12 Volt Battery Choice

[Doublegrat]

I've got room for 2 batteries on my sailboat. I'd like to know the difference in 2 12 volts hooked together to double the amp hours or 2 golf cart (6Volt) hooked together to double the volts? Will the golf cart batteries last longer? Which is the better way to go?

TIA,

John

[CaptKen]

If you use 2 12v batteries you need to hook them in parallel and you will effectively double your amp hours available. If you hook 2 6v in series you get 12 volts and also double the amp hours. So it comes down to what the batteries are rated at. As a general rule of thumb, for a house bank, the golf cart batteries provide more amp hours. You should also consider a seperate starting battery or carry a start pack in case you run the house down too far to get the engine started.

[gonesailin2]

Golf cart batteries (hooked in series to make 12V) are fantastic. Best bang for the buck, much easier to deal with than the big marine batteries (weight, size), and much more suited to regular discharge than automotive batteries.

[Arni]

You have only been given half an answer by the other posts. The reality is that ALL batteries have a theoretical ability to be discharged and re-charged, or cycled, a certain number of times, and this figure is also misleading because different battery manufacturers and also different countries, use a different formula. Say for example, if you quote discharges to 50% capacity, or 40%. Cranking batteries need to rapidly produce a very large current for a short period. They would normally never fall below 90% charged. Thus, they have many thin lead plates to release the current rapidly from a large surface area. However, these thin plates are damaged by too deep a discharge. Thus 'house' deep cycle batteries have fewer and thicker plates, which are more robust at repeated discharges, but can't deal so easily with sudden large demands.
Gel cell and AGM (Absorbed Glass Mat) are technologies that help to re-inforce the plates and make the battery more robust.
Now if you are looking at 'bang for the buck' you want to know how much the batteries will cost, and how long they will then last, i.e. how many cycles they will cope with. It is true that golf cart batteries can come out top of this equation, because although they will not cycle as often as good quality deep-cycle batteries, they are cheap. Don't expect them to last more than three years though, unless you are very careful with them.
It will also depend on how you plan to charge them. If you are the type of sailor who deeply discharges them and then charges rapidly at the dock from the mains, you'd better buy good batteries. If, on the other hand, you have onboard solar, wind and alternator so you need'nt let them get too discharged, then cheap golf cart batteries might be fine. You should never discharge any battery more than 50%. That, typically, means not less than 12.2 volts, but check for different types of battery.
If the engine starting demands are not too great, the house bank might cope ok, but this is risky as you might drain them. Much safer to have a separate start battery. You can still charge both banks from one source.
A smart charge regulator is pretty essential, and if you have more than one panel, a smart solar charge regulator also.
The best reference for all these questions is Nigel Calder's 'Boatowners Mechanical and Electrical Manual'. Buy it!

[harryrezz]

FWIW - I've used four 6-V Trojan golf cart batteries for my house bank and one Delco 12-V cranking battery on board La Nostra for several years in the Caribbean. I have, in addition to a 100 Amp. Bomar alternator routed through a "smart" regulator, two 80-Watt solar panels and a Kiss wind generator. I never run the engine when at anchor - even when anchored for several days. I have plenty of power to run a large fridge, lights, radio, water pumps, etc., even with charter guests on board. I get about three years from a set of house batteries (last cost was about $90 U.S. each), and the starting battery has ben in seervice for 5 and is still fine.
Capt. Harry Rezzemini
www.lanostra.net
CSY 44 W/O

[eatsleepsail]

I agree with most of what was said in this thread except something CaptKen said. When you connect 2 batteries in series you do NOT double the amp-hrs only the voltage. Connecting 2 batteries in parallel doubles the amperage but not the voltage. My understanding is that golf cart batteries are great because they can be cycled many times over their useful life. But if the amp-hrs of a single 6-volt golf cart battery won't supply you with your amp-hr needs then 12-volt is the only option in a two-battery boat.

[Dr.Theopolis]

I don't think anyone mentioned this yet. But if you have two 12v batteries and one goes bad out in the middle of nowhere, then you still have the other 12v battery to supply a full 12v. Though batteries usually degrade in performance gradually, it is possible for a battery to suddenly stop working altogether. Happened to me once.

By the way...I use marine batteries available from Advance Auto Parts. They are very affordable, last about four years for me, and I once anchored for nearly 3 days while powering lights and a small fridge off an inverter without running the engine.

[Merrick]

I have a Pearson 34 and recently installed two Odyssey 100 amp hour dry cell batteries onboard. I can run everything on the boat all night without worry. Next morning, I crank the engine off the 2nd battery and switch back to the first for recharging. The dry cell can take whatever your alternator can dish out. In addition, the odyssey batteries are rated at over 500 complete discharges. If they're good enough for vehicles, boats, tanks, etc., they're good enough for me. Battery troubles are a thing of the past now. I paid $300 for each one. Good for roughly 8-10 years. Good luck!