Wiring batteries in parallel vs. serial?
 

Ok, I've got two 12 volt 475 CCA batteries. Looking at the wiring, it's set up to be serialized. Do I lose anything by wiring them that way, or is it best to run them parallel and why?

If I need to run the latter set up, how do you suggest to wire? Thanks for the help.

Hi Brent

I'm not into electronics but I do know that if you hook two 12 volt batteries in series you will get 24 volts out put. ( not a healthy thing for all your electronics and wiring I would suspect). If they are hook in parallel, you will get 12 volt output and I think you will double the amps (but I not sure on the amp thing). Ask Petek he will know.

Paul

Paul, you're correct on the amps as well as the volts. Two in series gets you twice the voltage and two in parallel gets you twice the amount of time that a given current will be available.

David

Ok, so running them in parallel....Does that mean that the batteries will only give me a total of 475 CCA (that doesn't seem good as my old single battery in my last car had 850)? Electricity and me just don't mix well, so pardon my ignorance!!

As for the wiring. Is there a specific way to wire? I mean do I just splice the positive lines into one another and all is good or????

Paralell is:
Negative terminal to negative terminal.
Posit terminal to posit terminal.

So yes, "splice" the posits and negets together using jumpers or whatever good method you can.

Series is Posit to Neget criss cross. Get it right and you get 24 volts, get it wrong and you blow up the batteries and go "Oops".

I don't think "paralell" (still 12 volts) will increase or double the cold cranking amps output BUT it will sustain that output longer. Which is really what the CCA measurement is trying to tell you anyway. How LONG can I crank my engine before the battery goes dead sort of thing. That time is "doubled". Your current single battery CCA should be plenty to start even a mean motor!

OH!!!! Now I get it!!! Thank you Ernie!

I was thinking serial was positive to positive post and parallel I was picturing as the two positive posts running into a splice sort of thing, and I wasn't understanding how you would end up with double volts/amps one way and not the other. DUH!!!


Thank you!

brent,

I see you got it.
it's pretty much the same as if using your neighbor's car to jump start your car.
yes, the amps add in parallel.

although the saying is, if your battery is really dead you should connect ground from the neighbor to chassis or block.

dom, cape town

There are some really nice wiring kits out there just for this. I know Painless makes one and sure there are others. Look in a Summit catalog too. BTW, what are you trying to crank with 445CCA? a small lawnmower? Get you an Optima and be done with it.

If one battery delivers 445CCA then with the 2 batteries in parallel you will get 890 CCA at 12 Volts

With the 2 batteries in series you will get 445CCA at 24 Volts.

Voltage and current always work in a ratio.

Cheers

Brent,

All answers above are correct. Just putting the batteries in paralle doubles the capacity, and you see this on many heavy duty pickups with campers. As said above, putting them in series just keeps upping the voltage. Three in series would give you 36 volts and no gain in capacity. I use one battery with 750 CCA and have had it since 96.So unless you plan to run 12 or above compression, I really don't think you need all that capacity to start the motor.

Ron :)

Forgive me for wandering a little off topic here. ... During my very poor college days, I owned a 1951 GMC panel truck with the original 6 volt electrical system. As the engine became more worn out, I found it wouldn't start with the RPM the 6 volt starter provided. The cheapest temporary solution was to buy a second 6 volt battery. I wired it so I could change from parallel to series by flipping a gate switch. To start the engine, I'd put the switch in series and run 12 volts to the starter. As soon as it started, I'd flip the swith back to parallel so the 6 volt generator could recharge both batteries. ... That starter really spun up fast with double the normal voltage.

Seen this done on boats (power & sail) so probably you run into the same problem on cars, - shortened life of both batteries unless you add an "isolation" switching circuit. Try "Boatowner's Mechanical and Electrical Manual" (Nigel Calder). It's in most libraries and goes thru matching the batteries (parallel vs. serial) with the rest of your needs and equipment as well as the amps vs. volts issues. However marine batteries are expensive and in awful locations so replacing a set of car batteries frequently may not be an issue.

Batteries in Series

Batteries in Parallel

Prof. Ron :cool:

Good Basic Electricity reference...

Tommy, "back in the day" a common solution to the 6 volt problem was an 8 volt battery and an adjustment to the mechanical voltage regulator to support it. I haven't seen any 8 volt batts for sale for years and years now.

Good idea using a switch to make 6 into 12 just for cranking.

I aint got all that fancy learnin' like Prof. Ron got. :D

Backyard Ernie

Professor Ron, thanks for the pretty pictures... :)

The Kirkham comes with the appropriate wiring for this, I just needed to figure out where each wire went before I tried. Good thing I asked first! :D

Galaxy, the batteries fit behind the passenger seat and are much smaller than the optima dimensions, hence the smaller output of each.

I second the isolation switch , the one battery will try to "top off" the other battery because of the small differance in voltage between the two .(example 13.5 volt 13.8 volt ) this will cause one to always pull .3 volts .

I would think the chemistry of the batteries would normalize after a very short period of time so that you could have the two batteries wired in parallel without a problem.

One battery "topping off" the other would only occur for a short period of time (until the voltages equalized).

If one of the batteries was dead, then you'd have a problem. But if both batteries were in good condition, I don't think there would be a problem.

I used to work a lot with DC power systems with large banks of batteries in series and in parrallel.

What Pete says is right, the batteries will ballance themselves out over time. It's only a problem if one cell fails and the voltage of one battery drops in voltage it will tend to drag on the others.

Like any battery make sure you check the electrolyte levels regularly and keep them topped up.

Cheers

Just a explanatory note on "12 volt" batteries.

There is really no such thing as a 12 volt battery!

A battery is basically an electrochemical reaction between 2 different metals that form the + and - terminals and the electrolyte that flows between and separates them. In a car battery the electrolyte is a weak acid (no so weak if you spill it on your paint job). In a sense the electrolyte actually forms and contains the charge in the battery.

Is is a fact that the electrical reaction between the + and - terminals of ANY battery using the common materials used today is between 1.2 volts and 1.7 volts. This is true in nicads, lithium ion batteries, flashlight batteries and even the common garden car battery.

Voltages higher than 1.5 volts in a "battery" are attained by hooking up several batteries in series internally in the battery case to obtain the desired resulting voltage.

Remember the old 6 and 12 volt car batteries that were sealed in tar and had large straps running on top of the battery from cell to cell? And the separate filler caps on each cell? Each cell was an individual battery hooked in series giving a nominal total of 6 or 12 volts depending on how many cells were used.

What does all this have to do with the original question? IMO it doesn't matter if you use one large 12 volt "battery", 2 small ones in parallel or 2 large 6 volt batteries (if you can find them) in series. All you are doing is duplicating externally what the battery manufacturer has done internally inside a single battery. :) :)

1.2 to 1.7 must be theoretical. Fact is a normal 12 volt battery runs better than 12 volts, pushing 13 often. The indivdual cells in the 6's and 12 volts are 2 volts or higher that being the case.

The 8 volt battery I referred o earlier did indeed have four cells. The new optima I have, spiral cell technology, has in fact six cells. You can see them, you can count them!