Three (3) batteries?

[Jim K in PA]

I was planning to convert our Viking to two 6-volt GC batteries.  While investigating placement of the second battery (I have very little room where the current single 12v battery sits), I discovered that I can mount two batteries behind the propane tanks, and still have a battery in the original position in front of the propane tanks.

Has anyone tried running 3 GC batteries?  I can wire two batteries in parallel, then add the third in series to get my 12 volts.

     (+)---->(+)---->(-)

     (-)---->(-)---->(+)

If I do this, I might actually just go with three good quality 12v true DC batteries and just run them all in parallel.

Thanks for any input or advice.

[brainpause]

Good idea on paper. But there's something swirling around in my head about 2 parallel batteries and one in series, that doesn't sound right.

Somebody will talk more intelligently one way or another....

Larry

[AustinBoston]

Do not mix battery types or capacities.

The only way you could get 12V with three golf cart batteries would be if you left one disconnected.

If you connect two golf cart batteries in series, then put a 12V battery in parallel with them, then even a small charge difference would result in a HUGE amount of current flowing between the batteries in an attempt to equalize them.

You could wire them with a SPDT switch so that you are either using the two GC batteries OR the 12V battery, then just throw the switch as the GC batteries discharge.

I wouldn't put 3 12V batteries (or 2 12V batteries) in parallel unless I knew they were from the same manufacturing lot.

Austin

[Jim K in PA]

Thanks for the input guys.  Perhaps my post was not completely clear, but my intention would be to run 3-6 volt GC batteries of the same age, mfr. and lot.

Alternatively, I would run 3-12 volt batteries also of the same age, source and condition.

Many people run pairs of 6-volt GC batteries to gain a much greater AH reserve than a pair of 12-volt deep cycle group 27 batteries.  I just also wonder about the combination I have in mind.  Like you Larry, I wonder if I am missing something.

Some more googling is in order . . .  

[AustinBoston]

Thanks for the input guys.  Perhaps my post was not completely clear, but my intention would be to run 3-6 volt GC batteries of the same age, mfr. and lot.

There is no way to arrange three 6-volt golf cart batteries to give you 12 volts.  Everything in your camper runs off 12 volts.

You can use 2 GC batteries to give 12V because two 6V batteries in series add up to 12V.  But how do you hook the third battery?

If you put it in series with the other two, that comes to 18 volts, and will damage things in the camper.

If you put it in parallel with both of the other two, it would look like a dead battery to them and would draw so much current that a fire or even explosion could result.

If you put two of them in parallel, then the third one in series with that, you would get 12V, but as soon as the single battery (the one not in parallel) was discharged, the other two would try to push it into negative charge and destroy it.

Austin

[LimeJeeeep]

i concer with the folks who are discouraging you...i was going to remove the 1 battery from the front and remote them behide the axle and install a vented door.this would have given me a several things .(1)room for a second LP tank up front.(2) more 12volt capacity(3)and the possiblity for a 1or2k inverter 110volt.and to top it off a small generator to charge them during the day

[Jim K in PA]

If you put two of them in parallel, then the third one in series with that, you would get 12V, but as soon as the single battery (the one not in parallel) was discharged, the other two would try to push it into negative charge and destroy it.
Austin

Austin - what you stated above is what I was intending to do and I tried to draw the circuit (crudely) in my post.  What I was not grasping is why the battery in series would discharge at a different rate.  I think I understand now.  Bear with me and see if I get this right:

(Assume we are working with 6 volt 100AH GC batteries for mathematical simplicity)

One 6V/100H in parallel with another 6V/100AH would give you a source (potential) of 6V/200AH

So combine what is essentially a single battery that is 6V/200AH in series with another battery that is 6V/100AH and you initially get 12V/300AH.  BUT - as the single 6V/100AH discharges at twice the rate of the (combined) 6V/200AH battery, it will eventually draw down the potential in the larger bank.  What I would have created is a series pair of grossly mismatched batteries.  Not a good idea.

I think your idea of having a switch or some other means of using the "third" battery as an independent backup is an excellent idea, and would essentially give me the additional reserve capacity I was hoping for.

I was an EE undergrad for one semester, then switched to ME, and got my MS in Engineering Management.  Electricity is black magic I tell you! :confused:

Thanks for the responses and for everyone putting up with my techno-silliness.

When I get my setup configured, I will post a writeup and some pictures.  

[brainpause]

IMNSHO, if your tongue weight/space/etc can handle it, two 6 volt batteries and one 12 volt battery (being independent from the 6 volts) would give you great reserve for several days.

Having said that, buying all that, and the necessary heavy switches, will get you a good start on a solar charging system.

Larry

[AustinBoston]

Austin - what you stated above is what I was intending to do and I tried to draw the circuit (crudely) in my post.  What I was not grasping is why the battery in series would discharge at a different rate.  I think I understand now.  Bear with me and see if I get this right:

(Assume we are working with 6 volt 100AH GC batteries for mathematical simplicity)

One 6V/100H in parallel with another 6V/100AH would give you a source (potential) of 6V/200AH

OK so far.

So combine what is essentially a single battery that is 6V/200AH in series with another battery that is 6V/100AH and you initially get 12V/300AH.

No.  You get 12V/100AH.  All of the amps go through the single battery (6V/100AH), and only 100 AH will go through it before it is discharged.

BUT - as the single 6V/100AH discharges at twice the rate of the (combined) 6V/200AH battery, it will eventually draw down the potential in the larger bank.  What I would have created is a series pair of grossly mismatched batteries.  Not a good idea.

Close enough.

I was an EE undergrad for one semester, then switched to ME, and got my MS in Engineering Management.  Electricity is black magic I tell you! :confused:

IMHO, electricity follows known laws until you get into the world of microwaves.  Now THAT is black magic.  :yikes:

Austin

[scottykrug]

We are wired up for it but have yet to use them.  With that said, the three batteries I have were accrued over time and did not cost me anything.  I would probably go another direction (solar) rather than spend the money on that much lead and acid.  Up to this point we have not needed the extra reserve capacity- all our trips are short or have access to ac power.  

I agree, I would not mix different types of batteries.  Ours are all similar age, size, and rc, though all different brands.