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MG MGB Technical - New Battery - No Power?


My car is a 1973 MGB. I restored it several years ago and it has run like a fine tuned watch - most of the time.

When I restored it I changed to a single 12 volt Interstate battery. It was a five year battery and last seven years. About a month ago I had to replace the battery, so I bought a new Interstate battery. Also, after reading about it on this site I bought and installed one of the battery Tender units.

When I installed the new battery I made sure the terminals were clean and tight.

However, twice now, the second time being yesterday, I have been out driving the car and everything seemed to be fine. However, when I stopped and turned off the car, when I went to restart it, there was no power. No interior lights, no horn, no nothing. The first time I got out and checked the fuses and such, but everything was fine. Then I finally uncovered the battery and with my hand I hit the battery terminals. Everything started right up. When I went home I rechecked to make sure the terminals were tight and clean - which they were - and there is no corrosion.

Yesterday, it did it again and I just hit the terminals with my hand and the power came back on.

I am pretty knowledgeable about cars and mechanical things but this one is a challenge.

Suggestions? Is it a bad battery? What else could it be that I am missing?


Robert Browning


It is possible that the problem is the new battery, but it is more likely the connections, as you may have verified by hitting them.

Maybe the wire to battery clamp connection is bad. This is especially likely if it is the bolt on type, but can happen even with the soldered on type.

I think you will find a problem with the connections at the battery, though it is worth checking it at the battery to chassis ground point and at the starter.

Also, if you moved the clamps by hand, maybe they are not tight enough, but I suspect corrosion at the contact point or between the clamp and cable.

C R Huff

As Charley, and:
Sometimes acid gets inside the cable insulation and eats the wire.

Most of these things will show up by checking voltage drop across any suspect section, UNDER FULL LOAD. Get the system hot and stable by running at 2000rpm for 10 minutes with all electrical loads ON; measure in this state. Lights, heater blowers, etc. With a voltmeter read V over the section. Clamp to post, solenoid to batt+, chassis to neg post, etc. Any section involving fewer than a half dozen joints should read under 0.1V. Large sections, like Batt+ to headlamp high beam terminal at light should be well under 0.5V (that one won't be unless you have rewired with relays and min 14 gauge to lamps!) I usually can get full system drops down around 0.2V after cleaning and correcting all deficiencies; if higher you should be able to figure out why & where.

It is quite possible to have a bad battery - nowaday it seems that they fail without warning. Bad internal welds will do as you observe. Find somebody who can do a max load discharge test on it. Any real auto electric shop or the actual battery distributor should have the equipment. That should show it.

Two years ago a customer installed a new battery in his MGB, drove 40 mi to work, started home that night and the car died in flight. Zilch electrics. Flatbedded to me, totally dead. I put my charger on it and the charger started to smoke, The battery had a dead short internally, and it killed his alternator and melted the charge circuit wires.

FR Millmore

Next time it happens turn lights on (even though they won't come on) and connect your voltmeter to a) the battery *posts*, and b) the battery connectors. If both show no (or very low) voltage then it is indeed the battery at fault. But if the former shows normal voltage and the latter no volts then it is the connectors on the posts that is or are the problem. If both show 12v and you can do the same on the cables inside the connectors, that will test the connection between the cables and the connectors on the battery posts. But that still leaves the connection between the ground cable and its lug, and that lug and the body, and the battery cable and its lug, that and the solenoid stud, and the solenoid stud and the brown wires as possibles.
Paul Hunt


I will get my voltmeter and put it in my car so I will be able to try this when - if it happens again.

Thanks for all the suggestions. I will keep you informed.

Robert Browning

In keeping with the suggestions given by Fletcher and Paul, often the battery and the clamps of the battery cable can be in good condition and the connection can be good, but the cable itself can be corroded right under the insulation that will cause a large drop in voltage. This corrosion can develop under what appears to be perfect insulation and remain invisible to observation. Cheers - Dave
David DuBois


You read my mind in that I was thinking about going ahead and changing out the cables as a precaution.

Robert Browning

This thread was discussed on 06/07/2009

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