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Two 12-Volt Batteries in Parallel

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Crusty

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My BN7 has two 6-volt batteries located along side the differential, same as the BN1 and BN2, connected in series.
The location is awkward to say the least and it is easy to overlook the maintenance.
The two 6-volts batteries expired recently and I have to replace them.
The cost of each battery is about NZ$ 350, I am up for about NZ$750 for two.

A 12-volt battery that will fit in the limited space available is about NZ $ 175, which would be a big saving as well as the advantage of no maintenance. If I put a single 12-volt battery in where one of the 6-volt batteries goes, either the positive lead or the negative doesn't reach the battery terminals. As well as that, the CCA of the 12-volt batteries is 330, so I need two of these to give me sufficient CCA.

I thought that the easiest way to solve this would be to fit two 12-volt batteries in parallel. This would let me connect the positive and negative terminals and give me a combined 660 CCA.

I tried this out and the result was a failure. The ignition light comes on and the headlights work, but as soon as I hit the starter button, after turning the headlights off, the engine cranks over for a few seconds and the the ignition light goes out and there is no power.
I checked the fuses and they are all intact, so I am at a loss to know what is going on. All connections are tight.
I am thinking of reverting back to two 6-volts in series as this system worked perfectly well, but I am intrigued as to why the problem exists. It is almost as if the CCA are not double that of a single 12-volt battery.

Any suggestions as to the cause would be very welcome.

Crusty.
 
Just find a 12 volt battery with sufficient CCA. I’d check what mine is but I took all the labels off. There is nothing different about a BN7 compared to other Big Healeys.
 
In theory, this should work. Can you post a photo of your 2 x 12V batteries connected in parallel? Which 12V batteries did you find that will fit in the small 6V trays? Also, which 6V batteries can you source; all I can find in the 'States are 17H, which barely fit under the hinged cover and, yes, they are a PITA to service?

My SWAG would be insufficient grounding/earthing. I had similar problems with my BN2--even going through a couple sets of batteries--until I pulled the starter for rebuild. When I'd first installed it, I had used grade 8 flat washers under the nuts, as is my wont, but when I reinstalled I used split washers under the nuts. Then the starter would crank just fine and I realized, since all surfaces had thick, fresh paint on them the flat washer install did not permit sufficient grounding, but the split washers cut through the paint and did the job.
 
It is very difficult to get the batteries to be perfectly parallel at high loads. The two cables running between the batteries have to have the same resistance and the lead to the starter switch has to come off of one battery and the earth lead has to come off the other battery.

Do you have a battery cutoff switch? These are sometimes problematic when trying to start.
 
John Turney said:
It is very difficult to get the batteries to be perfectly parallel at high loads. The two cables running between the batteries have to have the same resistance and the lead to the starter switch has to come off of one battery and the earth lead has to come off the other battery.
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Click to expand...

At first I was incredulous at this statement, but I think John may have it right given the 'weirdness' of the symptoms. It seems there are potential/probable issues wiring batteries in parallel (but there's also a bit of debate):

"It is possible to connect batteries of same voltage ratings in parallel, but generally it is avoided, because some types of faults developed in one of the parallel connected batteries can lead to the large current circulated between the parallel connected batteries."

www.quora.com

What is the advantage of wiring car batteries in parallel versus in series?

Answer (1 of 15): With series you add voltage but capacity stays the same. With Parallel you add capacity but voltage stays the same.
www.quora.com www.quora.com
 
Several boats I have owned over the years had two banks of batteries--one for house and one for starting. Each bank had two 12's (230AH Surrette or Delco deep cycle batts) with the two batts wired in parallel. Never had a problem.

All that said, why not get a single decent size 12 V and use the other empty space for storage of your jack or some other parts you won't have to access often.
 
I have used two 12V lawn tractor batteries in parallel for years with no problems. I keep the terminals meticulously clean and rotate the position of the batteries at least once a year. At about $60 each and available anywhere, it is a solution that works for me. I recently swapped one out for a sealed battery and will do the same with the other when needed.
 
Just buy one battery of appropriate CCA and physical size …..your back will thank you . (y)
 
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