battery questions

[vtrich]

Hi,
I'm a new member but have spent lots and lots of time scouring posts. I'm the proud new owner
of a 1970 911T Targa. This forum has been a huge help in either validating what I thought I knew
or pointed me in the right place to get the info. I needed. THANKS to all of you!

So, I'm considering going from dual batteries to single. As I said, I've read many many posts related
to this topic, but I still have lots of questions. Primarily, my motivation is to be able to maintain the
battery while in the car. I have an '88 911, single battery, single Battery Tender through the winter
works great and is easy. If I retain the dual batteries, do you need to take them out of the car and
connect them to separate battery maintainers?....or, since they are connected to each other, can
I connect a single battery maintainer to the driver side battery and would it maintain both batteries
safely?

Ok,...so,...assuming that I can't do that and I decide to convert to single battery, is there a single
battery that will fit in one of the existing battery boxes (not smugglers, not interested in that) that
would replace the twin batteries. Not too psyched to install on it's side.

THANKS for any observations!!

Rich Pearce

[beh911]

If I retain the dual batteries, do you need to take them out of the car and
connect them to separate battery maintainers?....or, since they are connected to each other, can
I connect a single battery maintainer to the driver side battery and would it maintain both batteries
safely?

Rich, the latter. One battery tender to one battery will charge the dual system

[M_deJong]

While I agree that one tender would work, I think two tenders with the batteries disconnected from each other would work better as the tender would be adjusting itself to individual battery condition.

I used to do the one tender approach but it may have contributed to the premature death of one of my Sonnenschein batteries.

I believe Ed Mayo told me that you can install one battery in the left box with 912 straps which are longer and made to fit a single battery. Don't forget to remove or secure the right side cables if you do this.

I'm of the opinion that the car handles better with both batteries, so if you go to a single you might want to ballast the other side to compensate. Others will say I'm nuts.

[vtrich]

Thanks for the responses!

I guess for now, since both batteries appear to be functional, I'll try one tender for both batteries and
see how I make out. If, like Mikes Sonnenschein experience, it leads to early death,...I'll come back for more
advice.

In reading through other threads related to batteries, some have said the Sonnenschein's life expectency is pretty short,
it could also be a contributor to the early demise of yours.

Again, thanks for responding.

Rich

[304065]

The Sonnenschein is an AGM battery ideally intended to be charged with an intelligent charger, not a tender. The charging voltage and current may exceed that of a typical Porsche installation e.g. wheezy little 35 or 55A bosch alternator with a 14V set-point and a long copper cable running forward to an unheated compartment with plenty of high-resistance connections and oxidations thrown in over 40 years. If you do put one in your car, make sure to get the DEKA charger or equivalent.

Having used one Sonnenschein in my SWB, I went with a conventional lead-acid at 1/3 the price and 2/3 the weight and charge it with a Bosch charger. All connections are bright metal, alternator rebuilt, solid state VR, good ground straps etc. Works great.

Two batteries in the LWB cars are for handling: they serve no electrical purpose.

[vtrich]

Thanks 304065. To be clear, I currently have a pair of Interstate conventional lead acid batteries. I was looking at simplifying the setup. I'm sensitive to the
weight thing, but I can adjust that pretty easily using some form of ballast.

I've read a bunch of threads and it seems that your Sonnenshein will fit in my existing battery compartment in it's upright position,..is that correct? If I go that
route I would remove the secondary positive and negative cables, and would buy a charger designed for that purpose.

THANKS!

Rich

[M_deJong]

There is a little electrical to the second battery... one small battery doesn't have the same amp-hour or cold cranking capacity as two small or one big battery. That said, you could probably get by with one battery box sized Sonnenschein or other AGM but it might be a little weak at peak demand. I ran on one Son for quite a while after the first one died with no ill effects.

Unfilled Intersates make a convenient size ballast, without that annoying acid to leak out.

[304065]

There is a little electrical to the second battery... one small battery doesn't have the same amp-hour or cold cranking capacity as two small or one big battery. That said, you could probably get by with one battery box sized Sonnenschein or other AGM but it might be a little weak at peak demand. I ran on one Son for quite a while after the first one died with no ill effects.

Unfilled Intersates make a convenient size ballast, without that annoying acid to leak out.

Well sure, but the engine doesn't need that CCA to turn over. The stock batteries are 36 Ah each, 300CCA (specs from Interstate U911). For years I have been using a single Hawker PC680 which has 16 Ah, 220CCA, works great, in my 9,1:1 911E race car, which ONLY gets cranked over on cold mornings with no MFI cold-start system.

And don't get me started on the battery in my old '88. Maybe if one needed to jump-start a M-B diesel truck or start-cart a Dornier Do328, I can see it. Talk about a boat anchor.

To the OP-- the LWB cars have a double battery clamp. Undo the clamp, remove the cable that goes to the other side. Sell the cable to an "audiophile" as an "oxygen-free German-made interconnect" for $725. Mine is in the barn: the mice think it's a snake, which slows them down only slightly on the way to make a nest in my orange foam heater tubes.

[HarryD]

I have a single optima on it's side in place of the dual batteries. I did not notice any handling changes. The reason for the single Optima was cost and the fact I was tired of cleaning up the coorsion from the fumes of the Interstates I had originally.

[Gumby]

I just had a battery fail in a 2 battery set up,( using 1 charger ) I highly recommend using 2 chargers, just disconnect 1 ground strap