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ContinuousWave: Small Boat Electrical
Connecting Cables To The Battery
|Author||Topic: Connecting Cables To The Battery|
|Tom W Clark||
posted 01-19-2014 03:37 PM ET (US)
Many modern Marine batteries have threaded studs in addition to the conventional clamp-on terminals.
So if one is having brand new battery cables fabricated, should one have cable clamps put on or are ring terminals sized to the threaded studs adequate?
posted 01-19-2014 04:07 PM ET (US)
I'd use ring terminals on the cables. The large battery clamp and fat and soft post connections are a legacy from old automobiles and early batteries. Today many of the imported AGM batteries have neither big posts or threaded studs, but rather a threaded land or conductive area on the top, and cables are attached with ring terminals and a machine screw.
The threaded land for connections is seen in this Cabela's AGM battery. Those types of connections use a rather small hole in the ring terminal compared to a 0.313-inch or 5/16-inch threaded stud. If going with 4-AWG and a 5/16-inch stud, use a terminal like a MOLEX 0192210231. For 2-AWG and 5/16-inch stud use Molex 0192210233.
posted 01-19-2014 04:31 PM ET (US)
I think the Cabela's AGM battery lands are threaded for a M6x1 fastener. That is a 6-mm fastener. [Here I had to change my calculations due to some bad math and wrong equivalence; thanks to tmann45 for the hint--jimh] In inches, 6-mm is about 6/25.4 = 0.236-inch, or just under a quarter-inch. The 6-mm terminal won't fit on a 0.250-inch stud. And 5/16-inch is 0.3125, so the 6-mm connector does not fit on a 5/16-inch stud. You can find some specialized ring terminals with the hole size for the stud set for metric and the wire crimp housing sized for AWG. For example, a ring terminal for a M6 stud and 2-AWG wire is
posted 01-19-2014 09:26 PM ET (US)
Be careful of the ring terminal size, some batteries have 5/16" and some have 3/8" studs on the negative side.
1" = 25.4 mm [Thanks for the reminder--jimh]
posted 01-20-2014 08:21 AM ET (US)
Relying on the pliability of soft lead (or whatever those terminals are made of) to make a good connection does seem pretty "low tech". After a long enough time, many seem to ooze yellow crud.
But this discussion begs the question of why automobile batteries don't use some other form of connector.
posted 01-20-2014 07:01 PM ET (US)
Most car batteries now use side terminals, which have a large area of connection between the battery land and the wire terminal.
Re the wire terminal hole sizes for the post or fastener: it is actually quite hard to enlarge the size of the ring terminal hole without messing up the terminal. I recommend getting the terminal with the correct hole size for the threaded post or fastener that will be used. You can make up some difference by using large washers.
posted 01-20-2014 08:35 PM ET (US)
This might be helpful:
6-mm 0.2362-inch. Used on many AGM battery terminals.
1/4 0.250-inch. Found on some threaded terminal posts.
5/16 0.3125-inch. Common on threaded terminal posts.
3/8 0.375-inch. Very common on threaded terminal posts.
In looking at the drawing for a ring terminal rated for an M6 fastener, the actual hole diameter in the terminal is given as 6.4-mm, or 0.252-inch. It should be a snug fit on a 1/4-inch post. Another terminal, said to be sized for 1/4-inch studs, has a hole diameter of 7.06-mm or 0.278-inch. This is an easier fit on a 1/4-inch threaded post.
According to a Wikipedia article at
marine batteries are said to typically have different diameter posts for the positive and negative terminals, with the convention to be
POSITIVE = 3/8 x 16 thread
NEGATIVE = 5/16 x 18 thread
I have not made a wide survey to see if that is universally true. I have an Interstate Marine Battery on hand, and the two terminals are the same size; both are 3/8 x 16. Based on that exception to the supposed rule, don't automatically assume a marine battery will have terminals of either the same or different size; you should check the particular battery. Also don't assume the terminal polarity orientation will always be the same. The positive and negative terminals often change position on some models.
posted 01-20-2014 08:38 PM ET (US)
The concept of the Group Size is a bit frustrating to track down good information. The Group Size is set by the Battery Council International (BCI) but they do not publish their information on-line. They would rather sell you a printed copy of the specifications for $125. On that basis, I don't know of any really authoritative source of information about battery size, terminal location, and threaded post size as set by a BCI Group Size designator. (But try this PDF file; it looks interesting, if even you are warned against using it.)
posted 01-21-2014 10:04 AM ET (US)
The use of side terminals on an automotive battery is perhaps something which has mostly been adopted by General Motors, and the old-fashioned soft lead posts, which, by the way, come in a variety of diameters and tapers, are still in use on many brands of cars. I think my 2011 FORD has a battery with those tapered lead posts.
In the so-called marine batteries one often finds both lead posts and threaded terminals. Sometimes having two connections on the battery can be useful; for example, if you want to connect more than one conductor to the battery. You could connect one conductor to the lead posts, and connect a second conductor to the threaded posts.
posted 01-21-2014 12:32 PM ET (US)
Fundamentally, you are always looking for the most contact area to make your electrical connection. So a large lead post has it's advantages in large current (but low voltage 12-48 VDC) applications. For small recreational boating applications, screw terminals make much more sense out of practicality versus the theory of needed surface area.
If I was running a electric propulsion system on batteries, it would likely use lead posts, with 00 cables.
Some of my battery sets have dual posts. I have liquid taped a cap on the lead posts, to avoid touching them with a tool or something.
|Tom W Clark||
posted 01-21-2014 11:02 PM ET (US)
It sounds as if the Sears group 34 Marine Platinum Die-Hard is perfectly typical. The screw studs are 3/8-16 on the Positive side and 5/16-18 on the Negative side.
The battery cables we are ordering from genuinedealz.com will have 5/16" (M6) and 3/8" (M8) lugs. There is the option of ordering "Heavy Wall" tinned copper lugs for a trivial added cost. Any reason not to order "Heavy Wall" lugs?
posted 01-22-2014 10:37 AM ET (US)
Tom, I think the heavy-wall lugs are preferable. That being said, it is possible you may find a connection where the smaller, standard wall size fits better. I would suggest you plan to use the heavy-wall but get a few standard as backup.
This is a great instructional for cable choices - I followed it closely. (the recommended crimper is great also)
posted 01-22-2014 11:43 AM ET (US)
Tom--your mention of the Sear battery terminals reminded me: I had to run to the hardware store to buy new hardware for that 5/16-inch threaded post when I changed to the Sear marine battery from the Interstate marine battery. So much for consistency among "marine" batteries.
posted 01-23-2014 03:02 AM ET (US)
I use the ring style in tinned copper. They work fine.
I use the Costco Kirkland marine batteries and they still tested good from 2006. I changed them just out of paranoia that they would fail in the middle of the ocean.
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