Moderated Discussion Areas
  ContinuousWave: Small Boat Electrical
  Connecting Cables To The Battery

Post New Topic  Post Reply
search | FAQ | profile | register | author help

Author Topic:   Connecting Cables To The Battery
Tom W Clark posted 01-19-2014 03:37 PM ET (US)   Profile for Tom W Clark   Send Email to Tom W Clark  
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?

jimh posted 01-19-2014 04:07 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

jimh posted 01-19-2014 04:31 PM ET (US)     Profile for jimh  Send Email to jimh     
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

tmann45 posted 01-19-2014 09:26 PM ET (US)     Profile for tmann45  Send Email to tmann45     
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]

swist posted 01-20-2014 08:21 AM ET (US)     Profile for swist  Send Email to swist     
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.

jimh posted 01-20-2014 07:01 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

jimh posted 01-20-2014 08:35 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

jimh posted 01-20-2014 08:38 PM ET (US)     Profile for jimh  Send Email to jimh     
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.)
jimh posted 01-21-2014 10:04 AM ET (US)     Profile for jimh  Send Email to jimh     
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.

kwik_wurk posted 01-21-2014 12:32 PM ET (US)     Profile for kwik_wurk  Send Email to kwik_wurk     
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)     Profile for Tom W Clark  Send Email to Tom W Clark     
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 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?

russellbailey posted 01-22-2014 10:37 AM ET (US)     Profile for russellbailey  Send Email to russellbailey     
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) He discusses the heavy-wall versus standard wall in this instructional.

jimh posted 01-22-2014 11:43 AM ET (US)     Profile for jimh  Send Email to jimh     
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.
17 bodega posted 01-23-2014 03:02 AM ET (US)     Profile for 17 bodega  Send Email to 17 bodega     
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.


Post New Topic  Post Reply
Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.