Batteries

Having the power to start with!

For those of us who use our P6’s as daily drivers in all weathers, our batteries will take a hammering over the Winter months from regular cold starts and short journeys with all the lights and wipers in use. Some of us have even had the misfortune of having a battery go flat on us. There are some basic things you can do to help maintain the health of your battery, but sooner or later it comes to the point where we need to think about replacing it. This article aims to clarify the types of batteries that are suitable for our P6’s, and helps you choose the best one for your car and your wallet. We’ve also included some important information on how to maintain and protect your battery for maximum life expectancy, safety and security.

P6 battery locations and car polarities

Until 1968, all P6 batteries were located under the bonnet. For the V8 models, lack of space in the engine bay forced its relocation to the boot, where it can be found in all V8 engined cars. After the launch of the facelifted ‘New Look’ (series 2) cars in September 1971, this boot location was standardised across the P6 range where it remained until the end of production.

All V8 and Series 2 cars are negative earth. This means the body is connected to the negative battery terminal and acts as the negative side of the circuit, while the wiring carries only the positive feed.

Early Series 1 models had positive earth electrical systems until early 1966. Negative earth started with Suffix D cars, which began from chassis number 40037001. Electrical specifications varied for export models, but the differences were far too numerous to list exhaustively.

Understanding Batteries

Sizes

Modern car batteries are produced in a variety of common sizes, which are given reference numbers for ease. The correct sizes for the P6 battery trays are as follows:

Similar sized batteries can be fitted with some success to the engine bay tray, but the terminals need to have positive at the top right and negative at the top left. The boot tray can only accept 075 batteries or smaller, but the terminal arrangement is not vital – the battery can simply be rotated through 180 degrees as both cables have enough slack to reach the terminals.

Ratings

Volts (V)
If you’ve got a physics degree, it is the difference in electrical potential across a wire when an electric current of one ampere dissipates one watt of power. But for the rest of us, all you need to know is that you need twelve of them! All P6 electrical systems-  both positive and negative earth – are 12 volts.

Amp Hours (Ah)

This is just a measure of the amount of current the battery can supply in one hour, provided there is an electrical component connected capable of drawing that power. The bigger this number, the less likely you are to have dim headlights when all of the other electrical components in the car are switched on as well.

Cold Cranking Amps (CCA)

This is simply a description of how much current the battery is capable of supplying at a constant 12v for a 30 second period in zero degrees celsius ambient conditions. In other words, it’s a measure of how fast the battery can turn your starter motor, and how long it can maintain that speed.

Reserve Capacity (RC)

This states the number of minutes the battery is able to supply a constant 12v, 25 amp load at 80 degrees celsius ambient temperature. In other words, it describes how long your battery can keep your engine running if the alternator/dynamo conks out. RC is often regarded as a truer reflection of battery life expectancy than the Ah rating.

What rating do I need?

There is a lot of popular misconception about battery power outputs, and a lot of people are very concerned that fitting something that is ‘too powerful’ will damage their car. Whilst that is true to some extent, batteries do not ‘send’ electrical power through the wiring, they allow the electrical components to ‘draw’ the amount of power they require. So if your starter motor is only capable of drawing 30 amps, that’s all the battery will deliver, regardless of its maximum power output.

Worries about burning out the contacts in the starter motor are valid ones, but this isn’t directly caused by the power of the battery, but rather the amount of electrical arcing and heat generated on the commutator when cranking the motor for long periods. A higher CCA rated battery is likely to start the engine much quicker, and therefore offsets any concerns about burning out the contacts.

V8 and four-cylinder

There isn’t terribly much difference between the ratings required by a V8 and 4-cylinder. They share all of the same lights, heaters and major high-draw electrical components, and the general health of the engine and starter motor will be much bigger factors in affecting the cold start speed than the number of cylinders in the engine itself. In theory, a V8 will place greater load on the starter, but it is also likely to start sooner than a four due to it having twice as many strokes per revolution. Of the 4-cylinders, the 2000 TC is likely to be the hardest to start owing to its high compression pistons and relatively crude choke arrangement, but the difference between all models is likely to be reasonably small.

Amp Hours

60Ah is going to be sufficient for the average daily driver. For regular short trips on winter nights, you might consider 70Ah+, but it’s likely that old and worn starter motors and alternators will begin to fail in these conditions long before the battery runs down.

Cold Cranking Amps

Somewhere between 500-600 CCA is more than enough to spin a Lucas starter motor at a useful speed, although you could reasonably go as far as 700 without too much concern. If you have fitted an after market high-power starter, be sure the main positive cable is up to the job of handling the additional current draw before you turn the key. It’s best to replace this with a higher rated cable in this situation.

What battery should I buy?

Our advice is to always buy the best you can afford. As long as you find a battery with strong enough ratings in a size that fits the tray in your car, there is very little else to worry about. Generally speaking, the more expensive the battery, the higher its reserve capacity will be. But for us, the real clincher is how long the guarantee period is.

Exide and Varta batteries are currently very highly regarded by a number of independent motoring journals. We found several examples of each in a variety of suitable ratings and sizes for £65-£85, so it pays to shop around. Both of these are available from motor factors as well as internet suppliers.

Our Best Budget Buy is currently available on eBay. Simply search for  “075 transit battery” to find the listing from ‘Advanced Battery Supplies’. They are currently selling an 075 size (boot tray), 60Ah, 570CCA battery with a 4 year guarantee and free next day delivery for £50.00. Two committee members’ cars – a 2000TC and 3500 – have been running on this battery for 2 years with no reported issues.

Protecting your battery

Top up the cells

Many modern batteries are sealed units, but open cell batteries are still available. For these, it is worth periodically removing the cell caps and topping the cells up with de-ionized distilled water until the plates inside are covered. You should never use tap water as its mineral content will coat the plates and shorten the battery’s life expectancy. In the absence of de-ionized water, boiled water that has been allowed to cool will suffice in an emergency, as the boiling process removes some of the mineral content.

Smear the contacts

Smearing some vaseline over the contacts and terminals of batteries mounted in the engine bay will help protect them from damp, grit and corrosion, and maintain a good electrical contact.

Longer term protection

Isolator switch

Unlike older types, modern car batteries are only capable of surviving 4 or 5 complete discharges in their life before they begin struggling to hold a charge. If your car has an ‘electrical leak’ and your battery loses charge overnight, you should fit an isolator switch as soon as possible. In addition to protecting your battery’s charge, this also provides a vital safety function in preventing shorts that could lead to electrical fires. Many switches also come with a key, so as a third advantage it can act as a simple immobiliser when parking your car in less desirable locations!

The switch is traditionally mounted on the negative side of the battery between the body and the battery, as electrical arcing on the positive side will pit the switch’s contacts over time. But there is no outright danger to fitting it on the positive side in a car with a positive earth system if it is more convenient.

Battery conditioner

For cars that are taken off the road for the winter months, a trickle charger is an excellent way to ensure your battery stays conditioned while the car is laid up. You can buy low current battery conditioners that clamp onto the battery terminals, although a neater solution is to buy a cigar lighter adaptor and charge through the dashboard. All P6’s have permanently live cigar lighters, so you won’t even need to leave the key in.

Improving battery and cold starting performance

Earth straps and contacts

The biggest weakness in any electrical system is the joints and contacts in the circuit. These need to be clean and tight for optimum performance.

Remove the straps at both ends and clean the contact faces with some fine emery cloth. Also clean the securing washers and nuts, and the area that the strap’s contact will be touching. Check the straps themselves for wear. Braided straps should not have any significant tears or breaks, and should be replaced if they are in poor condition.

As well as the strap from the battery to the body, there is also one from the body to the engine block. This can be found on one of the engine mounting brackets. On the 4-cylinders it is on the offside mounting (behind the oil filter housing), and on the V8’s it is on the nearside engine mount close to the cross member. 

The engine earth strap is vitally important. Not just because it provides the earth for the engine ignition, but also because it earths the alternator, so a dirty connection here can be one factor in poor charging.

Words: Michael Allen

Originally published in Driving Force magazine

May 2013