Dual Battery Options

[justacoupe]

In a Pelican posting, a Rayovac battery was mentioned as one option for dual battery cars (sold at Batteries Plus). That battery has been replaced at Batteries Plus by a model branded as a Duracell. It is the same battery (model number SLI70). Mine cost $71.99 each (after credit of $9 for each core). Including two pairs of charging posts and sales tax it was just over $170 out the door for the pair.


The side tabs need to be trimmed off (utility knife works). I also had to trim a bit off the bottom edges on each side due to battery box's radius'd edges on the bottom.













Here's how it looks installed on the passenger side in our '73E (original owners... 66,000 miles). I'll be adding protective covers to the positive side terminals:




Compare that $170 total to what Stoddard wants for their "original style" dual batteries (their part number 999-611-036-20). Those run $ 169.86 each but no sales tax = $339.72. Add estimated shipping of $25 and battery acid (shipped dry) at $16.00 for a total of about $381. Link: http://www.stoddard.com/999-611-036-20.html

Or, the Sonnenschein option: http://www.batterysales.com/downloads/A512-40.0-G6.pdf Their model number is DF75, and the cost is $195.95 each plus shipping so about $425. Plus, terminals or cable connectors need to be converted so maybe a total of $440.

Regards,

Bill
Grand Rapids, MI

[super9064]

Nice option, I like that the posts are on the side. Thanks for posting.

[Mocker]

Just to update and reinforce what Bill wrote: The model number for this battery is SLI-70 ("I", not the number "1"). My local Batteries Plus had it in stock. My plan is to go with the single battery, keeping it on a tender.

The other thing to reiterate is that it's a very tight fit! Very! I spent a few hours first with a file, and finally settled on a Dremel, to remove the bottom platform off both ends of the battery in order to clear the radiused (not square) corners of the box. Still took some hammering and persuading on the corners of the box itself. Once in, it looks like it really belongs!

[mohrgan]

Another option is this AGM battery for a Mazda Miata. I have two of these in my car and they need to be spaced out a couple of inches with a piece of 2 x 6. These are AGM batteries so the likelihood of them leaking is very small.

http://www.batterymart.com/p-12v-maz...a-battery.html

[speedo]

Great timing as I was just about to post the same question, although I am deleting the passenger side battery. Are the Duracell and AGM powerful enough to only use one battery? Are there any optimus or oddessy alternatives? I am +++ on the front facing terminals.

Speedo

[304065]

I used to use a Hawker PC680. You delete the starboard battery by removing the cable from the port clamp.

It doesn't look like it but it would start my 2,2 race car from stone cold in the mornings at the track. The power density is amazing. The aviation approved version (3x the price) would happily swing the 74" prop on my Citabria under the same conditions.

Once you deviate from concours, might as well go with a nice light solution. It doesn't help the polar moment of inertia however. . . .

[pcivalero]

Mocker - agreed VERY tight fit!! Concerned - how the heck do we get the damn thing out in a few years?? Kinda serious about that question - I had to jam it in pretty good - though it does fit.

[Zedone]

Has any one tried the AGM battery from Power Sonic? Almost same size as the Interstate 911 (L 8.19 in, W 6.875 in, H 6.88 in) and Sonnenschein A512/40A (L 8.26 in, W 6.9 in, H 6.9 in) and seems to be at a very reasonable price.

Here- length 7.76 in, W 6.5 in, H 6.69 in

http://www.techbatterysolutions.com/...7827_a_7c02348

[merbesfield]

I have used several times now the AGM battery sold for Mazda Miata's with much success. Easy to find and not too expensive. Just bought one from O'Reilly Auto Part last week. $110.00 Part no. MIATA

[G69]

Whenever two batteries are connected in parallel like the two 12v batteries in longhood 911s the battery with the higher state of charge will discharge into it's weaker sister. This goes on until the batteries are charged or destroyed. It was not a problem for daily drivers. Like the oil pooling up past the oil pump and leaking through the rocker shafts onto the garage floor, this problem is a result of our cars being "classics" and not driven daily. Two 6 v batteries in series will not have this problem. Some modern batteries utilize gel cells. The gel cell is less likely to leak because it is sealed. It DOES have an emergency vent. When charged at too high a voltage the gel battery will vent the electrolyte and it's electrolyte is not field replaceable. Gell cells must not be charged above 14.4 to 14.7 volts for a 12-volt battery. You will find the stock alternator regulator on many 911s will not hold this tolerance. 15 and 15.5 volts are a "normal" charge rate for old-school relay type alternator voltage regulators and will cook a "sealed" gel cell. Absorbed Glass Mat batteries and Gel Cell batteries are great to precent corrosion in the front of out cars. But- be aware of the strict charging regimens required by these new designs. Once the "vent" is compromised the battery is junk.

Disconnecting one ground strap from one battery when the car is left for over a few days will prevent this "race to the bottom". A smart charger like the "Battery Tender" or Harbor Freight trickle charger will keep the old-school lead acid batteries ready for action. I use one battery shoved sideways into the driver's side (will not fit against spare tire on passenger side) and it works perfectly. It is NOT for those who nit-pick tool kit screwdrivers or hood badges...

Modern lead acid batteries starting in the Seventies (lead acid is OLD tech) use much thinner plastic cases compared to the rubber cases in old days. Therefore bigger lead plates can fit into the same package. I believe a couple of modern lead acid riding mower or motorcycle batteries would have enough current to start a 2.4 liter engine in moderate temperatures. That is the answer I'm looking into.

The motorcycle batteries also have a nice acid overflow tube to drain any acid overboard or into a small catch can. Those two-wheelers do not like sulfuric acid spilling on the back tire at speed...