What's Better, Two 12V Batteries In Parallel, Or Two 6V Batteries In Series

Joined
Sep 18, 2005
Messages
2,415
Location
Willamette Valley-Oregon
While reading the current thread "HJ60 starter system issue" I started thinking about dual batteries in a 12 volt system. This is something I've wondered about before, but never could get any definitive answers for. My question is this: which can be stronger/better, dual 12V batteries wired up in parallel to stay as a 12V system? Or dual 6V batteries wired up in series to make for a 12V system? (I think I've got those two terms correct) I realize that the outcome can vary, depending on what batteries you start with. My big question is which set up will be better for the long life of the batteries, and work better for the starting system.

My application is not Toyota, but I think this information could benefit many Landcruiser owners. I have a 1994 Chevy K2500 with the 6.5 diesel motor. Specs call for two group 78 batteries (10.3" long X 7.1" wide X 7.3" high) of at least 770 CCA capacity. The 6.5 is a power hog when it comes to starting, and this may be true for Toyota diesels, although I think most are single battery for the 12V systems (have never owned a Toyota diesel). First, the glow plugs take a big draw (comparable to a starter), then once the glows are done you use the starter. The two together make for a huge draw on a good pair of batteries, and if your batteries are old and marginal, you may have trouble getting the motor to crank over fast enough for a diesel to start easily.

I currently have two Interstate Megatron group 78 800 CCA side post batteries (one also has top posts for easy hookup for jumpers or winch) which are about 4 years old. At this age they are probably close to the end of their life for this motor, and I want to be prepared with good information when I need to replace them. I also replaced the battery cables with larger size welding cables for better currant capacity. I do realize that welding cables are not as good as automotive cables because of oil resistance for the insulation, but they do have some real good currant carrying capacity.

I have rebuilt one battery tray to be able to hold a larger battery (Ford, group 65, 12.1" long X 7.5" wide X 7.6" high), and although not done yet I have most of the parts to do the other battery tray.

The two group 78 batteries i have add up to 1600 CCA, when new. Two of the better group 65 batteries can add up to 1700 to 1900 CCA. But, these are some real expensive batteries, up to $335 each. For that kind of money can I get two 6V heavy duty (industrial?) batteries that will give me more/better/stronger life than two 12V batteries? Or, am I better off to stay with 12V batteries?

Opinions and speculation are fine, but real life experiences will be much better.

Thank you, Don
 
Joined
Sep 27, 2008
Messages
2,942
Location
El Segundo or Gardnerville!!!
Typically, pairs of 6v battery's are used where loads are constant, such as motor homes and solar power storage. They are not used as starting battery's.

The main advantage of a 6v in series is the battery's design and construction. The plates are much thicker, so they withstand hundreds, or more, deep discharges that would kill a regular 12v starting or dual purpose battery.

I run 8 6v battery's in a 12v system charged by a 600w solar system in our motor home. With good maintenance, a good charger, and proper use, our battery power will last over a week of dry camping with lights, stereo and TV. I have a similarly sized system in our boat also.

When sizing 6v's, if the battery's are 220ah, when 2 are placed in series to make 12v, the amp hours are only 220, but if you install 2 12v's in a 12v system, you will get the combined rating of both batteries,since you started with 12v. 6v's are rated by amp hours, as they are storage batteries, where most 12v's are rated in Cold Cranking Amps, so it gets confusing.

There are many links about using 6v's, google it. Many good articles about using 6n's in Solar, boating and RV forums.

Hope this helps a little.
 

78fj40mg

TLCA #23510
Joined
Sep 13, 2008
Messages
4,072
Location
PNW, WA.
Website
forum.ih8mud.com
I'd recommend two 12 volt batteries wired up in parallel for the following reasons:
a) If you use two 6 volt batteries and one goes weak on you then you'll be bellow 12 volts,
b) conversely with two 12 volt batteries if one goes weak you still have a 12 volt back up.
 
Joined
Jul 22, 2003
Messages
19,005
Location
Perth Western Australia
I think the best way is to have 2 12v batteries,as big as you can afford connected by battery switches in parallel. Use one for starting. If it gets weak,you still have a good fully charged battery you can fall back on.
If you are using both all the time and they get run down ,you have no spare
I had a solenoid switch on mine so both were getting charged once the engine started.
I switched the batteries around every month or so.
I used a couple of 700cca Chinese batteries,only $150 each but I got 4 years from one and five from the other.
 
Joined
Sep 18, 2005
Messages
2,415
Location
Willamette Valley-Oregon
Typically, pairs of 6v battery's are used where loads are constant, such as motor homes and solar power storage. They are not used as starting battery's.

The main advantage of a 6v in series is the battery's design and construction. The plates are much thicker, so they withstand hundreds, or more, deep discharges that would kill a regular 12v starting or dual purpose battery.

I run 8 6v battery's in a 12v system charged by a 600w solar system in our motor home. With good maintenance, a good charger, and proper use, our battery power will last over a week of dry camping with lights, stereo and TV. I have a similarly sized system in our boat also.

When sizing 6v's, if the battery's are 220ah, when 2 are placed in series to make 12v, the amp hours are only 220, but if you install 2 12v's in a 12v system, you will get the combined rating of both batteries,since you started with 12v. 6v's are rated by amp hours, as they are storage batteries, where most 12v's are rated in Cold Cranking Amps, so it gets confusing.

There are many links about using 6v's, google it. Many good articles about using 6n's in Solar, boating and RV forums.

Hope this helps a little.

What you are saying here definitely makes a lot of sense to me Doug. Thank you!

Mike and Rosco, some good advice that you give. You bring some real good food for thought. My pickup is a real battery hog, so I think it needs two 12V batteries in parallel just to have enough CCA to start the motor. Open real estate under the hood of the pickup is hard to find, but I think I have room for a third battery. I'll have to do some careful thinking/searching to see if wiring up three in parallel for even more CCA is expedient and cost effective.

Thank you all for your info.

Don
 
Joined
Nov 21, 2011
Messages
263
a lead acid cell is 2 volts, a battery is generally 2 or more cells connected in series or parallel to build voltage or capacity, however there are single cell batteries; usually a single cell with protection components to make it safe for the end user.

Cells in parallel also increase the maximum deliverable current as long as the connecting components are correctly sized. bad/loose or high resistance materials used in connections and too small connecting components add resistance to the system. Resistance in the system wastes energy and that waste energy creates heat.

A 12v lead acid battery is literally 6 lead acid cells in series.

A 6 volt lead acid battery is 3 lead acid cells in series
 

Users who are viewing this thread

Top Bottom