6 volt battery question

DougD · #1 (**permalink**) 08-12-2010, 12:05 PM

I have two six volt Optima batteries in my Cobra. They're wired in series to produce 12 volts.

My question is this - I'd like to wire up a Battery Tender to keep these high-dollar batteries happy when the car's in storage. My Battery Tender is a 12 volt model. Can I hook it up to the two 6v batteries at once and charge them as if they were one 12v battery?

Skuzzy · #2 (**permalink**) 08-12-2010, 12:09 PM

As long as the batteries are in series you can use a 12V charger to charge them.

If you have any doubts about it, you can open up any 1000VA, or larger battery backup system and note the two batteries they use in series. 24V out, and 24V in using 2 12V batteries in series.

Ronbo · #3 (**permalink**) 08-12-2010, 12:34 PM

Each "cell" is 2v, so a 6 volt battery has 3 cells wired in series, a 12v has 6 cells wired in series. (essentially you have two "halves" of a 12v battery in seperate casings)

As long as you don't hook up across a single 6v battery it will work fine.

Don · #4 (**permalink**) 08-12-2010, 06:02 PM

Additional info:

http://batterytender.com/resources/c...s-chargers.htm

Excaliber · #5 (**permalink**) 08-12-2010, 06:19 PM

Virtually all battery "cells" are 2 volts. How many "cells" you have determines the end voltage (could be 6,12, 24, 36, 48). The difference with external and internal cells is that you can screw up more easily with external cell combinations (like two 6's for 12 volts). Putting a 12 volt charger on only HALF (6 volts) of the cells is bad news. But you could use a 6 volt charger on each half of the combination and that would be OK. Charging ONE of the two batteries at a time. I wouldn't recommend that though, use a 12 volt charger and get them both at the same time with a single charge.

twobjshelbys · #6 (**permalink**) 08-12-2010, 08:21 PM

Yes, just connect the + to the + of one battery and the - to the - of the other. After hearing horror stories of people jumping across one of the batteries I've considered covering the two inside terminals so that they can't be touched.

mickmate · #7 (**permalink**) 08-12-2010, 08:34 PM

I just read something in a car mag about the modern chargers not charging where they couldn't sense voltage on sealed batteries gone flat. The trick to that was to hook another battery in parallel for the first hour to get the charge to start. You're doing the best thing for them by trickle charging.

twobjshelbys · #8 (**permalink**) 08-12-2010, 08:43 PM

I also find that I don't have as much trouble with the batteries if I flick the kill switch when it's going to be off for a while. I drained them when it was in storage over the winter. In my case I think it was the DFI controller as everything else is so 60's technology that they don't draw any standby current. Possibly some portion of the MSD6 box too. The battery tender I have I got from my mechanic who got it from his Interstate Battery guy. It's the same exact unit with 20 different labels on it. He passed it through for no cost add. The margin/markup on these is very high.

Skuzzy · #9 (**permalink**) 08-13-2010, 05:58 AM

Quote:

Originally Posted by twobjshelbys

Yes, just connect the + to the + of one battery and the - to the - of the other. After hearing horror stories of people jumping across one of the batteries I've considered covering the two inside terminals so that they can't be touched.

No, no, no. That is a parallel connection. For two 6V batteries, you have to connect + (on battery 1) to - (on battery 2) then connect the charger to the - (on battery 1) and the + (on battery 2).

That is, if you are charging them with a 12V charger.

You do not want to charge a single 6V battery with a 12V charger.

Also note, a fully charged automotive battery contains 2.5V per cell. If you measure the voltage on a fully charged battery not connected to the electrical system, it should measure 15V.

Bob In Ct · #10 (**permalink**) 08-13-2010, 07:15 AM

I would expect the batteries would hold a charge for a long period of time if there was no draw on them. Do Cobra guys really need battery tenders?

I've had the same Interstate for 10 years now with long winters and no tender.

Bob

Excaliber · #11 (**permalink**) 08-13-2010, 07:21 AM

Well I don't know about 15 volts max, 13.6-14 maybe. At 15 I'd be wondering whats wrong with my regulator.

That's a valid point Bob, but a lot of cars do have a slight drain that over the winter could cause a low voltage condition.

Skuzzy · #12 (**permalink**) 08-13-2010, 12:26 PM

Excaliber, notice I said "fully charged" and "not connected to the electrical system".

Yes, if you were getting 15V out of the regulator, your regulator had stopped regulating.

It does not hurt to disconnect a battery for the winter. It usually is a good idea to trickle charge them once a month while they are not being used though. That is not to say the battery will die if it is not done. I am just saying it is a good idea. Definately will not hurt.

Then again, it also depends on what the battery design is. Some battery designs are more immune to breakdown.

twobjshelbys · #13 (**permalink**) 08-13-2010, 08:39 PM

Quote:

Originally Posted by Skuzzy

No, no, no. That is a parallel connection. For two 6V batteries, you have to connect + (on battery 1) to - (on battery 2) then connect the charger to the - (on battery 1) and the + (on battery 2).

That is, if you are charging them with a 12V charger.

You do not want to charge a single 6V battery with a 12V charger.

Also note, a fully charged automotive battery contains 2.5V per cell. If you measure the voltage on a fully charged battery not connected to the electrical system, it should measure 15V.

That's what I said. Battery pic:

+ (connect + tender or charger here)
(battery 1)
-
*
* (connect - of battery 1 to + of battery 2)
*
+
(battery2)
- (connect - tender or charger here)

(Sorry for the primitive graphics, but the forum software removes extra spaces so old-style VT220 graphics don't work)

twobjshelbys · #14 (**permalink**) 08-13-2010, 08:48 PM

Quote:

Originally Posted by Skuzzy

No, no, no. That is a parallel connection. For two 6V batteries, you have to connect + (on battery 1) to - (on battery 2) then connect the charger to the - (on battery 1) and the + (on battery 2).

That is, if you are charging them with a 12V charger.

You do not want to charge a single 6V battery with a 12V charger.

Also note, a fully charged automotive battery contains 2.5V per cell. If you measure the voltage on a fully charged battery not connected to the electrical system, it should measure 15V.

Oh, I see what you read. I saw it perfectly but stated it badly. I should have said "just connect the +[of the charger] to the + of the first battery and the -[of the charger] to the - of the second battery.

Furthermore, the +'s are to the primary hot lead and the -'s are to the frame ground lead since "battery 1" or "first battery" are also ambiguous.

English is such a crappy language for technical specifications.