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MG MGB Technical - Battery Charging


I'm planning on charging the MGB's battery with a constant (always on) charger. I have a battery isolation switch connected to the earth lead of the battery. My question is : if the isolation switch is turned off, am I safe to charge the battery, with this type of charger leaving the positive connection from the battery to the car in place. The reason for this is so that I can leave the battery compartment lid in place an run a lead directly from the battery terminals to a connector in the boot and then to the charger.


D Beardsmore

Should be no problem.
John H

On a related note, it's always said that you must charge batteries in a well ventilated area. I imagine this is less important with the sealed batteries of today, plus the battery bins on the B are exposed under the car (not enclosed). Is charging like this safe? Does it depend on whether it's a "full" charge or just a trickle top-up charge?

T Jenner

I charge my car from under the bonnet (OK hood). I clip one wire on the solenoid where there is a spare tag and earth the other usually on a cylinder head stud. That circuit is designed to take the 500 Amp stall current of the starter motor so a few 10s of mA is not going to hurt it. As noted above there is plenty of ventilation around the batteries, there has to be they are charged while you drive the car. I have never had any problem in the 20+ years I have done this. I have considered putting a 2mm jack socket in to make this easier and more importantly to ensure it just disconnects if you forget and drive off but it is so easy to clip on and the tail to the socket would be one more thing to go wrong.
Stan Best

Any idea what the typical charging current is (from the point of view of fitting a charging jack for continuous use)? The wires from my charger are quite thin so it can't be that high.

T Jenner

It makes no real difference whether you have a battery cut-off switch or not. However having such a switch prevents problems developing in the alternator or anywhere else and causing a battery drain or even worse a harness fire. I use switches without a bypass fuse in both my cars so they are completely isolated.

But you should not charge batteries constantly with a conventional trickle charger as it will boil them dry. For long-term storage where you want to keep the batteries up you should use one of the more modern 'conditioning' charges which sense battery voltage and vary the charge to suit. If you charge with the lid in place, particularly with a conventional trickle charge, you are likely to get accelerated corrosion of the terminals. Even worse if you use those accursed 'battery boxes'.

If you want to have an easily connectable and disconnectable charger then the simplest way is to put a cigar lighter adapter on the end of the charge leads and plug it into the car's cigar lighter. From 68 (North America) and 72 (UK) these were powered from the purple circuit i.e. always on but fused and so are perfectly safe, but you would need to have any cut-off switch left switched on of course.
Paul Hunt

Paul is right about "dumb" chargers. Keeping the batteries charged full of electrolyte and corrosion free was a time waster before voltage sensing chargers existed. I checked mine before the season and as well as being up to 14.5 volts they had fluid levels exactly where I left them in all cells and no corrosion anywhere so the Oximiser has paid for itself, several times over. He is aslo spot on on battery bins, although you can use them as storage space if you go down the single 12 v route.
Stan Best

At the moment I have a Halfords Fully Automatic charger which also has an automatic maintenance mode. Thanks to this thread I was Googling and found a safety recall notice for it - so thank you for that!

However, this also leaves me with the choice of either replacing it with another the same or getting a completely different one. Any suggestions on what the best charger is for long-term connection? The latest Halfords model of the one I have came out top in an Autoexpress group test:

However I've also heard good things about the Accumate:

T Jenner

I have a small trickle charger called "Battery Fighter junior" this unit has automatic cut off and the unit and plug are in one unit approx. size 50 x 30 x 50 deep, the unit came with a plugged lead. Short lead is permantly connected to the battery with the lead coming out under the rear battery cover (70 MGB).
If I do not drive the car for 2 weeks I just plug the unit into the power point and connect it to the cable fitted to to the car overnight, it has been very sucessful and is a lot easier than connecting a battery charger. I had to extend the lead and you have to make sure you do not forget to disconect it.

David Levy

It shouldn't need charging for a 2-week layup, mine goes more than a month with barely detectable change in cranking speed. That is with no alarm system or audio connected, the former in particular *can* sap the battery in a couple of weeks, and even if it doesn't drain it enough to prevent starting next time the repeated partial draining will drastically shorten battery life from years to months. The answer to that is fit a cut-off switch (without bypass fuse), which stops the drain as acting as an imobiliser, preventing any damage from any shorts while unattended, and also allows you to cut power very quickly if a short should develop while driving.
Paul Hunt

I have used a unit called the Super Smart Battery Tender for several years. This link is the cheapest price that I have for this unit. Currently $34.97 USD

Ed Emery

I have had an accumate on mine for the last 2 years, with the flying lead coming out of the battery tray.

I have a 2001 Suzuki SV650 that has had an optimate connected for 7 years now, and still starts off the original battery.

Do get the wall bracket/holster with accumate/optimate. makes it easy to check the led is on green, as they are hard to see if it is flat on a shelf.
Martin Layton

I also use a Battery Fighter Junior and that seems to work well. Since my car is under restoration and not driven I find it is the best way to keep the battery all topped up.

Simon Jansen

If you have a 68 or later North American MGB or a 72 or later UK model, or an earlier model that has a cigar lighter conencted to the purple circuit, you can put a cigar lighter plug on the end of the charger and simply plug it into the cigar lighter on the car.
Paul Hunt

This thread was discussed between 20/06/2008 and 03/07/2008

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