Dual Battery Questions - Help!!

jwernquist · 04-26-2008, 08:59 AM

Hey guys it's been way to long, My 73 was converted to a single battery many many years ago.

My new 72 has the connections for dual batteries still.

1: Do you need 2 batteries in order for the car to start? I thought so I just can't remember. One battery just fires up all the ignition etc, and the 2nd battery works the starter correct? So if you only have 1 it will not start?

2: I cannot find any replacement batteries locally, except 1 that might fit.

Any advice would be greatly appreciated. My memory is failing

JW

Bench-Racing · 04-26-2008, 09:12 AM

The two batteries work together, not on separate circuits. Just be sure to tape off the positive terminal of the second battery if you use only one and leave the wiring for the second battery in place.

Gogar · 04-26-2008, 11:54 AM

+1 you don't need 2. Undo the passenger one if you like and make sure you have the positive end safely insulated or taped up or something.

A Mazda Miata battery will fit in the battery boxes, but it's a little shallow. You can put a hunk of 2x6 behind a Miata battery and use your stock battery hold down hardware.

Of course, a 911 is a harder 'start' than a Miata, so you might be stressing the battery/alt a little bit more than the stock setup.

fastfredracing · 04-26-2008, 12:11 PM

One of my customers has been using tractor batteries for the last 3- years( he owns a lawn equipment company) , but I think that he just found optima's that will fit. . Hope that helps

FPB111 · 04-26-2008, 06:31 PM

I have had dual SEALED Miata batteries in my 1972 3.2S for 4 years with no problems.

FenderBender · FenderBender's Garage

Odyssey pc1200. Plenty of cranking power, completely sealed, can be mounted sideways also. Costs approx. as much as 2 miata battery's and is much better in my opinion. (Battery's plus)

Disconnect the second positive cable from the drivers side positive clamp. No worries about tape wearing through. I left mine lay in the trunk.

fram914 · 04-27-2008, 09:33 AM

Interstate makes a replacement for the OEM dual battery

Shoepop · 04-27-2008, 09:54 AM

Interstate part #?

fram914 · 04-29-2008, 07:52 AM

Interstate number U911-50

pwd72s · 04-29-2008, 09:06 AM

Also, Sonnenschein, a german battery maker, produces gel cell totally sealed "exact fit" batteries for the dual battery early 911...available thru Ft. Lauderdale battery...sorry, lost their web address.

kenikh · 04-29-2008, 09:47 AM

There's a lot of info if you Search, but I use the Sonnenscheins and really like them.

HarryD · 04-29-2008, 09:26 PM

Quote:

Originally Posted by HarryD

The battery is not a "standard size".

Per Ed Mayo (http://www.early911sregistry.org/forum/showthread.php?t=16781&highlight=battery - August 2007): "The source for the Sonnenschein batteries is www.batterysales.com You want the DF-75 w/SAE posts, and the cost is $129.00 I have three 911's with two of those in each car, best money I've spent."

A search will turn up a wealth of information.

BBShriver · 07:22 AM

oops, replied in wrong thread!

JerryL · 03-19-2010, 09:40 AM

As above, any 1 of several batt's will work BUT would like to add a word of caution on that additional hot lead.
Tape may work for a while is very dangerous. The end of the 2nd cable ties into the +clamp on the drivers side, REMOVE IT from the batt post clamp do not leave it in the system.
And if you want a clean look it is lead arounf the gas tank area inder the lip of trunk, VERY EASY to remove and take the other ground cable off the stud with the butterfly nut and clean up the trunk and it wont look like a kludge, as well as being SAFER.
That is what I did with my 71.

Regards

chiroracer · 03-25-2011, 10:08 AM

I am converting my 71T back to dual batteries and would like a pic of the correct connectors and diagram to be sure I do it correctly .I think(hope ) I saved all the parts .Thx in advance .Steve

Gogar · 03-25-2011, 10:21 AM

I'm with Jerry,

Remove the Pass. side battery completely, and undo the thick positive crossover cable from the terminal post clamp on the Driver's side completely, tape both ends just because, even though you don't have to.

cmcfaul · cmcfaul's Garage

buy two batteries, it's the way your car was designed.

Chris

73 911 E

abisel · 03-25-2011, 12:41 PM

Quote:

Originally Posted by cmcfaul

buy two batteries, it's the way your car was designed.

Chris

73 911 E

What Chris said.
For the 69 to 73 cars, they designed it for two batteries for corner balance reasons. Stick with the dual battery setup and use Sonnenschein.
A 500
get two of the 40 amp batteries in parallel for a total of 80 amps.

69911e · 03-25-2011, 01:19 PM

I have run 1 Interstate for quite a while (7years)in my '69 with a 2.7L. I expect I will need to replace it soon.
Only downside I see is it cranks too slow when below 25deg, but I don't drive it in winter due to salt.

E Sully · E Sully's Garage

Here is a good thread. I personaly use the 2 Westco Miata batteries. They have been working great
Early 911 Battery Options