Join Date: Jan 2003
Location: Maine Coast
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The 6V wet cell battery is a tried and true work horse, if you have the space, height wise. There is no good reason to switch from 6 6V to 6 12V batteries. The 12V will usually not last as long, will generally cost more and will take up more foot print to get the same number of Ah's. The only time I will use 12V batts for a house bank is if I only have space for an odd number of batteries. For instance if you only have space for three batteries then three 12V deep cycles will give you the most Ah's for the space constraints, as you would need two or four in 6V to do the same thing. The entire golf car industry and industrial machine industry relies heavily on 6V batteries, often of the GC2 size which is very common amongst cruisers.
Nearly everywhere on the planet these robust batteries are widely available. The reason many cruisers use 6V batteries is for increased cycle life, $$ per discharge, and $$ per Ah of capacity. The plates are thicker, taller and more robustly built than a 12V deep cycle of the same capacity. Their foot-print is also often smaller for a given Ah capacity.
The 6V battery will have only 3 cells where the 12 will need 6 cells thus thinner plates in the 12V battery. 6V wet cells win on price, Ah per dollar, and value every time. They do lack high acceptance rates, which with many charging systems on sailboats is a non-issue because they can't actually benefit from this due to HP and belt restrictions, they lack ability to lay them on their sides, and they do require maintenance and can "gas".
A bank of four 6V batts at 450 Ah's can accept charge current of about 20-25% of the 20 hour Ah rating. a 450 Ah wet bank can take the full output of a 100 AMP alt when deeply discharged but this won't last for very long as they charge begins to taper due to acceptance once at or near 80% of capacity or so. If you want to go bigger than 100 amp you will spend a lot of money on custom pulleys and brackets to fit a larger alt. In contrast that same bank in AGM could theoretically take well over 300 amps IF you had a way to feed it that, which most don't.
Testing has been done between 6V and 12V longevity but it does not take a scientist to see why a typical 12V battery with plates that are roughly .040-.060 thick can't compete longevity & cycle life wise with the typical 6V battery that has plates in the .105 -.110 thick range.
Here are typical plate thicknesses from four of the leading US 6V battery makers:
US Battery US2200 - 232 Ah, +Plate Thickness .110
Crown CR225 - 225 Ah, +Plate Thickness .105
Superior GC1200 - 220 Ah, +Plate .115
Trojan T105 - .110 (Trojan will not disclose this data but US Battery has done destructive testing on the Trojan's and confirmed the plate thickness)
My current favorite battery from the list above is the US Battery US2200 if you have a distributor near by, we don't have one in Maine so going to MA to get them is a pain, cron or Superior are great substitutes. US Battery competes directly with Trojan and the US2200 is their "T105 killer" yet costs significantly less.
Before asking, "is 500-600 amps enough", actually Ah's not amps, you should ideally know what the baseline of consumption is before you can know what "enough" needs to be. I would always suggest a battery monitor so you know accurately what your system is using in terms of consumption and what your charging sources are actually returning to the bank over how long a duration.
The Victron BMV-601 from Jamestown distributors is only $175.10 and a very good value but the Xantrex Link Lite is another great battery monitor though slightly more expensive. Buying this first, before major bank upgrades, can actually pay for other upgrades because you may find your calculations were way off and that 600 Ah bank you thought you needed, would really be more like 675 Ah's if using 6 6V batts, really only needed to be a 450 Ah bank, thus paying for the battery monitor. Every boater I have installed a battery monitor for raves about them and can't believe they did not do it sooner.
In short, a bigger bank is better. The bigger the bank the shallower the discharges will be. Deep discharges kill banks faster. If you have a bank you regularly cycle to 50% SOC (state of charge) vs. one you cycle to only 75% SOC then the bigger bank only dropping to 75% SOC will outlast the one regularly being discharged to 50%. 50% is the lowest you generally want to discharge to.
In my opinion 6V wet cells would be your best bang for the buck, should last a good long time if properly cared for, and you don't need a fancy regulator to run them unless the bank size at a 25% acceptance dwarfs your alternators output in which case alternator temperature sensing and an external regulator is a good idea.
Also, remember that when charging off an alternator you will rarely charge the bank back beyond 80%-85% of capacity due to time constraints and basic battery acceptance curves so this is where your solar and wind can help in a big way.
-Maine Sail / CS-36T
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