I have a small single 12volt battery fitted to my 1970 MGB GT which is limited in Ah capacity because of its small physical size, my question is will it benefit me to use a second 12volt battery (which I have) housed in the vacant battery compartment and connected in parallel, or are there more beneficial alternatives?
Answer
This raises the fundamental question of why should you need to consider the change from 6v to 12v and there are a number of aspects that need to be covered.
Obviously the main issue for owners is the cost of replacement batteries with modern 12v batteries being much cheaper than original 6v, but that question usually only arises following a failure of one of the original 421 series 6v batteries, and then having to face the need that both will have to be changed, otherwise the old battery will drag down the new one. Another consideration is when a battery failure may occur away from home and where can a replacement 421 6v be obtained on a Sunday afternoon, whereas a common 12v should not be difficult to find.
It was also true that for quite some time in years past many of the replacement 6v batteries on sale failed to provide good service, and by good service I would consider a minimum of five years a reasonable time, although double that has been seen. However, not all replacement batteries in those previous years have given anywhere near that length of service and some spurious supplier’s products may have only just made it past their 12 month guarantee!
Today the widespread use of intelligent battery management chargers has significantly extended the average battery life by removing the high peaks and low troughs of voltage variation, that for a car battery shortens the batteries life, and this is especially so on a classic that has infrequent use and long periods of standing, more so if there are any continuously active electrical items.
D025 - AA Smart Battery Charger Maintainer 1.5A
The availability of 6v batteries today, and specifically the 421 series for MGA, B, C and V8, pre Rubber bumper, has seen most of the cheap spurious options disappear and what can be found are often more expensive than MGOC Spares 421 offerings, which are 80Ah (Amp Hour) rated which is 40% more than the original standard MG supplied batteries. It’s worth noting that having a larger battery capacity means that in use the battery will be less stressed and that usually extends battery life.
D004WX2 - Batteries 421 6V 80AH 600CCA Wet Pair. Also sold individually under part number D004W
Nevertheless the issue of higher 6v cost compared to mainstream 12v offerings has driven the growth of owners choosing to look down the single 12v battery route, but they often do not appreciate the additional considerations and conversion aspects. Additionally some look to use two smaller 12v batteries that have a similar physical size and weight to the original 421 6v batteries, one in each 6v battery box, but that then has its own individual conversion requirements.
There is much more to choosing a battery than just price and the other considerations in order of priority are, physical size, type and configuration of the battery terminals, battery weight and related structural condition of the original 6v battery box, connection of the new battery to the cars wiring, and finally correctly securing the battery in the car.
Looking at battery size first and the 6v battery box is small and designed to comfortably take the approximate 175mm x 175mm square footprint of the 421, whilst any standard ‘off the shelf’ single 12v replacement with enough power reserve is oblong in shape. (Note; Square footprint single 12v batteries don’t have enough reserve, see more later.) The 6v battery box is bigger than the 421 6v footprint and it will just accommodate a long battery length of 212mm, with the battery sitting in the same orientation as the original 6v. This dimensional limit means in practical terms common battery choice is restricted to the 063 series that generally have long side dimensions of around 207mm, with the same short side dimension as the 421 at 175mm. Meanwhile heights are generally common at around 190mm. That bigger dimension also means that you will need to configure a new form of battery clamping as the original 6v clamps will no longer fit.
Rubber bumper single with enlarged battery box for traditional oblong battery of similar reserve to the paired twin 6v batteries.
It’s also important to consider the weight of the battery to be used and compare it to average weight of a 421 at between 10 and 11 kg. The commonly used 063 12v has a weight between 12 and 14kgs, so a potential 40% increase in battery weight demands careful checking of the battery box condition as it is often overlooked and I have seen 6v batteries start to drop out as rusted frames give way!
Typical 063 12v battery conversion in 6v battery box with arrow pointing to front of the car.
The next important consideration when swapping a pair of 6v batteries for a single 12v in is the battery reserve in terms of the amp hour (Ah) rate and its cold cranking amp (CCA) capacity. Original MG standard spec 421 batteries for the MGB were rated at 51 to 56Ah with alternative heavy duty ones offered at 63 to 65Ah with approximate cold cranking amp ratings for the standard and heavy duty of approximately 360CCA and something over 400CCA.
As a general guide for a 12v conversion I see a minimum of 50Ah and over 500CCA (ideally higher) as being the minimum if the battery is not going to be stressed to a point that could reduce its service life, and more practically expose the owner to greater risk of battery reserve running out if the engine doesn’t start and the starter has to be repeatedly used.
At this point it is very pertinent to remind you of the 421 batteries offered by MGOC Spares are rated at 80Ah and 600CCA that provides the simplest ‘old 6v ones out’ and ‘new 6v ones in’ battery change, and importantly MGOC Spares is able to deliver acid filled batteries to your door in England and Wales, so overcoming many suppliers who can only supply dry batteries and leave the customer to find acid locally. That high reserve means that to compete with it when changing to a 12v battery demands a physically much bigger battery that can’t easily be fitted in a 6v battery box without significant modification, or adding complication of needing two 12v batteries, with a different wiring set up.
Following on from battery reserve is the physical ability to be able to connect the battery to the MGB wiring and here this starts with looking at the main underfloor battery to engine bay cable at its battery end terminal. Here if the car is pre 1968 and still wired positive earth then the underfloor battery cable will have the slightly smaller negative terminal fitted to it that attaches to the negative post of the new 12v battery.
Obviously if it is a later car or if the cars polarity has been swapped, then the terminal on the battery end of underfloor cable will be the slightly larger positive type to fit on the new 12v battery. This detail demands that your replacement 12v battery not only has the right round type terminal posts, but that the orientation of those terminals allows for the connection of that main underfloor cable.
By comparison dealing with the body earth cable needed for the other battery terminal is much simpler because in the original left 6v battery to body connection position the cable won’t reach the new right side position of the new 12v battery, so if it is in serviceable condition you either have to remove it from the body mounting and clean up the convenient unused hole in the right side battery box frame and bolt it there, or buy a new cable.
Chrome bumper MGB twin six volt batteries in their battery boxes.
I mentioned earlier the other option of moving from the original paired 6v batteries to a modern pair of 12v batteries (e.g. 202) that in paired format have sufficient Ah and CCA capacity and also have the same footprint and weight of the original 6v, plus you may also be able to use the original battery clamping arrangement. The biggest change needed here is to change the wiring from the original 6v arrangement of ‘in series’ that sees one 6v battery positive terminal connected to the other battery’s negative terminal, so you get 6v plus 6v giving a 12v output. The other choice would be to fit two of the common 063 12v batteries often chosen for single battery conversions as the long side dimension of the 063 just fits in the 6v battery box.
However, with two 12v batteries wired the same way as the original 6v would deliver 24v, so to maintain 12v the batteries now need to be wired ‘in parallel’, with both batteries positive terminals are connected together and both negative terminals connected together, so all that occurs is that the actual electrical reserve is doubled but voltage remains at 12v. This makes wiring connections a little more awkward as essentially the main underfloor cable connection has to connect to the same terminal on both batteries, (which terminal depends on the cars polarity), whilst there needs to be two separate body earth cables.
Whilst this subject is on the ins and outs of a 12v battery conversion, mention must also be made of the close relationship the generator, (alternator or dynamo) has with the battery, and the need to ensure it is working as effectively as it can, otherwise the battery will soon run flat and draining a car battery is very bad for its long term health. This also shows there is battery life benefit in increasing the charging ability of the generator and there are several options.
Cars with an alternator are the simplest to deal with as uprated original pattern alternators delivering 55 outputs (originally 34 to 45amp) is a simple change. Cars with a dynamo can have them upgraded but not easily, so conversion to an alternator is the most common route along with a polarity change if needed. Those looking to keep the dynamo look can buy an alternator that is made to look like a dynamo, to provide all the benefits of an alternator and these come in both positive and negative earth, but at a significant cost.
GEU216 - Alternator, 16/17ACR 55amp
GXE3101 - Dynamo MGA/B/Midget New, Negative Earth
With the battery and the closely related charging system in tip top order you can expect many hassle free reliable years and many miles of pleasurable motoring.
Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
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