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Boxes of Power

Everything you wanted to know about car batteries...

By Michael Knowling

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Most of us regard car batteries as humdrum in the extreme - it's a maintenance cost we'd rather put toward buying go-fast gear. But we'll show you not all batteries are the same and that taking the time to choose the correct battery for your purpose can actually be rewarding...

The Different Types of Batteries...

One of the primary decisions when selecting a new battery is whether to go for a conventional lead-acid job or a deep cycle battery. What's the difference, you ask?

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Well, both deep cycle and conventional lead-acid batteries use the same principles of operation, but they're optimised to suit different applications. Deep cycle batteries are often used in off-road four-wheel-drives and camper vehicles where there's a need to power electrical appliances for extended durations. A conventional battery, on the other hand, is designed to deliver a large current 'burst' over a short duration. This is ideal for starting high-compression motors that, once running, rely on their alternator to provide the necessary charge.

Due to the short length of time that a conventional battery is undergoing a high current load, they are rarely drained more than twenty percent of their capacity - they aren't intended to fully discharge. In comparison, a deep cycle battery is capable of repeatedly running down to a very discharged state. They can withstand several hundred full charged/discharged cycles, but they cannot deliver the instantaneous surge that a conventional battery can.

Inside a Car Battery

A lead-acid car battery has two terminals - a positive and negative post.

Inside the battery, a chemical reaction produces electrons, which collect on the negative terminal. Electrons are produced at a rate dependent on the battery's internal resistance. The metals and electrolytes used inside the battery determine its voltage. In one cell of a typical lead-acid car battery, there's one plate made from lead and another plate made from lead dioxide. These are immersed in sulphuric acid. Each cell is normally good for around 2-volts, with six cells used to form a 12-volt battery.

The lead acid batteries used in cars are based on 100+ year old technology. Not surprisingly, therefore, they are hardly ultra-efficient - they have quite poor charge density per kilogram mass. However, over other battery technologies they do still win out on a cost basis.

Battery Ratings

There are two rating systems used to grade car batteries - CCA (cold cranking amps) and RC (reserve capacity).

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CCA is an internationally recognised industry standard to determine battery starting capability. The Cold Cranking Test is conducted at minus 18 degrees Celsius to simulate cold, difficult-to-start conditions. The battery's ability to perform is assessed by measuring the current it can deliver over a period of thirty seconds, while maintaining a voltage equivalent of 1.2 volts per cell or higher. A battery's CCA number shows this current.

Note that a true deep cycle battery has only around half to three-quarters the CCA of a conventional battery. Depending on its compression ratio, a typical 4-cylinder engine usually requires a battery rated at 220 - 305 CCA, a 6-cylinder needs 380 - 520 CCA and a V8 typically requires 520 or more CCA. Diesel engine batteries, on the other hand, have a CCA of 550 - 900 - ideal for turning over that high compression, competition-built V8!

The RC rating represents the number of minutes at 25 degrees C that a battery can supply a load of 25 amps and maintain a voltage of 1.75 volts or higher per cell. In the event of a charging system failure in cold winter conditions, a battery's reserve capacity must be able to supply a minimum electrical load for ignition and fuel injection, headlights, windscreen wipers and demister.

Note that a deep cycle battery has around two to three times the reserve capacity of a conventional battery.

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A very rarely used battery rating these days is the Amp Hour. The Amp hour rating represents the current a battery can supply for a period. For example, a 50 Amp hour battery can supply 2.5 amps for 20 hours. That's because 2.5 amps x 20 hours = 50 Amp Hours.

And what about the number of battery plates? Well, in the past, it was assumed that the more plates, the greater the power. This is a myth; the surface area of these plates is much more important. The area of plates determines the amount of starting power, and the weight of active material determines the battery's reserve capacity. Therefore, a large number of thin plates that are very small in surface area are likely to have less cranking power and less reserve power than a battery with a small number of thick plates that are large in area.

Battery Life

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Under normal day-to-day operating conditions, the average life of new car batteries has increased to 42 months. However, factors such as storage, vibration, temperature conditions, overcharging and cycling can reduce that life considerably. Excessive vibration loosens active material from the battery plates, which may lead to shorting and can also damage the structural integrity of the battery; it's a good idea to look at the mounting system of the battery if these sorts of problems arise. A high ambient temperature (often artificially created by under bonnet temperatures) requires higher charge rates, which leads to an accelerated loss of active material from the positive plates. (Many cars have air ducting to the battery to keep battery temps down.)

As part of good battery maintenance, you should always keep the battery levels topped up (using demineralised water in most cases), keep the terminals clean and make sure the top of the battery isn't covered in crud, which can potentially find its way inside the battery (particularly during servicing).

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Once disconnected from a car's electrical system, a lead-acid battery maintains good charge for a fairly lengthy period. The rate of charge loss depends whether the battery is low maintenance or maintenance free, and the temperature condition it is left in. Battery charge is lost quicker under high temperatures and/or humidity. It is claimed a low maintenance battery stored at 20 degrees C loses approximately 0.5 percent charge every day, while this rate doubles at an ambient temperature of 30 degrees C. Maintenance-free batteries lose their charge more slowly. Note that storing a battery with low charge for an extended period will cause irreversible damage.

Cool and dry storage conditions help keep the loss of charge rate minimal. Furthermore, recharging every four to eight weeks will keep batteries 'as new'.


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The ideal approach to battery charging is with a slow-rate charger, which features automatic resetting. A slow rate (aka 'trickle') charger maintains relatively stable temperature through the charging process and the automatic resetting feature ensures the battery isn't overcharged - the battery is constantly 'topped up' just enough to keep it at maximum charge.

Note that overcharging can permanently damage a battery; furthermore, we're told it's not unknown for more basic 'rapid chargers' to boil the battery dry and buckle its plates.

Battery Relocation

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Remote location of the battery is often done to provide necessary clearance for, say, a new set of monster intercooler pipes, to adjust a car's weight distribution or simply to improve underbonnet appearance.

The practice of relocating your car's battery should be tackled with some caution as there are a few pitfalls. First, in general you should not mount a conventional lead-acid battery inside the cabin with occupants - during charging, lead-acid batteries emit gasses that can be harmful when inhaled. However, batteries specifically designed for in-cabin mounting area available - they use a design with an external venting tube.

Next, don't skimp on the quality of the power cables you use to run from the battery into the engine compartment. Steer away from using small diameter jump-start cables running from A to B - these are not designed for continuous use and the threat of melting the external insulation is very real. Furthermore, the relatively modest diameter of jumper leads will induce a significant voltage drop - this means you may need a higher CCA battery to do the work of an existing battery.

The best-suited cabling to use is known as 00 BNS and 000BNS - this is heavy-duty electrical cable (around ¾-inch thick in the case of the more serious 000 BNS), which can be purchased from most truck parts outlets.

Oh, and - lastly - when you're routing the cables through your car, make sure you take every pain to ensure the cable is protected and all holes through which the cable passes are heavily rubber grommet'd.

Gel Batteries

A new breed of car batteries that don't contain acid is known as 'gel' batteries. The most common gel battery sold for cars in Australia is made by Odyssey.

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Odyssey claims their product has numerous advantages over a conventional item; they are completely maintenance-free, use non-corrosive copper alloy terminals and they're much safer to handle. In addition, they can apparently withstand more than four hundred cycles at 100 percent depth of discharge, and are able to be safely 'fast charged' in less than an hour. They can also be stored for two years at 25 degrees Celsius and maintain more than 50 percent of the original charge.

One of the biggest benefits of an Odyssey is its packaging and mounting flexibility. A typical Odyssey battery is around half the size of a comparably rated lead-acid battery and can be safely mounted in any position - such as on its side. Unlike a lead-acid battery, a gel cell will not emit gasses during charging - this enables you mount it inside the cabin without any health issues.

So now you know everything you should know about car batteries - it really is more than just a matter of dropping into your nearest service station and picking up the first box of power that looks like it'll fit!

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