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MG MGB Technical - Accumate-Chargematic 6/12

Good afternoon all,

Sorry to be such an intermittent poster and then when I do arrive to ask a daft question, but car electrics are simply a dark art to me. So can I ask my daft question, please?

I have an MGB ('66 GT), which is entirely standard other than having two 12V batteries in place of the original twin 6V batteries. This conversion was done by the previous owner.

My question is this: I thought it would be a good idea to buy a trickle charger/conditioner to use in the garage in between using the car (although I do drive it a good distance each week).

I have seen the Accumate-Chargematic 6/12 on the MGOC site, and it looks good for what I need, but my question is this: would I need two, and hook one up to each battery? Or one, and alternate it between the two every couple of days? Or one, and hook it to the positive terminal on one battery and the negative on the other?

Apologies for seeming such an ignorant. Car electrics are my kryptonite.

Thank you,

Piers Colver

I won't go into the pros and cons of having two 12v batteries, but to answer your questions:-

You don't need two chargers.

You can just connect your charger to the positive and negative of the same battery - either of them - or positive of one and the negative of the other. Theoretically, it shouldn't make any difference.
Dave O'Neill 2

Thank you, Dave.

Yes the pros and cons started a well-rehearsed string on here previously!

Thank you for the response.
Piers Colver

"You can just connect your charger to the positive and negative of the same battery - either of them - or positive of one and the negative of the other."

Doesn't the first part of this require one to charge each battery separately at 6v?

Surely better to charge both together at 12v, and rather than having to access the batteries with the connections made to, say, the fusebox in the engine bay in the shape of the purple, and a good earth on the engine.

"Doesn't the first part of this require one to charge each battery separately at 6v?"

Not with two 12v batteries.
Dave O'Neill 2

Piers, your two 12 volt batteries should be connected in parallel (otherwise you would have a 24volt battery if they were connected in series) and where you connect your charger is important as you need to balance the voltage drop evenly across both batteries and each wire during charging. This is not critical for lower current chargers, but it is best practice.

If you have a cigar lighter fitted, that would be a convenient place to connect up to. That is providing you have the optional cigar lighter plug with the charger. If you are just using the battery clips, then one charger lead should be connected to the battery with the main wire going to the solenoid i.e. +ve on a negative earth car, or -ve on a positive earth car, depending on if your car has been converted to negative earth or not. The other charger lead should be connected to the common battery earthing point which connects to the chasis. Although this charger is fairly low current at 1.2 amps, you should also ensure that the batteries are sufficiently ventilated when charging.


Andy Robinson

Ah, missed the two 12v batteries.

But for Andy's comment, since the cables between the batteries are (presumably) capable of powering the starter, it is irrelevant which battery an Accumate (or similar) charger is connected to.

I agree a cigar lighter is the most convenient, but some years have this on the accessories connection of the ignition switch so the ignition key would need to be in and turned to the first position, and is unfused Early years would be dealer provided so could be wired any old how. It's only from the 72 year-on that they are (or should be) wired to the purple circuit which means the key isn't needed, and it is fused.

But twin 12v batteries could confuse a conditioning charger if they have aged at different rates or have different characteristics. It can only deal with them as an average and may under-charge the weaker and over-charge the stronger.


Paul, if I connect a battery charger to the cigar lighter socket on my '78 BGT will that charge the battery. Will the ignition need to be turned on? would a smart charger be the way to go if I can connect through the lighter socket?

Sorry for all the questions, it seems a very good way of keeping the battery charged if I have got it right that is.
Phillip Burbank

Hi Andy R, thank you for the extra response. I don't have cigar lighter at all, so will be using the clips that the charger comes with to connect direct to the batteries. So thank you for the clarification of where to connect them - wouldn't want to blow anything up! And yes, all well ventilated - a stone-built double garage in the Peak District is always pretty drafty, much like most places in the Peak District, and I have the battery lid off and the rear seat out.

Thanks for the tips, all. Electrics and I are not friends. I promise that overall I'm not as impractical as my posts on this string make me sound!!

Piers Colver

I'm assuming both 12v batteries by definition would have to be pretty small and thus quite probably relatively low amperage. As you've got to remove the cover anyway, why not not charge one conventionally overnight and then charge the other one the following evening?

Fairly early on into my my ownership, the hassle of maintenance of those two 6V batteries got to me (as it would with two 12v ones in parallel) and I relocated one 12v battery to the boot. In those days this was any battery I could get hold of cheaply s/h. These days a compact 063 does the trick nicely and affordably. The redundant space vacated can be used for spares etc if suitable boxes made up.
Peter Allen

To answer Philip, is your cigar lighter socket live with the ignition turned off and the key removed? By that I mean can you plug something in and it will work with the key out? If the answer is yes, then you can charge the battery via the socket - make sure you get the polarity right!. Remove the key.

If the answer is no and the socket is only live with the igntion turned on, then you will need to connect your charger elsewhere, either the battery terminals themselves or the big lead on the starter solenoid and a suitable earth, as Paul has already said. You must not leave the car for hours with the ignition turned on.

Referring to Peter's comment about charging the batteries separately, you would have to disconnect the cable that links them. If you don't you will be charging both at the same time anyway, which is what I would do. My V8 has a single 12v battery in the left hand battery cradle. It's not a huge battery but it has turned over the 3.9 litre engine perfectly happily for 9 years now.
Mike Howlett

Charging twin 12v batteries separately would need them to be disconnected, which seems an awful faff, unless you have two cut-off switches.

Phillip - the cigar lighter on a 78 should charge the batteries without the ignition being on, or even the key in. You can confirm this by plugging something in that will show whether it is live or not. If it shows live, then you can charge that way. If not, then something is wrong or has been changed.

To achieve a balanced charge on both batteries you should connect the positive lead of the charger to the first battery and the negative lead to the second battery.

Charging through the cigar lighter would lead to unbalanced charging.

Consider this explanation:
Steven Rechter

That's with a 50 amp charger. The voltage differences with the batteries interconnected with starter-grade cable and being charged at not much more than an amp, will be infinitesimal.

That's true with a trickle charger, though it's not what happens when both batteries are depleted due to headlights being left on, etc., and the 45amp alternator tries to recharge them.
Steven Rechter

What you say is correct but as the current a lead acid battery charges at is dependant on the difference between the charging voltage and the voltage of the battery, as the battery thatís taking the most current charges itís voltage will increase and current decrease, this will allow the second battery thatís at a lower voltage to take more of the charge. If the charging is stopped before the batteries are fully charged, the battery thatís charged the most will have a higher voltage and therefore transfer some of its charge to the other battery until both are at the same voltage.

In practice the batteries will have separate earth straps and the cable between the batteries is so short and hopefully large enough, hopefully at least 25mm squared, for this not to be an issue.

Personally I question the need for 2 batteries unless youíre planning to run accessories for long periods without the engine running or trying to get home after your charging system has failed and it could be argued that as modern batteries at the end of their life just seem to die rather than fail gracefully, 2 batteries doubles your chance of having a flat battery.
R.A Davis

Going to the battery lead off the solenoid will be a lot easier. No need to take out the back seat and open the battery cover.

I use that post when I have my BGT in the garage for any length of time.
Bruce Cunha

Going to the brown - ideally purple - on the fusebox is even easier!

"... when both batteries are depleted due to headlights being left on, etc., and the 45amp alternator tries to recharge them."

In that case the alternator will only put back about 50% of capacity anyway.

What I done on another car was run pos and neg wires directly off the battery(ies) to a socket on those wires that tucks , out of the way, at the corner of the carpet that the Accumate directly plugs into. Accumate sat inside the car where it was slightly warmer and out of the wet and damp (hopefully).
Nigel Atkins

Bruce's plan is the easiest
One leed to earth One leed to the power terminal of the starter solenoid
Easy to get at and decent sized wiring
William Revit

One point to remember when installing additional wiring either directly to the battery or any other place, is the wiring will be permanently live. In the case of wires going directly to the battery and routed under the carpet to a convenient location it would only take the insulation to chafe where it passes any metal and youíd possibly have a fire on your hands.

If I were doing this Iíd put a fuse in the positive lead (assuming negative earth) where it connects to the battery. In line fuse holders are readily available and with most of these charges only putting out a few amps a 10A fuse would suffice.
R.A Davis

"One point to remember when installing additional wiring either directly to the battery or any other place, is the wiring will be permanently live."

Exactly why using the purple at the fusebox (if you don't have a cigar lighter on that circuit) is both easiest and safest. Putting unfused wires under the battery lid and carpets is rather risky.

I was rather thinking the job would be done properly, unusually I was giving a brief description - as always all wiring and connections would have to be of the corrects size (the Accumate wires are quite small and so the sockets can be), all wiring secured and protected where required (fixings, grommets, insulation, fuse). A little bit of thought and effort would still give an easy an discreet installation.
Nigel Atkins

if you drive the car a good distance each week you're already giving the batteries activity and recharging them so unless you have some big drain you don't need a battery conditioner (or two 12v batteries). Getting a charger would be like getting a dog and barking yourself.

I always think/remember(?) (this should cause debate/discussion) that after 15 miles you've got the power back from starting the car.

Why introduce something that's not need.

My midget is outside 24/7, 365/6/a year sometimes a couple of weeks have passed without use but I've not used the trickle charger, I got for a previous car that had an immobilizer, to keep the battery charged in over 10 years.

You're doing the right thing for the battery and car by driving it regularly on reasonable length journeys, well done. :)
Nigel Atkins

I connect my battery conditioner to the brown wire under the bonnet. In fact I ran a short fused lead to a barrel connector and made up a matching extension lead that the conditioner is left permanently connected to. This arrangement would be fine for 2 12v in parallel. When the last set of 6v failed about 4 years ago I went single 12v. The idea of 2 in parallel terrifies me, I lost a car once while it was stranded due to an internal short on the battery.
Stan Best

Similar to Nigel the only time I've needed a charger of any type is when I've run the battery down through my error, and on cars with an alarm drain that I stopped using every day, until I fitted battery cut-off switches, including to a 'modern' MG. On a few occasions I've left a car with a cut-off switch for three months and it's started just fine. Conditioners are sold on the basis of fear, uncertainty and doubt. If you don't have a drain you don't need one. If you do have a drain then find and fix it, or fit a cut-off switch, which have safety and security functions as well as battery saving. If you leave a conditioner on all the time, as purchasers are told to do, then they cover-up a failing battery until such time as you are away and don't have the conditioner connected, and it won't start next morning. About the only time you need a conditioner is when leaving the car for many weeks and for one reason or another it's not feasible or advisable to disconnect the battery, which isn't applicable to classics of our era.

Wet lead acid batteries will eventually die if you dont keep them floated. It takes years, dont ask me how I found that out.
Both my MGs have a drain for security reasons and with the theft of classic cars seeming to become more common they are staying that way. I keep them both on a conditioner.
It is true that for a few months you can just disconnect the battery and that works fine, however if you have declared the car as fitted with a tracker or alarm/immobiliser then your insurance company may see a glimpse of wriggle room.
Stan Best

Stan I'm not getting at you as the reason I bought a battery conditioner was because the car had an immolisher and alarm and I had another car to use.

Having two or more cars/classics reduces your opportunities to use the car/classic and disconnecting batteries and/or putting cover(s) on them (as I did) makes you relucant to just hop in them for a drive.

Best is to leave the battery(ies) connected and use the car on reasonable length journys ever two or three weeks.

As my car(s) are always outside for the one with the alarm I had an all-weather cover which I couldn't put on if the car was wet and wouldn't put on unless the car was clean. This would mean after I went for a run often I'd have to leave the cover off until I'd the time and opportunity to clean the car, which in winter could be days later. Then once I'd cleaned the car and put the cover on I became reluncant to nip out on driving opportunities because of the hassle of when I'd be able to clean it and put the cover back on.

Moral - don't use an outdoor cover and enjoy the car more instead.
Nigel Atkins

"disconnecting batteries ... makes you relucant to just hop in them for a drive"

Not if you use a cut-off switch.

Outdoor covers are not a good idea. I borrowed one for the V8 for a few weeks after moving house and it was perpetually damp underneath even though it was a 'breathable' cover. For a hard-top you are better off letting the air get to it, for a roadster just put a storm-cover over the top half but remove it in dry weather.

My son had one of his BMW saloons under an all-over cover for a couple of years and he had the same dampness on the outside, also a noticeable damp smell inside. He bought a tent-type garage with a tubular frame for yet another of his cars, which was fine damp-wise, except mice chewed the wiring. He fixed that but sold it as he wasn't using it enough, put the 'outside' car in the tent, but with another drive-in car-coon type tent inside, and now it's dry, mouse-free, and with a couple of zip flaps he can drive off.

Funnily enough the car I used a cover and battery conditioner on had a cut off switch installed loose on the main wire by a PO which I didn't want to use as it was a pain to get at and it was easier to use the plug I'd set up for the battery conditioner. The battery way in a bolted down carrier in the passenger footwell which also had the fuses and relays just out of reach and the ECU and wires just tucked at the side of the battery area. Needless to say this was a British made car. :)

I've used a top cover too but again you don't want to put it on a wet and/or dirt roof and over enough years the straps could affect the paint. You'd keep fit if you removed it in good weather and put it back on when it rained. :)

As Cover Systems isn't far from me they came out to fix a pop-up garage tent but found the car door opened too low to go over the tent centre hinge which is at ground level.

No need for a cut-off switch if the car is used more often - if you want one or it's for safety that's fine but you don't need one for road use. I did think at one time about getting a cut-off switch but I knew if I did it'd go wrong or break at the very least convenient time and I won't be able to start the car, one of my pet hates which is why I like to keep the batteries in good condition, by driving the car, not that I get enough chances.
Nigel Atkins

To use the B I have to remove the cover from the TF, refit the aerial disconnect the conditioners from both cars move the TF getting it off its chocks and then I can get the MGB out.
We are supposed to be enthusiasts
All the comments about car covers are right. Good ones breath but evetually lose their proofing. Fabsil will give you another year but its easiest to just buy a decent new one ever 4 years or so.
Stan Best

Not enthusiasts for shuffling parked cars. ;)

Couldn't you put the B where the TF is so you don't need to move the TF to drive the B.

Sounds like you might have the luxury of having a vehicle or two in a garage, itís not so much fun moving cars outside when itís raining and playing with a heavy or very light covers with the wind blowing hard and your glamorous has very sensibly gone inside.
Nigel Atkins

This thread was discussed between 19/02/2018 and 03/03/2018

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