Amp-Hour ratings for 6V Batteries

[dick]

<div>I have noticed a few people claiming their golf-cart (6V)
batteries as having a certain amp-hour capacity. Typically the
amp-hour rating for these is around 100-120 AH each. However,
this is at 6V, which is half the power of an equivalent amp-hour
at 12V. When computing the total capacity for onboard use, the
batteries must be considered in pairs. So two 6V batteries of 100
AH each will provide 100 AH at 12V, not 200 AH. You can't simply
sum the individual capacities -- you need to divide the total by
2. Consider that these batteries are not magical. Capacity is a
function of plate area, so there won't be much difference in
terms of volume whether you go with golf-cart, 4D or 8D batteries
(of course there is plate thickness to consider, but that is
secondary). So it doesn't make sense from that standpoint to
believe you are getting this phenomenal capacity from golf-cart
batteries in half the volume of conventional marine/truck
batteries.

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[budfet]

I totally agree. The big advantage to good, deep cycle batteries is the size and weight. With 6 volt batteries you can get them aboard your boat easier, put them in a variety of locations, and increase plate size. The important thing to remember is to monitor all batteries and not deplete them more than 50% before recharging.

 

[bschnur]

6 Volt "golf cart" batteries come in a great variety of sizes, and therefore capacity, which frequently makes the topic of battery capacity a sort of apples and oranges discussion. The Trojan Battery Company (which was mentioned by somone earlier as having reasonably priced deep cycle batteries) makes 14 different deep-cycle 6 volt batteries for electric golf carts, utility vehicles, etc. At one end of the spectrum is a 6V, 107 A-H battery that weighs in at 31 Lbs. At the other end is their 395 A-H battery, weighing in at 121 Lbs. The 6 Volt battery that Trojan says is most often used by sailors is their "T-105" deep cycle product, which weighs 61 Lbs. The 105 is a reference to the golf cart rating (initially105 minutes at 75A, but is now 115 minutes due to better technology). The rating methodology that is generally used for sailboats, however, is the 20 hour rating, and at that rating the T-105 battery is good for 225 Amp-Hours. Put two in series and you have a 12V, 225 A-H battery bank.
It's useful to note that Trojan defines the endpoint of discharge as 1.75 Volts per cell, or 10.5 Volts for a 12 Volt bank. They also say that the T-105 battery will sustain 754 "Lab" cycles at this rate and depth of discharge. In the real world it would be less. Discharge cycles of less than 50% will dramatically improve the lifespan of deep-cycle batteries.
On a related topic --
For cruising purposes, experts now say that you should build and use a single, large, house battery bank and try to cycle it no more than about 30% of capacity. A large flooded-cell battery bank of, say 600 A-H, will allow a substantial bulk charge rate of 150 Amps and make the charging cycle fairly efficient. Compare this to recharging a 300 A-H bank at 75 Amps. It would take twice as long to put back what got taken out. Also, a 30% or less discharge cycle would allow perhaps 1500 to 2000, or more, cycles, depending upon a whole host of variables. This translates to many years of use from one set of deep cycle batteries!
FYI, the Trojan Battery Company has a toll-free number, 1-800-9-TROJAN.

 

[budfet]

For a great reference on electrical for your Morgan as well almost all other areas that you might want to know, consult Nigel Cailder's "Boatower's Mechanical and Electrical Manual: How to Maintain, Repair, and Improve Your Boat's Essential Systems". This book should be in all your libraries.

 

[piperdoc]

What is an amp hour