If I have two batteries and charge one, does the other get charged?
 

If I have two batteries and charge one, does the other get charged?

I could be wrong, and will soon be corrected, but my understanding is that the two batteries are wired in parallel (pos. to pos., neg. to neg.) and act bascially as one battery. Thus, charging one battery is really applying the charge to both at the same time.

Yes.

If a tree falls in the forest and no one hears it did it make a sound?









Yes.

If you leave the 'w' out of the word 'two' because you're typing quickly and trying to get your kid to school on time do Pelians stil no whut u meen?

We speek Pelican here. :)

If the batteries are in SERIES and not parallel - ie. adding up to 24 volts and not 12 - they will usualy not charge equally unless you use a charger designed to equalize the batteries.

Parallel acts as one battery, just bigger, as posted above.

-kevin

Are you using (2) 6 volt batteries? In series?
or (2) 12 volt batteries in Parallel, Either way both are charging.
Bob

If you are talking about using a Regular Battery charger, not the cars charging system and you have (2) six volt batteries in series, you must hook the charger up in series also. So clip the positive lead to the Plus lead on the battery that has the main positive cable on it and clip the battery chargers negative lead to the negative on the other batterys negative terminal (The one that is grounded) This will send the currtent through both acting as if it is a single 12 battery.
Bob

'72 911 originally had t(w)o 6V batteries in series.

The car has two 12V batteries.

Lee please make sure those are in Parallel, That means the the Positive terminals are connected to one another and the nevatives are connected to one another or both grounded. If they are in series using the cars original wiring, you are blasting the ignition starter and your whole electrical system with 24+ Volts actually closer to 30 volts rather than the 12-15 volts it was designed for.
Each cell of a battery puts out roughly1.25 volts so in actuality a really healthy 12v battery puts out 15V. in series 2 12v batterys would be double that....
The wiring could overheat and cause a fire and your starter, radio, bulbs and other electrical parts will fail prematurely. Please check wiring
Bob

Another issue with using an inexpensive AC (Wall) powered battery charger with 2 batteries in parralel is that the circutry that prevents overcharging is confused and can cook the batterys or do an inadquate job charging them. Often one gets cooked and the other never gets fully charged. I have had this problem in my sailboat. I ended up buying a multi bank marine battery charger, some battery isolators and some additional make before break (Perko) switches. Real pain, but I was destroying 4 pricey marine batterys each season.
bob

If the batteries are both 12V, they must be connected in parallel. If they were in series your car would be seeing 24V which would certainly do some damage. I would charge them separately so with one disconnected from the other. Otherwise, unless they are perfectly matched, they will not receive balanced charging current from a single charger. If it were me, I would then look into a single 12V battery with the proper rating.

Sorry friend, but my '73 (which is wired identically to the '72) has always had 12 v batteries. Look a the wiring diagram an it is clear.

The conversion to a single battery is problematic in an early 911. This is largely due to the lack of space for a standard battery unless you get an Optima and lay it on its side. The standard battery for a 911 is not a standard size like you will find at most stores.

If running the batteries in series and using a wall charger, its better to charge each battery individually. Try it. Charge two 6 volt batteries in series with a 12v charger and then measure the individual voltage after charging. I would be very surprised if they are not different.

Batteries, even though they may be identical models, sizes and ages, are almost never the same electrically. They differ in internal resistance due to minor manufacturing irregularities in plate size, electrolyte differences, etc.

If charged in series mode together, typically a weaker battery will charge less than a stronger one. This perpetuates itself as the slightly stronger one will get more so and the weaker one will get weaker. Often the low side battery - the one that has its negative terminal grounded - will become the weaker one while the high side will become over charged.

If charged in a vehicle, it is recommended to swap battery positions fairly regularly to reduce this potential imbalance. One should also replace batteries used in series together in matched sets. Though it is unlikely that one would ever be used, a six volt accessory (on a 12v series system) should NEVER be powered by a center tap on one battery unless a voltage balancing device able to cope with the current draw from the accessory is used. Even a very small draw will weaken that battery so that it will negatively affect the charging balance. The same applies to a 12 volt accessory on a 24 volt system.

-kevin

That's a fact. I went to three auto parts stores today. Only one had anything that would work and that was the Miata battery for $110. It had vents on it that would have been facing the interior of the battery box, so I passed. I ended up ordering one of these:
http://forums.pelicanparts.com/uploa...1218951142.jpg
https://www.motobatteries.com/p-185-ptmia.aspx
Based on the dimensions quoted it should fit as well as the battery that came out. The Miata battery I saw today was rated at 330 CCA where this one is 475. Also, the terminals were reversed. The one I bought came from Ebay so the delivered price was $86.

I had one of my batteries tested at Autozone and it was showing around 6 volts under load.

Sonnenschein makes exact fit 12V batteries to fit the dual battery system of the early 911.

Will one of those work or are two necessary?