Battery Tender on Dual Battery Setup

[G69]

I would always replace 911 batteries in pairs.

[Haasman]

Interesting thread ... OK, I went to the the Battery Tender site and found this http://www.batterytender.com/connect...el-one-charger


With no electrical prowess, I have to say I find this quite interesting. The current crossing across both batteries seems to make sense. But that Battery Tender suggests this orientation relates correctly balances recharging through both batteries.

Additionally I offer their reminder: "Note the blue wire designated W1 in the diagram. The purpose of this wire is to balance the voltage drop evenly across both batteries and each wire during charging. This is not critical for lower current chargers, but when you start to get into the 10 amp and above range, the voltage differential can be significant. The blue wire W1 must be connected to the opposite end of the battery pack as the black wire at the top of the battery pack".

Seems a bit complicated .... I am fine with 2 sealed batteries and the battery tender connected to the driver's side battery.

[Steve Meltzer]

I want to slightly hijack this thread, but first, is there any harm in using a trickle charger on each battery? Are the cheap Harbor Fright units OK?

I just replaced the dual batteries on my '69E. Got the Mazda Miata units from The Battery Mart. They are about two months old and won't hold a charge for more than a few days. I have several questions here: 1) If one battery is moderately discharged, will the car fail to start? I would think so. B) The tech at the vendor said that if the battery is on the charger (Schumacher ?? Spelling) and it shows the green light of a fully charged unit, a reading of 12.6 is not good enough, and that it should be 13 or 13.2. He questioned the condition of my charger, tho' it's a good one and seems to have always worked fine, as far as I can tell. iii) what is the best battery for these dual battery 911 cars? thanx steve

[melanmack]

Usually, people need two batteries on big cars. For example, I have two batteries for my GMC Van. One is for the engine and the electric system, and one is for the TV for the people in the back seats. It was hard for me to find the right Batteries for my car. Luckily I found all the information I needed on cararac.com. I manually installed everything and even connected a PlayStation 4 to the TV, and I am pleased that everything is working alright. Now it is a good car for long trips with children and not only.

[72targa]

Why do you need a dual battery?

Welcome to the Early 911S Registry. As to your question, I always understood that this was for weight balance on these early cars. Others may chime in.

[Theunz]

The weight balance theory doesn’t hold water in my opinion. With even a 100 lb driver (and most likely 150lb+) the drivers side would out weight the passenger side by about 60lbs. Sure, with 2 of the same sex in the car the dual batteries might help the balance. So, perhaps 5% of the time it’s balanced. The rest of the time you are more often driving by yourself or with someone who weighs significantly less you. To me it would make more sense to locate 1 battery on the passenger side.
As for melanmack’s question, perhaps he is unaware that some of the early cars came from the factory with dual batteries.

[frederik]

It seems to me that the "trick" to attach the charger to the positive terminal on one battery and the negative on the other battery only makes a difference when the charging current is high enough to make a significant voltage drop over the cables that connect the two batteries. That will never be the case with a trickle charger, so it really shouldn't matter how you hook it up.

[Bahman]

I solved this much-debated problem by fully disconnecting both batteries from the car and using two CTECKs to charge each battery separately. May take few minutes more to set up, but not a problem for me, especially for prolonged storage periods. Chargers/maintainers less expensive than CETEK will also work.

[Arne]

The weight balance theory doesn’t hold water in my opinion.

My understanding is that it's not balance, but where the weight is located. SWB cars got a pair of cast iron weights added at the corners of the front bumper for high speed stability. LWB cars went to a pair of batteries in much the same locations rather than the crude weights.

[Steve Meltzer]

I'm sorry that I have not responded earlier but I've been ill. Finally tracked down the poor battery charging which was secondary to a parasitic current draw. Ultimately, I put battery tenders on each of the two batteries and I've had no trouble since that time. Thank you very much all the great information I learned a lot and I appreciate the time it took for you all to write. Steve