House Battery Bank Size

[islander28capt]

I am wondering if anyone has a simple formula for a minimum house bank battery capacity. I am going to beguin rewiring and updating the electrical system on an islander 28, w/o refridgeration or anytype of hvac. I have a small chart plotter, depth, vhf, fm, raymarine tiller pilot and the standard lights (non led but will be switching over a little at a time) the only things i plan to add are a 12vdc fresh water pump, and a small inverter for occasional use. (laptop, guest cell phone chargers.......) My only source of charging is the bal-mar alternator,but am considering supplimenting this with a solar source.
So anyways looking for a little advice or input on battery capacity!

[Captain Bill]

I have a friend with a similar load, but he also has a small isotherm refrigerator supplemented by 220 watts of solar. He has never been plugged into the grid and seldom has to use the alternator. His house battery is a single 255 amp hour 8D agm, without the refrigerator you can probably get by with a 4D 210 Amp hour pretty easily. He can go about three days of very cloudy weather even with refrigeration before he has any problems and refrigeration is most of his load.

[MoorOrLess]

Thanks Capt Bill, good information. I am about to start similar task as islander28capt, and ask the same question.

[Pete7]

We have a similar power usage to you (including a small fridge) and use 30 amp hours a day in harbour. we use 2 x 110 AH 12v batteries (cheap flooded lead acid type) and manage perfectly well. we have 45w solar panel to top up but must run the genny every few days to make good our consumption.

I suspect we might use slightly more at sea with the Raymarine ST2000 but not huge amounts.

Pete

[islander28capt]

Thanks for the input so far! Im also leaning towards two smaller batteries over one big one such as an 8d. I like the idea of being able to isolate one if i do have a problem. Any opinions??

[capt_douglas]

The best number I can suggest is that you want a battery bank with a capacity of 4x the total load of the systems. There are spreadsheets out there where you can determine you daily power requirements, at sea or on the hook. Once you get that number you can decide the batttery capacity.

Then you get to ponder charging sources, types of batteries, places to buy them, and how to maintain them.

As far as batteries go, I'd suggest 2 banks. You can parallel them most of the time but if one bank fails, you can separate it from the house and still have power.

You might also consider weight and size. A 4D/8D is huge and weighs a lot. A comparable 6V golf cart alternative might cut the weight into 4 more easily handled items but take up a bit more space.

[islander28capt]

thanks capt douglas, seem like 4 6v batteries can be found cheaper than two twelve volt so that is something to consider. I am leaning towards to staying away from one large battery mostly for the pease of mind that comes from being able to isolate one of a failure does happen.

[Bash]

Originally Posted by capt_douglas

As far as batteries go, I'd suggest 2 banks. You can parallel them most of the time but if one bank fails, you can separate it from the house and still have power.
.

+1 on the value of redundancy. I was once in a situation where a gel battery shorted out, and it was nice to be able to cut it out of the system without having to lose all my electronics.

[cfarrar]

4x your daily energy consumption in amp-hrs.

In terms of daily consumption, I use considerably more amps while under sail, on account of plotters, autopilot, radar, instruments, etc.

[Captain Bill]

I wouldn't argue against the redundancy aspect of two batteries too much except to mention that depending on the failure mode with two batteries in the system the failure of one battery is not always obvious until the second one also fails. Need I tell you how I know?

[CharlieJ]

Most new designs are following the lead set by Nigel Calder; the house bank is a single bank of batteries. Less complication.

Using good quality (Trojan T105s are the gold standard, but there are others) deep discharge golf cart batteries for a house bank is far better than using 4Ds or 8Ds as they are generally not deep cycle batteries but starting batteries with rope handles!

Charlie

[goboatingnow]

The best number I can suggest is that you want a battery bank with a capacity of 4x the total load of the systems.

Theres no doubt that traditionally we have sized batteries based in potential consumption of electrically. I would like to suggest that you consider the alternative and size the batteries based on generation ability.

Consider what you typical engine alternator might generate daily and add other charging sources. That gives you a potenial total amp hours of charging available, Then size the battery to take that. The fact is its somewhat silly to use comsumption as an indicator without considering charging ability. Otherwise like a lot of people , you end up with a bank you cant recharge.

Dave

[roverhi]

I've got pretty much the load you have. Have a 12volt group 27 engine starting battery that I use for just that. The house bank is two 6 volt golf cart batteries in series for 225 amps at 12 volts. On a recent transpac, the instruments, lights, AIS and plotter took about 12 days before I had to recharge. An autopilot, refrigeration, etc. would have greatly increased that load on the batteries. I have 260 watts of solar panels but didn't see the sun for all that time. Because of the overcast, battery recharging was minimal and definitely didn't keep up with the use. Fortunately the sun began to shine on the 12th day so didn't have to run the engine to charge the batteries back up.

Unless you don't like your back, use a combination of golf cart batteries. They are relatively light, if any battery could be called light, take a lot of abuse and most of all, cheap.

[Bucky205]

Put a standard Digital multimeter between your hot lead and battery. most will do up to a 10 amp reading. Measure what you expect your average load to be. In general you never want to discharge your bateries below 50%.

So if your average load is 5 amps and you have 2 105 amp hour batteries. You would have 21 hours before you needed to charge. If your charger is a 20 Amp charger 1 hour puts back roughly 20 amp hours of power. Same with solar, wind and alternators. good DC systems are a combination of space available, charging capabilities and load.

[Jeff Millar]

Go Boating now. Your suggestion is the most intelligent suggestion I have read in a long time. For a long time FOR THAT VERY REASON I thought it would be better to have 2 house banks. However I accept the opinion that 1 House Bank is better, for the reasons explained.
Question
Which Battery to select. Odyssey PC1800 FT ( 12V, AGH ) OR ROLLS ( 6V ) FLOODED ( WET CELL ) MODEL # EIGH - 262.

Which would you select and why ?