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MG MGB Technical - Secondary battery (for radio)

I only really use my B at weekends, and during the winter not even that often. This seems to kill the batteries fairly quickly, so I am planning to fit a cut off switch so I can disconnect them when Im going to be leaving the car for a while.

However, Its a real pain to re-program the radio and so Im wondering if I can install a secondary battery that will just provide power for the radio memory while the main battery is disconnected. I would want this secondary battery to get charged up while im driving.

Is this a feasible idea? I can imagine that I will have to find a way of stopping the secondary battery from providing power to all the normally live parts of the harness.
Can anyone suggest a way to connect something up so that the secondary battery (Im thinking about a very small batery - like in a cordless drill) gets charged when the car is running, but does not run flat excessively quickly when the main battery is disconnected?

Thanks
Richard.
Richard Knowles

I think the alternator would cook a small battery. You can add a second full sized battery with a cutt-off solenoid in between, but I think this would tax the alternator too much. This link might give you a possible solution.


http://www.mgcars.org.uk/electrical/switch.html

I cannot see a radio memory draining a battery in a few months. You can also buy trickle charges that you can mount to your car perminently. You just plug them in when you park it, and it will charge the battery as needed.

John A

....or a solar charger, if it's left outdoors or in a spot with no electricity available.
R. L Carleen

Most radios have two power inputs, one for the main power, and one for the memory. If you wire directly from the battery to the memory power input, and add a battery cut off switch on the main power line, you can save the battery, while having a minimal drain from the memory power. I think you could put a fuse in the line that was only a couple of miliamps so as to protect the radio should the battery short in the dead of winter while sitting there unused...

Justin
Justin

Hi..

The best solution really is to provide a trickle charge for the battery while the car is not being used, I charge mine via the cigarette lighter socket.

It is important not to connect a high current charger in this way, as the battery and / or the alternator may be damaged.

The life of a lead-acid battery is significantly reduced by sulphation of the plates, which occurs whenever it is less than fully charged, and lead-acid batteries do self-discharge even if they are disconnected. Existing sulphation can be broken down to some extent by prolonged trickle charging.

I do have a solar charger (approx 12" square, giving 300mA (0.3A) in good sunlight), but the UK winter weather and short daylight hours result in a daily charge of typically about 1/4Ah (Amp hour), which I suspect is not really enough.

Don
Don

Justin was close to my solution.

I have a battery cutoff switch which I use primarily to demobilize the car (as opposed to an alarm, which tells you that it's being stolen). The switch is on the negative side of the battery.

I also have a small wire which goes directly from the negative battery clamp to ground, with a 1/4 amp fuse. At this low amperage, the fuse will blow if there is any attempt to start/run the car.

That fuse is sufficient, however, to keep my radio's memory, operate my paging alarm, and illuminate the courtesy/map light.
Dan

I second Don's suggestion of a trickle charger. I use a very small "smart" charger http://www.vdcelectronics.com/batteryminder.htm
I have a pig-tail attached to the battery terminals in my '67 MGB GT. If I'm not going to be driving for more than a week, I just plug it into the charger's matching quick-connect and leave it. When the battery is fully charged, the device just floats and waits for the voltage to drop, then kicks in again.

Regards,
Jerry

Jerry Causey

If in fact your battery goes dead if left hooked up and stays charged when its disconnected you have a "draw" problem. the first thing you want to do is to check the system for a draw Not to talk down to you but in case you don't have an amp meter an easy way to check it is to disconnect one of your battery cables[on a two battery system, not the center conector cable]useing a simple test light connect one end to the disconnected cable and the other end to the terminal from whence it came.If the test light lights you have a draw, leave the light in place and start checking circuts until the light goes out when you cut power to the circut you are checking.If the light is not lit when you first connect it,[turn the headlights on they won't light but the test light will until you turn the headlights off] you have no draw and disconecting the battery won't help enough to worry about.
RIC
Ric

Dittos to Rick, My 67 keeps charge for months, even when I'm not using it. If yours doesn't, you probably have a leak somewhere. Good luck in finding it.

Bud
Bud G

Richard,

Use a small 12V lead acid battery ( get from Maplins) and wire it in to the radio memory - run a wire from the main brown wiring harness ( at the fuse box or the starter solenoid ) to the new battery and fit a diode in this wire so that current only flows to the battery. You can get the diode at Maplins as well - they should be able to advise a suitable unit if you explain what you are doing.
Chris Betson

How long is fairly quickly? My roadster with no clock, radio, alarm etc. can hold its charge for a couple of months. The V8 with clock and alarm, even when not set, only lasts a couple of weeks so I have recently fitted a cut-off switch with a fused bypass for the clock. With only a radio you should not have a problem with the battery, have you checked for other drains on the system? Remove the battery ground strap and connect a voltmeter in its place. Disconnect known drains like clock, radio and alarm. If you see just a few volts on the meter that is probably the alternator - disconnect it and confirm - but is the normal reverse leakage of the diodes and should not be a problem. If you see a full 12v registered you have some other drain, you will have to start pulling fuses and disconnecting browns to see where it is coming from. Check the boot light first.
Paul Hunt

Do you have the permanent feed to the radio connected to more than just the memory circuit? I used to have this problem and it was compounded with an external amplifier being kept on as a result of a wrong feed to the radio. Finally put the amp power feed through from the ignition circuit and probem fixed. (Couldn't be ar*ed to pull the radio out again as was real pain to clear heating controls).

Good luck
Stuart Robson

Richard, treat the cause, don't treat the symptoms. Adding an additional battery is a very bad idea.

My B sits for weeks or more and starts with out a problem. And the radio stays programed.

1st a diode as richard suggests will loere the voltage to the radio. Diodes typically have a .7V drop when forward biased (conducting current.)
2nd if the "radio" battery is discharged it will look as a dead short to the alternator and draw as much current as it can when you reconnect the main batteries and start the engine. With luck you will only blow a fuse. With bad luck the battery can over heat and will spew acid fumes or worse.

Sources of power drain on a parked car:

1. Bad diodes in the alternator.
2. Dirt in 30 year old switches that no nonger turn all the way off.
3. internal leakage in the battery.
4. Trunk lights that stay on.

there is more to the list but you get the idea. If you have the twin 6V batteries take them out and clean them and the battery box. Put it all back dry.
Put a meter in line with the battery and start unplugging things until the current drain is isolated. Lifting fuses will isolate a lot of possible sources.

Lots of luck
Doug Gordon

Thanks for all the comments so far.

Heres a little more information. When I replaced the twin 6v batteries about 4 years ago they worked fine even after a couple of weeks without use. At the start of this winter they would not start the car after a week of disuse.

With the battery connected there is 12v across the terminals with the ignition off. When I pulled the radio fuses (main power lead and 'memory' lead) this went down to 5.6v, which I believe is acceptable for the bosch altenator I fitted (last summer).

So - Ive taken this to mean that my batteries are dead, and so sitting on the garage floor is now a single 12v battery waiting to be fitted (when I get the dash re fitted).

Is my only real solution to buy a house where the garage is not 100 meters away, and so has power to it, so I can trickle charge?

Would the altenator really fry a second battery - I thought they were load sensing?
Given my options consensus seems to be to put a 1/4 amp bypass around the cut off switch and try to reduce any other draw on the system (1970 - so no clock or boot lights) - although I still like the idea of being able to completely disconnect the battery.

Sorry for the ramblings.
Richard
Richard

FWIW, 0.7v volt drop is neither here nor there when the operating system voltage of the vehicle varies from 12v to 14.5v.

An alternator won't fry a second battery, as far as the alternator is concerned there is just a little more load than usual. Car/caravan systems allow the charging of the caravan battery from the cars alt but the caravan will not discharge the car battery. Common sense dictates that fuses will be used so if conditions *were* such that the battery was going to take excessive current the fuse will blow.

But having said all that if one connects a diode from the existing brown circuit i.e. battery/alternator so the alt can charge the radio battery, and it is the radio that is currently flattening the car battery, then when the radio battery goes more than 0.7v below the car battery the car battery will start to discharge through the diode and the radio, which seems to defeat the object!
Paul Hunt

How old are your batteries? No matter what the mfgr claims for a warrantee, you can only reasonably expect them to last for about three years, maybe less...depends on usage. All those 60 month guarantees are for are to get you to go back and buy the same brand again, as the warranty is prorated, so that you only get a few cents back on your remaining "life".I drive my MGB (80) maybe twice a month, and never have starting problems...BTW I don't live in the subtropical part of Fl, but rather in the NW tip, where freezing night temps are common Dec-Feb.
R. L Carleen

Hi,

First off chuck the "2 6 volt batteries" idea from Mr.Lucas and put in a decent 12 volt battery (1) and then chuck the puny alternator(thanks again Mr. Lucas) in the trash, then install a 55 amp Delco alternator. My B sits for months outside and starts on first or second crank of the starter. Make sure you have a proper ground and wire connection. Clean the battery posts and install those little felt deals around the posts of the battery. Install new ends on the battery cables. Good to (always) go.
Alan

Hey...

You are looking for a battery isolator

will charge 2 batteries with your standard
lucas alt. Same issue here. With a good amp
radio, rally computer, and lights and you'll
kill that battery quick.

The aux batt allows one to switch to another
battery to start & power the car. I'ts a cool
set up we use for handicapped vehicles as an
aux batt. Did the same in my B' with dual 12v
batt's.

Both charge from the alt. Prime battery
is wired as normal the aux batt just powers
accessories. Flip a switch and the aux
batt will power the car.

Last time I posted this info everyone cried
about the "extra weight" no need etc. etc.

You're on the right track if you are running
several accessories. However if this is just
a small radio with a clock/preset this is an
unusual drain on your system.

HowY

A cutoff switch may help. My BV8 is off the road for 5 months each year, and it has a master kill switch -- the kind that cuts the ground once and for all -- there's no thin wire to keep the battery pre-sets etc. Though the battery is several years old, each spring it fires up without any issues when I turn the switch back on. So I don't think the battery is likely to degrade all that much just by sitting there; seems to me if you need to top it up by trickle-chargine it unless the car's been off the road for one heck of a long time, there's some kind of leak in the system.

There's one really elegant, simple solution to radio pre-sets. At, e.g. Pep Boys, they sell a little gizmo which connects a regular 9V transistor battery to your cigar lighter. The idea is, 9V is enough to keep the radio presets. You just stick it in there before cutting the master kill switch, and life is good.
Ted

Hi..

A few years ago, I had a car with both an ammeter and voltmeter fitted. When I arrived at my destination, I would switch off all lights and other electrical loads while the engine was still running, then run the engine at a brisk tickover until the ammeter reading dropped to approximately zero, which was usually a matter of about 30 seconds. I still do this, and always get at least 7 years life out of a battery.

Don
Don

Richard,
I had the two battery/isolater set-up in my B several years ago. I went thru about an alternator a year, a bigger unit probibally would have helped. In all, it was not worth it. It's a pain to install as you have to mount the isolator by the alternator, cut wires and run a wire to the new battery. Overall, I had twice the battery/corrosion problems I had without the second battery. If you have 2 6V batteries, might be one is bad, which would cause the car batteries to be too low to start the car after a short while. You should check the voltage of the individual batteries.
I found a single 12 volt is a better set up over 2 6's anyway. For starters you have two terminals instead of 4, and one of the best ideas I got from this board was to put a bettery box in the second battery box and use it for spare parts/tools. Really cleans up the trunk from tools/parts/brake fluid ect..
If you still have a drain in the system after you put in new battery, you can use an ammeter to narrow it down and see how bad a drain it is. Realistically I think a new battery should power a radio memory for a few months. If you still have problems you can always add the second battery later.
John A

Richard,
I had the same problem on my 78 and thought it was the radio running the battery down. I found that the PO had changed something in the wiring and the gauges were staying energized all the time. I was never able to find the problem, but I put that circuit on a relay controled by an ignition hot wire and have cured the problem. My car seldom sets for over two weeks, but the battery stays charged. Before it would go down in about 4 days.
Jimmy Chew

Throw the radio away and listen to the exhaust note , it sounds better than any music.
Dave Bates

After reading all these posts I think I have decided to dump the idea of having a supplemental battery. It seems that the general idea would work with a diode, but the radio should not actually drain the battery significantly anyway.

As the dash is on its way out of the car I will take the oppurtunity to clean up the connections, and see if anything else is drawing current.

If I want a battery isolator in the future for security reasons I may just put a 9v battery into the cigar lighter (Ok - no cigar lighter, so ill add some wires) to power the memory.

Richard.
Richard

Richard, are your batteries really sitting on the floor? An experiment my high school electronics teacher had us do. Set a charged battery on a concreet floor for a week and measure its current capacity. Then recharge the same battery and set it on a bench for a week. Don't ask me why but a battery will discharge at a significant rate when setting on concrete. You have two batteries, give it a try.

You secondary battery may not be overcharged by your alternater but is the electrical circuit that feeds it upto to current the battery could draw from the alternator? Hate to fry your wireing harness after puling your B form hibernation. Those twin 6s are natorious for self dischage, especially if they have the external on the top of the battery buss' to connect the cells. Accumulated road grime and a litle acid saturation makes the top of the batteries conductive. Try it with a volt meter, you may be suprised how far away form the posts you can read voltage.

As I said before, keep them clean and dry when you store the car and most of your problems may disapear. Of course trading both in for a modern 12V will work just as good. Just depends on how much of a purist you are.

Cheers, Doug
Doug Gordon

Doug,

Thanks for the warning - my new battery (12v) is indeed sitting on the garage floor. Ill put it in the shed when I get home - so its on a wooden floor raised above the ground.

The car itself has just come back from a respray, so the old batteries are indeed filthy, and do have the external cell busses. - but I used to keep them fairly clean which is why I thought they were at the ned of their life.

Cheers, Richard
Richard

This reminds me of a long argument elsewhere, when someone posted that putting a battery on a concrete floor damages it. Much discussion and argument as to *how* a concrete floor could damage a battery, then the original poster eventually came back and said "NO no no, it is the concrete floor that gets damaged by the battery". How we laughed ...
Paul Hunt

This thread was discussed between 23/02/2003 and 28/02/2003

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