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MG MGA - Battery and Alternator Revisited

Like Jeff Bennett in his coil thread I am trying to get the the old girl started - but she complains of a headache all the time!

The car also would not start yesterday, no power in the battery. Switched on the onboard reserve (wired in parallel, both with isolator switches) and away she went. Charged the 'dud' battery overnight and she fired up a charm this morning. Took the battery out and went down to my parts shop to have it tested. 90% okay for a 5 year old battery. No obvious dead cells etc.

This leads me to consider the charging circuit which is alternator driven. I installed it about 3 years ago using Barney's instructions so I have heavy duty additional wires in the output side. Having only a basic, logical and practical understanding of all things wiggly amps, What checks can I do to confirm that enough trapped smoke is getting to the batteries? I have a meter and understanding of some of its functions.

This is also the area that is puzzling me because I seem to have no problem keeping the other battery charged up.

Steve
Steve Gyles

Have you tried swapping the batteries over? That should let you definitely confirm whether it is the charging circuit or battery that is suspect.

Malcolm
Malcolm Asquith

Malcolm

I thought of doing that. Unfortunately, my 2 identical batteries (separated by 2 years in age) have different terminal posts - one set round and one set spade!

Steve
Steve Gyles

You cheap skate Steve, what would the problem have been to buy 2 identical batteries.

OK it is difficult to check the ability of a battery but easy to check that it is charging.

Get you VOLT meter and put it across the selected battery. What is the voltage? hopefully about 12.3 volts

Now start the engine and see if the voltage risesafter a couple of minutes it ought to settle out with about 2 thousand revs on at 14.5 volts. If all this checks out fine then you will have to suspect that you have a discharge. Doesn't take much suprisingly to make a battery go flat in these wintery times.
Bob (robert) Midget Turbo

I have been happy with generators for decades. A year ago I installed an alternator on my MGA because I got tired of repeated control box failures (and I had a spare alternator handy). I switch wires around on the control box to bypass it in the normally prescribed manner, using the failed control box as a terminal block for wire connections.

Then there was a problem that the ignition light would stay on after engine shutdown, which would ultimately cause battery discharge. A little burst on the starter with ignition off would drop the system voltage a bit, the ignition light would go out, all was then normal, and the battery would not discharge.

This was obviously some issue within the control box. I have no idea if it is a common result with a healthy control box or if it was a result of the way mine had failed. I finally disconnected all wires from the control box, then connected the appropriate wires with twist-on wire nuts, and the ignition light and discharge problem went away.

If you're still using the old control box for a terminal block, it may be a good try disconnect it.
Barney Gaylord

Thanks Bob. I will go and do that right now and report back. The only thing I would add is that I have hard wired the radio into the suspect battery (bypassing the isolator). This is so I don't lose the electronic radio memory everytime I switch off (not a 1950s radio!). This is because I always ensure both batteries are isolated whenever I shut down the engine. Am I right in saying that the radio memory should only be a very small drain on the battery?

Steve
Steve Gyles

Barney

Yes I am using the control box as a terminal block. Ignition light works correctly.

I have just done Bob's check. Battery was showing a bit over 12.5 volts at rest, rising to 14 volts after engine start, so all would seem ok in that department. It may just be all these short runs to work during the winter months (3.5 miles each way) that I do twice a day, normally with lights on as well, do not give the battery a chance to recover.

I am confident all the connections are clean and tight.

Steve
Steve Gyles

I am not au fait with these little rewiring jobs when converting to alternators.
It should be a simple mater of a couple of wires from the large terminals on the alternator to the battery, (normally via the starter solenoid). Then a small wire to the ignition light at the opposite side to the wire going to the ignition switch! How hard is that?

Anyway each to his own, but Steve if you are isolating your batteries then a wiring problem will not be the cause.

However one or two of the radio cd [layers we have now do take a significant draw overnight simply to maintain the memory. You may want to isolate this and see the difference. Although wouldn't be simpler to use the other battery for a while. I have a feeling your battery is not what it should be. What is its number?
Bob (robert) Midget Turbo

Bob

They are Vartas, rated at 43amps. I do rotate their use, but I guess the one connected to the radio is not getting a long enough charge, so I will remedy that.

I just went and ran up the engine again. Fully loaded the system with headlights. Still got 14 volts at the terminals. I will see how it goes for the next few days.

Barney's site and also Moss have very easy to follow guides for alternator conversions:

http://mgaguru.com/mgtech/electric/ac101.htm

www.mossmotors.com/graphics/products/PDF/130-078MGA.pdf

Thanks for your assistance

Steve
Steve Gyles

Steve, While you are at it, check the current draw of the alternator with the engine off. A leaky diode inside the alternator ( or other internal electrical fault) can easily discharge a battery overnight. A leaky diode will charge the battery while running but quickly discharge it when the engine is off. This as happened to me on another car. It would explain your symptoms.

Chuck
C Schaefer

Chuck

How do I do that?

Steve
Steve Gyles

Steve, I am not familiar with the alt conversion that you did but I offer this: Since most modern cars with alternators are wired to the battery all the time, be very carefull. Disconnect the heavy wire from the alt. Set your DMM to its highest DC Amps setting. Most DMMs have a range of around 10Amps. Connect the leads of the DMM between the alt terminal and the wire just disconnected. There should be no reading. While still connected, switch to the next lower scale and look for a reading. repeat again. If you end up with a couple of milliAmps the diodes are OK.

With all the discussion about alternators earlier, I rushed to assume that was the problem. You could also have a low current leak in your electrical system somewhere else drawing curent from the battery. The same check can be done at the battery terminals using the DMM set for DC current. A std MGA in good electrical condition with generator, will show zero millamps draw from the battery with the engine and ignition off. If you have any other accessories, there may be some residual current in the 10's of mA. If you are drawing your batteries down overnight, then your problem will be in the range of an Amp or more.

Hope this helps.

Chuck

C Schaefer

Thanks Chuck. I will check things out.

The car started a treat this morning. The 'dud' battery has not spun the cold engine so well in months.

Steve
Steve Gyles

Before you start taking things apart remember that if the "spare" battery is charging OK there is unlikely to be anything wrong with the alternator. Similarly, if it doesn't go flat there is unlikely to be a stray circuit draining things.

If the battery is not faulty or it was not a one time problem, the fault must be on the battery side of your changeover/isolator switch. You have taken the battery out to have it tested so there is a good chance you have fixed a problem with a high resistance battery connector

Malcolm
Malcolm Asquith

I am suspicious of the very short distance/time of the regular trips Steve makes. 3.5 miles is only a few minutes. He states that he is normally running lights, etc. He also states a voltage of 14 which is significantly lower that 14.5 when you are "on the edge" of keeping a battery up on very short charge cycle times. Put all of this together and I think I would add one of the small battery maintainer chargers that you plug in at night.

FWIW

Larry
58 A
72 BGT
Larry Hallanger

Larry

I am inclined to agree. We are shortly moving our office to the nearby City of Preston. I will then be about 8 miles each way.

In the meantime, batteries are a minor problem this week. I am thinking of installing an inflatable dinghy and oars. Had to bale out the inside yesterday! The British Isles are sinking.

Steve
Steve Gyles

Hi Steve, Yes I too am with Larry on this- your short journeys are not enough to charge your battery with lights and wipers on. Even my Rover only just started over Xmas on an icy morning after 2 weeks away and a 20 mile run did not charge it enough so had to put it on charge for 24 hours. I would trickle charge them every weekend - good luck bailing out - I've only had 2 days use so far this year on the last 2 weekends - definitely a fair weather car! - cheers Cam
Cam Cunningham

I may be out of touch but 43 amps sounds like not enough oomph in a battery for an MGA. Group 24 batteries are in the 300 +amps cold cranking area.
Sandy Sanders

I suspect that the 43 amp rating mentioned may be a 43 amp-hr (storage capacity) not CCA or CA as is normal for auto batteries. 43AH would mean that a 1 amp load would fully discharge the battery in 43 hrs. "Storage" batteries are usually spec'd as a 20hr rate which would equate to a 2.15 Amp load.
A 43 AH battery would equate to an oversized lawn tractor battery. But a bit smaller than a group 26 battery. It should have enough oomph to turn an MGA in good weather.

I just bought a group 26 battery for my PT Cruiser with 525 CCA (cold cranking amps). This is about as low a CCA rating as you can get and is less than a Group 24 battery.

Chuck
Chuck Schaefer

3 days on and all is turning ok.

I am speculating that this is a unique problem due to the double battery system I am operating and the short journeys in winter. I rotate the use of the batteries by switching the isolators and, combined with the short journies, they do not get the opportunity to fully recharge. Also I tend to bias battery usage to the 'dud/problematic' battery because I am conscious that it has a permanent, albeit small drain, due to the radio. The drop off in performance has, therefore, been gradual and not really noticable day to day. Eventually, when all went pear-shaped I started looking for faults in the wiring system that probably were not there.

So, what are the messages?

1. Drive the car long distances!! Preferably all year and hood down (take note Cam).

2. Two batteries may not be advantageous, although when I started having problems (and the reasons could be various) it was great to be able to flick a switch and fire up and not have to abandon the car for other transport nor call out roadside recovery.

3. It's great fun having a few problems because that's what keeps the hobby going, keeps me active in the garage and gives all you guys a chance to hit the keyboard.

Many thanks

Steve

Steve Gyles

Steve

Some 30+ years ago I had a VW camper with dual batteries, one for thcar and one for the accessories including a small electric refrigerator. It had a dual diode battery splitter that allowed charging both batteries at the same time. Only downside was the small voltage drop across the diodes. I am sure that something similar is available today in the motor home / camper market. One example is at www.atkinsonelectronics.com/product_pdfs/DBCM-75A.pdf. This could give you the best of both worlds.

FWIW

Larry
Larry Hallanger

This thread was discussed between 13/01/2008 and 16/01/2008

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