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MG MGB Technical - Voltage at Batteries

Hi All,

After fitting a new generator (neg. ground polarized to pos. ground) and voltage regulator to the '63 B, I'm not sure I'm getting the charge I should. The ignition warning light has a faint red glow for awhile, then finally goes out after a few laps through the woods. If it starts to glow again, will go out with a blip to about 3,000 rpm.

I measure the voltage at the batteries with the engine revving about 2,000 rpm as such:

No accessories on: 12.38 volts

Parking lights on (switch in 1st position): 12.22 volts

Headlights on (switch in 2nd position): 12.08 volts

The voltages remain steady, and do not drop over time.

Do I just have a cruddy ground at the batteries or something? Surely, it's something simple. And don't call me Shirley.

Thanks,
Greg
Greg Van Hook

Greg, could be any number of things but the voltage at the battery when charging should be in the region of 13.5 to 14 volts. Sounds like a regulator fault to me. Was it new or just a replacement from another car.

Tony
Tony Oliver

Could be a bad cell in the battery. Externally charge the battery. Measure the battery with engine off and all lights off. Should be at least 12V to 12.5 when charged.
werner haussmann

Oh yeah, sorry, it was new from Moss, supposedly Lucas. They also have a less expensive "replacement". Maybe I should try that.

OK, I'll do the external charge and see what I get. Good idea.

Cheers,
Greg
Greg Van Hook

Greg. A well charged battery, in good condition, should show a reading of 12.5-13.0 volts at the terminals and under a no load condition. Your alternator (or a generator) will charge the battery at above battery voltage with 13.5 to 14.5 volts being the commonly used specification.

The fact that the system voltage does not rise to the proper levels indicates that the alternator is not charging properly.

There are two possible reasons why the alternator will not operate the system properly--bad alternator and/or bad hook up.

The alternator can be tested at most auto parts stores.
If it tests bad, have it replaced.

If the alternator tests out good, we would need to know what instructions were used in the wiring of it. Then, perhaps we could spot a problem with how it was done.

I would remove the alternator and have it tested while recharging the battery (ies).

Les
Les Bengtson

Just wondering - instead of measuring 12V across both batteries, would it be helpful to measure 6V at each battery? Maybe that'll tell me if one of them is bad?

Thanks,
Greg
Greg Van Hook

Greg - I would suggest that you talk directly to Bob Jeffers bobj50@comcast.net Bob is definitely the number one expert on automotive electrical problems, especially charging problems. He can tell you what the best tests are to run on the system and how to interpret the results. Finally, if it turns out that your regulator is bad, he can convert it to solid state and make the case look pretty while he is at it. Cheers - Dave
David DuBois

Thanks Dave,

Yeah, now I'm just flailing and spending money. I just sent an email to Bob Jeffers.

Cheers,
Greg
Greg Van Hook

If I do need to get new 6V batteries, can anyone recommend a "modern" battery available here in the States (Interstate, Die Hard, etc.) that is the original size? My batteries now are just slightly too big to allow installation of the hold-down clamps.

Thanks,
Greg
Greg Van Hook

Greg,


MGB with RB340 regulator should supply the following voltages ranges @ 3000 RPM and following ambiant temperatures values:
10C (50F) => 14,9 - 15,5V
20C (68F) => 14,7 - 15,3V
30C (86F) => 14,5 - 15,1V
40C (104F)=> 14,3 - 14,9V
data from AutoBook.
I hope that help.
rgds.
Renou

Yes, you can check the individual batteriees to determine which is at fault.
werner haussmann

Greg - Is there any particular reason that you want to spend all the extra money to stay with two 6 volt batteries? A single, 26 series 12 volt battery will fit (albeit, tight) for the price of one 6 volt battery. Unless you are competing in concourse, there isn't any compelling reason to stick with the 6 volt batteries. Cheers - Dave
David DuBois

Hi Folks:
Recently lost my twin 6 volt Interstate batteries, which were approaching 7 years of service; and replaced them with the Moss Miatia 12 volt battery. The battery has 475cca compared to the twin 6's @ 360 cca. Not cheap!
I could have gone to the Interstate MT 26 for less than half the price, but I have had terrible experiences with 12 volt wet cells in a Mark 1 MGB with a generator. Destroyed several Sears batteries before returning to twin 6's and again having great service for the next several years. The Moss (Westco) battery is a Absorbed Glass Mat (AGM) battery and has a higher H2SO4 concentration than conventional wet cells and prefers a higher charging voltage. Great for Mark 1 MGB with a generator/RB340 regulator, where charging voltage can peak at 15.5 vdc in lieu of 14.5 vdc from an altenator equipped MGB. The Westco battery is small and fits nicely in the battery box. You will need to run some plastic hose from the vents to purge any hydrogen gas. 4 months with no issues so far.

Good Luck: Rich Boris
67B roadster
Rich Boris

Hi Rich,

Really intersting this report on various MGB models and batteries compatibilties. Could you please explain what is cca standing for, Is that max amp. delivery for a short time?
Regards.
Renou

Renou. CCA stands for "Cold, Cranking Amps". Back in the old days, it was generally understood that larger engines required a larger capacity battery and the measure of battery performance was cranking amps. The rule of thumb was to have a battery having one cranking amp (or more) per cubic inch of engine displacement.

Today, cranking amps is often given as cold, cranking amps or the amount of amperage available at some temperature below "standard temperature" i.e. 20 deg C.

The higher the cranking amps, or cold, cranking amps, the more powerful the battery will be, the longer it is capable of cranking the engine over or the longer the lights can be left on and the battery will still be strong enough to allow it to turn the engine over.

But, considering that a two liter engine is about 124 cubic inches in displacement, almost any battery available today should have sufficient power to operate an MGB engine.

Les
Les Bengtson

One of the reasons MG chose to use two 6 volt was to overcome the current drop on the long battery lead, single 12's would have been too big, given the battery technology in 1963 !
A 63 "N" will have a dynamo and external voltage regulator. Take the top cover off the regulator and clean and set the contact gaps as well as the spade connectors, then see what happens!
By the way each cell in a lead acid battery gives out 2.2volts, hence 2.2 X 6 = 13.2 volts!
AL
Allan Reeling

Last year I replaced my twin 6 volt Interstate batteries after 13 years of service. I replaced them with Interstates, of course. RAY
RAY

Thanks Les for the cca accronym explanation, very bespoke and pedagogic as usual
Allan,another aim to choose twin 6v batteries was to give car an axial weight balance too ?
Rgds.
Renou

I've used a Gp 26 12v in my 65 with generator and positive ground for years. I think I get about 5 years out of each one. Latest is just a battery out of Walmart. It's a tight fit so you can't use the standard battery clamps. I just use a buckled nylon strap. I do run my idle a little high so the warning light is out at idle.

Of course when at idle with blower, wipers, and lights on the turn signal flasher runs slow. If I did a lot of in-town driving I'd probably go to an alternator.

Twin batteries do give you the advantage of more amp-hours capacity. More important with a generator than an alternator as there can be longer periods of discharge with the generator. Almost any battery will give you the all cranking amps you need unless you're starting in really cold weather.
Robert McCoy

Robert,

I have to disagree with you, Twin 6V batteries in serial connect supply 12V no more current intensity is strictly the same for both, if either you need more AMP-Hours, you must link together twin batteries in parallel for 6V or 12V, or install a bigger capacity one.
Regards.
Renou

Perhaps you are correct. However, two batteries either 12 v in parallel or 6 volt in series have more acid and more plates than a single battery. I think amp hours is related to this physical size.
Robert McCoy

Robert, I'm with Renou here. The number of plates in the battery determine it's voltage. As Allan said above, 2.2V X 6 plates gives 13.2 volts (which we all call 12V for some reason).

3 plates gives 6.6 volts and that is how many plates there are in a 6(.6) volt battery. Two 6V batteries in series have a total of 6 plates.

Cheers

Tony
Tony Oliver

It would be nice to think MG thought about balance so much that they used 2 batteries! At the same time they had already scrapped, on price grounds, coil springs and trailing arms for the rear axle! They shifted the tank to the drivers side and didn't think to either put it on the passenger side,(on the grounds that sports cars are driven a great deal of the time with only the driver), better still in the centre, or try to balance it out with a 12v battery on the opposie side. I've worked on MG's long enough to realise that whatever the designers, engine and suspension engineers came up with was compromised by the, "we've always done it like that and people still buy them" way of thinking, and we all know what ultimately happened to that concept. Then, of course, the "bean" counters also had their say.
Allan Reeling

For comparison, I have recently removed the dynamo from my 1965 B and installed an alternator and single 12V battery.

THe reason I did this was reliability and maximum power at tick over for the recently installed electric fan.

Before I did the replacement I fitted the fan and found it to be slow at circa 1000rpm due to the low voltage at the battery from the generator. Anyway, I tried a new voltage regulator from Moss in the UK (branded Lucas). The voltage at tick over was actually worse with the "New" item. A chat with moss revealed these items are not necessarily new, they are original stock and are not checked before distrubution!

From memory, at 2000 rpm, i was over 13 volts at the battery terminals before I removed the dynamo and control box, the new control box caused this to drop to 12.5volts.

Thus it may just be these replacement control boxes!
Chris B

From what I've read over the years MG used two 6 volts batteries, indeed, for front rear balance. The fuel tank was biased toward the right to allow clearance for the exhaust. The solid rear axle was retained due to the fact that the test cars, fitted with trailing links and coil springs, had too much roll. Their test drivers all voted to keep the solid axle as it gave a much more predictable ride. RAY
RAY

Tony, I've always understood that there are 6 cells i a 6v battery and 12 in a 12 volt. Approx 1 volt per cell. But, I think you can have a differing number of plates in a cell. Batteries get more capacity from bigger and more plates.

But, I'm no expert.

Robert McCoy

Robert. The lead/acid cell produces 2.2 volts. Every type of cell has it's own voltage. It is just the nature of the beast. Nickel/cadmium cells produce 1.2 volts from memory. The good old torch battery cell produces 1.5 volts. This of course assumes that the cells are fully charged.

Bigger plates probably (I'm no expert either) produce a larger amp/hour capacity but this would obviously require a larger battery case.

BTW. I recently shifted my 12V battery to the passenger side. Not for any performance reason, I just wanted the fuel pump to live in the drivers side for protection from rough Australian roads.

Cheers
Tony
Tony Oliver

You're absolutely right 3 cells on a 6 volt. It's been so long since I've looked at a 6 volt battery I forgot they only have 3 caps.

For what I do the single 12v on the right side has served me well. I put it there for balance plus the cables fit.

What you did makes sense.
Robert McCoy

This thread was discussed between 13/06/2009 and 22/06/2009

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