they do not make 6 volt AGM batteries Lifeline, Fullriver and Rolls do. I am leaning towards Lifeline.

Sizing Your House Battery Bank | West Marine

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Battery Brands

Jim Hawkins.................are they flooded or AGM and what Ah rating?

forums.sailboatowners.com

 

[Davidasailor26]

144 aH sounds like a pretty high estimate for those loads over 9 hours. That’s 16 amps average. In a typical 9 hour day with the same loads as you (minus radar) we generally burn less than half that. We don’t use much autopilot when sailing, more when motoring, but assuming you have sails reasonably well balanced that shouldn’t be that big a difference.

Having said that, 2 pairs of 6V’s sounds ok, but you could probably get away one pair if you’re not worried about having the extra redundancy built in. We have 2 group 31’s, for a total of 210 AH and in our full day sails we don’t really tax them below 70% of full. I sometimes would like to have more for overnights, but for daysails it’s totally adequate.

 

[sail sfbay]

Davidasailor26, thanks for the feedback. Below is how I calculated the daily consumption while motoring the entire way on autopilot (worst case ...no wind).

 

[dlochner]

If you plan to motor then consumption should not be much of an issue because the alternator will supply plenty of power.

When dealing with battery replacement, it is important to look at the whole system, generation/charging, storage, and consumption. AGM batteries have some important charging considerations, if they are not cared for properly they die and early and expensive death.

There is a wealth of good information on MarineHowTo.com on batteries and charging. Worth reading before plunking down hundreds of dollars.

Batteries Archives - Marine How To

Marine How To is the premier site for DIY boaters to get in depth articles for maintenance, upgrades and insight from a professional. I sell products that I know work, because I've worked with them. I tell it like it is. No sponsors, except my readers!

marinehowto.com

 

[sail sfbay]

Hopefully the Magnum Energy ME-ARC Advanced Remote Control, which will be programed for charging Lifeline batteries, will control the charging rate.

 

[Davidasailor26]

+1 for dlocher’s reminder that if you’re motoring then your alternator should be able to more than keep up with the 20 amps that you’re talking about drawing.

Partial state of charge for AGM’s can be bad news, but if you’re able to recharge after each sail that probably won’t be too big a problem.

 

[jssailem]

How about considering 3 of the 12 V Dyno Batteries. Dyno Battery - Premium Quality Battery Manufacturing - Made in the USA

The 30TMX is a true deep cycle Flood Acid Battery 135 amp hours. Weighs 65 lbs. 3 of them will meet your 144 AHr need with power to spare.

 

[sail sfbay]

jssailem............thanks for the suggestion, as they are half the price of AGM batteries.

 

[Jim Hawkins]

The 30TMX is a true deep cycle Flood Acid Battery 135 amp hours. Weighs 65 lbs. 3 of them will meet your 144 AHr need with power to spare.

I checked out their site but couldn't find prices. How nice are they in dollars?

 

[sail sfbay]

$168 from this site:

Dyno Batteries 30TMX | Fisheries Supply

Save More on Your Dyno Batteries #30TMX DEEP CYCLE GROUP30 BATTERY at Fisheries Supply. Excellent Customer Service, Ready to Ship. Marine Supplies Since 1928!

www.fisheriessupply.com

 

[Benny17441]

I think it would be very adequate with redundancy built in. Make sure you precool the refrigerator on shore power before getting underway.

 

[jssailem]

I have paid $150 at the company business office in Seattle.

 

[RoyS]

Your autopilot will not draw anywhere near the value you used here. In your calculations half of your load is allotted to the autopilot. Autopilot electrical loads vary depending on the force needed to turn the rudder and the time it actually has to run. You do not need to assume that it will run constantly or that it will run at full load.

 

[sail sfbay]

RoyS..............I used the current draw from the 36 year old autopilot owner's manual which specified continuous/peak of 6/10. I guess the real consumption will be determined by a battery power monitor.

 

[dlochner]

What are. your plans for the long term with the DC electrical system? Are you planning significant upgrades to the charging system, new charger, high output alternator, solar? That would be an important consideration.

If the goal is get you home safely for a 9 hour sail, then a couple of Group 27 batteries from the discounter of your choice would work as they will be essentially temporary disposable batteries.

 

[sail sfbay]

dlochner...............plans for the DC electrical system is to have a marine electrician inspect the system adequacy and to restore non-functional dc and ac power-on lights to operational status on the switch panel . Boat has a Magnum Energy 12V Pure Sine 2800 watt MS2812 inverter/charger with a ME-ARC Advanced Remote Control. The engine has a 50 amp alternator. Assuming these are adequate maybe I can minimize cost by replace the house batteries with initially two (2) 6v 220 Ah batteries in series.

 

[JohnShannon]

Probably the first step is to get a good battery monitor and then sail around for a bit to get an idea of your actual consumption. The only issue with a giant battery is cost and weight other than that it is all good. Have you looked a Firefly carbon foam batteries? They are supposed to be tolerant of prolonged partial state of charge operation. Buy some and tell us how it went.

 

[leo310]

I have 4 6volt golf cart bat. and before installing solar panels I could go 2 days with out charging. This was using AP, CP, AIS, VHF and stereo for 8-10 hours. the only time I used the engine was to anchor. I now have 300 watts of solar and I don't need to charge the bat. when docking and I now can charge my laptop, Iphone, Ipad and I don't need to worry about the bat.

 

[sail sfbay]

Probably the first step is to get a good battery monitor..............The only issue with a giant battery is cost................Have you looked a Firefly carbon foam batteries?...............

Agree with your battery monitor recommendation and cost. Looked at a Firefly carbon but they do not manufacture 6v batteries, which I plan to install due to their low weight.

 

[Jim Hawkins]

6v batteries, which I plan to install due to their low weight.

they may weigh less, but you need 2 to make 12 volts so what are you comparing them to weight-wise?