Classic cars forum & vehicle restoration.

[Ray White]

HOW GET MORE AMP HOURS WITH TWO 6 VOLT BATTERIES
Wiring two 6 volt batteries in series is a cost effective way to obtain a higher amp hour rating and extend battery life between recharges.

When it comes to starting an engine, cold cranking amps (CCA) is likely the most important rating of an automotive battery. A large engine theoretically requires a battery with more CCA than a small one. Likewise, a high compression engine (such as a diesel) requires a battery with higher CCA than a low compression engine.

Amps" refers to nothing more than the instantaneous current draw of a load (motor, light, etc) completely independent of time. Amp-hours refers to the total storage capacity of a battery; how many amps can be drawn for a period of 1 hour before the battery is completely drained.

PARALLEL VS SERIES
There are only two ways to wire multiple batteries - in series or in parallel. Connecting batteries in parallel doubles amp-hours while voltage remains constant (two 12 volt batteries in parallel produce double the amp-hours but remains a 12 volt output).

Connecting batteries in series doubles voltage while amp-hours remain constant (two 6 volt batteries in series produce 12 volts but retain the same amp-hour rating of a single battery).

You'll find that 12 volt batteries have significantly lower amp-hour ratings than 6 volt batteries

To wire two 12 volt batteries in parallel, the negative terminal of the first battery is wired to the negative terminal of the second battery. The positive terminal of the first battery is wired to the positive terminal of the second battery. The output voltage of the system will remain unchanged, however you will achieve double the amp-hours (amp-hour rating of first battery + amp-hour rating of second battery).

To wire two 6 volt batteries in series, the positive terminal of the first battery is wired to the negative terminal of the second battery. The remaining negative terminal on the first battery and positive terminal on the second battery become your 12v negative (ground) and 12v positive outputs, respectively. The voltage of the system is doubled while the amp-hour rating remains that of a single battery (a pair of 200 Ah batteries will produce a total rating of 200 Ah once connected in series).

[ukdave2002]

Hi Ray

Wiring batteries in parallel is not a good idea; unless the internal resistance of both batteries is exactly the same, one battery will drive the other ( a bit like putting 2 engines together; unless both engines produce exactly the same power, one engine will tend to "drive" the other.

Lead acid batteries are in series as standard; a 6 volt battery is 3 cells in series, a 12v battery 6 cells in series, the fact that they may be in different cases makes no difference.

A 60AH battery in theory should be capable of supplying 10A for 6 hours or 5A for 12 hours, although its not quite as simple as that as the battery internal resistance needs to be factored in to the calculation to get a truly accurate figure.

Dave

[peter scott]

Series connection is better than parallel because it avoids one battery charging the other when there is a difference in EMF.

Peter

Oops! Dave and I posted at the same time.
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[Ray White]

Thanks Guys. I just cribbed the info from somewhere on the net and posted it here to see what you all thought.

My quest is to establish whether the single 12 volt battery as fitted to my MG TC is an improvement over the previous - and subsequent - models which had two 6 volt batteries connected in series.??

I looked at the figures and came to the conclusion that two six volt batteries are better for amperes than a single 12 volt.

But what about the CCA? Surely that is more important?

This is linked to the options I have of battery location. I am tempted to stray from TC practise and fit a heater to the battery box ....and fit two batteries in the same place as a TA; that is in two boxes under the rear decking.

If, cost aside, there is a practical disadvantage to having two 6 volt batteries over one 12 volt then I will keep with TC convention.

[ukdave2002]

I have ditched the 2 x 6v batteries in my MGA in favor of a single 12V "Duracell" thats fitted in one of the 6v cradles without modification, its 45AH, less than the 57AH original spec, however the cranking capacity (CCXA) of the Duracell is about 50% more than the 2 x 6v.

https://www.duracell-automotive.com/en/Products/Car-batteries/Duracell-Starter/2-DS-44#

The 6v batteries might offer better AH on paper, but they are generally older technology, cost 2-3 times as much and don't last as long and have a lower. cranking capacity. As you are fitting an alternator a modern technology sealed battery is fine.

AH and CCA are confusing; if you were buying a battery to run a lamp AH would be the priority, if buying a battery to start a car CCA is more important.

If the batteries were visible I would have retained the old style rubber 6v's, but they are situated under the car behind the seats....

Dave

[Ray White]

Thanks for that, Dave. From what you say the CCA is generally less with 2 x 6v batteries than with one 12v.

I shall not be changing. It does make me wonder why MG reverted to the previous set up and continued with it for so many years.?

Then again, MG made so many odd choices I should not be surprised.

[ukdave2002]

I was just editing my reply when you replied!

I was editing to make CCA importance clearer:

AH and CCA figures are confusing; if you were buying a battery to run a lamp AH would be the priority, if buying a battery to start a car CCA is more important. Its a bit like HP and Torque.

Dave

[petelang]

In older cars Ray, batteries were traditionally larger than today's modern equivalent as the internal plates were bulkier, particularly in premium batteries. Nowadays you can get a lot more performance out of a considerably smaller casing and lighter too.
Of course, older vehicles with minimalist output Dynamos needed a large AH capacity to ensure they had plenty of "starts" in them as short trips may not have replenished the charge, hence Dave is managing sufficiently with a lesser AH now but, I assume, with alternator charging, recharging the cells effectively.
Other factors also come into play such as voltdrop on cable length and size. If your battery was a long way from the starter you needed substantial cable to prevent volt drop resistance.
To effectively "size" your battery, you need to know the cranking draw of the starter under cold conditions, vs the possible recharging capacity of your generator device.
I once had a very good CAV book about this based on commercial applications but long since lost, but it clearly identified case examples.
Your local battery specialist can probably help.
Peter

[peter scott]

Batteries in the rear gives a better weight distribution.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[Ray White]

[lowdrag]

[Ray White]