Hi Doug and others,
1. I have a battery cutout switch on the floor under the driver’s seat in my ’34 convertible coupe (ok because it’s easy to lift the small seat); and on the short vertical riser board under the front of the front seat in my ’33 convertible sedan, nearly out of sight because the board is recessed.
2. Regarding batteries -The most important spec to me is Ampere-Hours (A-H). It is directly related to the size of the battery, and how LONG it will crank. (Almost any battery will start a car several times.) Example: a 50 A-H battery will supply 200 amps (typical starting current) for 15 minutes, 50 amps for one hour, or 1 amp for 50 hours. The more modern spec of “Reserve Capacity” (RC) is related to A-H by specifying the number of minutes a battery will last when supplying 25 amps. So a 50 A-H battery will have a RC of about 2 hours, or 120 minutes.
Single Optimas have a rating of 50 A-H, and a RC of 100 minutes (=42 A-H, due to different levels of discharge involved in the specs).
Double Optimas thus have a 100 A-H and 200 minute RC rating.
I use an Interstate group 4xhd commercial battery (about $140) in my V12, which has an RC rating of 295 minutes.
So, I can crank the engine about 3 times as long as a single Optima, and about 50% longer than a double Optima. If my generator stops charging while I’m driving on a mountain road at night (25-30 amps draw including brake lights) I have almost 5 hours, vs. 3.3 with double Optimas, or 1.6 hours with a single Optima. Of course, I have to check water level every 6 months. (I do use single Optima batteries on other cars because of their low current draw.)
The 4xhd A-H rating is not listed, but is estimated to be 150 A-H. The 1933 P-A salesmen’s “Green Book” lists the battery for the 1933 V12 as “…Willard 160-165 ampere-hour battery”.