Dual car batteries.

[mikebyrne]

Anyone had any experience in using two 12 volt batteries in parallel to get better cranking speed ?
Mike.

[Joe Schiavone]

In my opinion all is needed for best starting is a high torque starter. My car starts with one of these starters and only the one battery starts the car within 1/2 second each time. But there is or are benefits with a duel battery system. RacerJoe of

[RockyUSA]

Agree with Joe.

Years ago, I did this with 2 6V Batteries (1946 Plymouth) and didn't see improved performance - the voltage across the starter will only be as high as the lowest voltage battery. You are adding weight and complexity.

Get a high-torque (gear reduction) starter.
Rocky

[Keith]

Fellas, there might not be a high-torque starter available for the engine that Mike is on about......

[slotcarone]

Just get a battery with more cranking amps. That should help.

[AH1951]

Certain cars, like the BMW 850 I owned, have two trunk-mounted batteries wired in parallel.
This is a good idea, for that particular car, as it keeps the batteries in a cooler place and allows for their proper weight distribution. And there is not much underbonnet space available.
BMW use a capture tube on the tops of their trunk-mounted batteries to lead any hydrogen generated during their charging, out of the car to prevent a dangerous build-up of that explosive gas in the trunk.
Perhaps having two batteries helps compensate for any extra voltage drop which results from having extra-long battery cables, and with those cables, the thicker, and the higher their copper content, the better.
As the Jensen worked perfectly well with a single battery mounted in the location chosen by the factory, I would suggest you make sure everything is in good order, use the best battery, starter motor and other components available, and forget about installing a dual-battery system.
Also, clean all battery terminals and cable contacts using a wire brush to scrape away any dirt or corrosion and re-tighten them fully.

[RockyUSA]

Also agree with AH -

Often, lower than desired cranking speeds are due to insufficient grounding.

I like to run three major ground wires..

Battery to Frame
Battery to Motor (typically at/near Starter Mounting Bolt)
Frame to Motor

Rocky

[AH1951]

Yeah, the battery/ground to engine cable is often overlooked.
I know of a Wolseley 1800 which cooked its throttle cable when that became the only current path.

[DaveT]

Mike has a Jensen PW - originally fitted with two 6V batteries in series!

Mike,

there are a few caravan related discussions about this subject. I typed "problems installing batteries in parallel" into google and got some useful ones, such as how to physically connect the 2 parallel batteries to minimise the problems of one battery charging the other due to small voltage drops in the connecting cables.

https://caravanchronicles.com/guides/ho ... -parallel/" onclick="window.open(this.href);return false;

http://www.amplepower.com/pwrnews/parallel/index.html" onclick="window.open(this.href);return false;

I think I would also consider including an isolator switch (or switches).

The other users of such a configuration would be VW campers and boat users. I am sure there are some useful, more technically oriented threads somewhere on the internet.

In the Alvis, I fitted one 12V battery in place of a 6V battery and used the other 6V battery space for a spares box. I have never had any problems turning over the relatively low compression 3.5l Alvis engine and, as you already know, a single 12V battery can manage very successfully with any properly set up 4litre 541 engine (often with more aggressive compression ratio). (The PW has a lower compression version of the 541 Austin engine).

hope this helps

Dave

[mikebyrne]

The car in question is the Jensen PW.
This was originally fitted with two 6 volt batteries in series and these did not put out enough power in my mind for the starter. I would have liked to replace the two small ones with one large 12 volt ,but problem there is that the battery has to be lowered through the seat frame into its cage. With that in mind, I thought that best method would be two 12 batteries volt in parallel, the same size small cases as the six volt ones. (or slightly larger as there is room on the cage for small increase in size)
A Lucas isolator is in place and was fitted to the car originally. All leads and connections are good.
Mike.

[AH1951]

That sounds like a very good idea, Mike.
I would still be tempted into looking at installing thicker, high copper-content cables, all-round, even if the existing ones appear to be in good condition.
It does make a difference.
As a test, just for starting the engine, you could run thick, high-quality jump leads in parallel with the existing cables.
I remember one particular gentleman installing thinner section cables in his car, as he believed they provided lower resistance!
I think the problem of small circulating currents between batteries is that battery life is reduced, but by isolating one or both batteries when the car is not used, you have eliminated that problem.

[kees]

The original starter motor is probably well capable of cranking the engine when in good order. No need for a high torque starter motor even if you can find one fit for the purpose.
Dismantle the starter motor, clean, very gently skim the collector on a lathe, undercut the insulation about 0,5mm and renew the brushes and bearing bushes. Rebuild. Simple. You will be amazed how much difference it makes.
Make sure all wiring is sound and connections to battery and chassis are clean and tight.
Using the biggest single battery that will fit the provided space is always a good idea. Kees Oudesluijs

[Chris_R]

Connecting 2 6-volt batteries in series gives you double the voltage but not double the capacity. So if you have 2 x 6-volt 10ah batteries in series you will get 12 volts @ 10ah. If you connect them in parallel you get same voltage but double the capacity, so now 6 volts @ 20ah.
The series connected batteries in the PW only put out as much power as one of them. Furthermore, cell differences or weak cells in series connected batteries affect the capacity to deliver power. If that power output was less than was ideally required by the car then it would feel insufficient when trying to start the car.
The amount of power a lead/acid battery can deliver is determined by the amount of lead in each cell in the battery. A 12v battery of the same physical size as a 6v battery will deliver less power than the 6v because each cell is about half the physical size in order to fit in the same case dimension.
We don't know what physical size battery is needed but looking on the internet I found a 6v type 421 with 77ah. 2 of these in series will give you 12 volts @ approx 77ah. By contrast similar physical sized 12v batteries would only deliver a maximum of 44ah. So 2 of these in parallel will give you 12v @ approx 88ah, almost 15% improvement.

[Michael Richardson]

Of course,the MGB has two 6V Batts under the rear floor over the axle. I always thought that this (a) spread the weight,and (b) left room under the bonnet for heater box. I agree that the further away from the s/motor loss of power will result. The Mini had same system with one 12V,and putting the starter button on the floor by the driverseat reduced any excess routing. Many Interceptor owners have fitted high torque starters,and they seem to work well.

[Chris_R]

Many Interceptor owners have fitted high torque starters,and they seem to work well.

Indeed, but this question is in relation to a Jensen PW and I doubt the Interceptor high torque starter motor will fit!