How do AGM Car Batteries work ?

AGM means Absorbed Glass Mat.  AGM technology has been developing quickly over the past decade.  An increasing number of cars and vans are making use of the technology - so what is it all about ?

AGM car batteries were created to solve a basic problem that arose when certain car electrical systems became too demanding on ordinary lead acid car batteries.

Many cars made since the millennium have incorporated 'stop-start' technology.  Stop Start means that the vehicle engine stops running when the vehicle is stationary, only to start again once the accellerator is pressed.  Stop-start systems are used by BMW, Porsche, AUDI, VW, Peugeot and Kia to name but a few.

Why do cars that use stop-start require such a specialized battery ?

Normally the car engine continuously recharges the battery via the alternator.  In most non-stop-start vehicles the engine is running constantly and therefore constantly re-charges the battery.

In cars that use stop-start technology the vehicle electrical systems e.g. air conditioning, radio, sat-nav, heater etc will continue to draw energy from the battery even when the engine switches off at the traffic lights.

So, why are ordinary car batteries not up to the job ?

An ordinary lead acid car battery will work fine unless you exhaust it beyond a certain point.  If the battery is drained below a certain percentage it will struggle to recover fully, e.g. it will not recharge back to full capacity and its future performance will quickly degrade.  The lead plates inside it whilst converting energy become 'sulphated' which means that the sulphuric acid surrounding them leaves a sulphur deposit on the surface of each lead plate.  Eventually, with enough sulphation covering the lead plates the battery will fail.

Can't I just use a deep cycle battery - they're cheaper than AGM ?

The majority of deep cycle batteries are not very good for starting engines.  A deep cycle battery is designed to deliver a constant flow of power.  A car battery is designed to throw plenty of power into the starter motor and ignition circuit when you start the vehicle but will not function well in the long term if you draw too much power from it.  A leisure battery (even one with starting capabilities) may fail before its guarantee due to overheating if used for starting the engine week-in week-out.

What then makes an AGM battery superior to an ordinary car battery ?

Absorbed Glass Matt car batteries contain slices of a fibreglass composite that sit in-between the row of lead plates inside the battery.  These help the battery to distribute available power more evenly as well as ensuring that the lead plates do not become overly sulphated.

Can I use an ordinary lead acid car battery as a replacement for an AGM battery ?

Doing so would not save you money in the long term!  An ordinary car battery would fail prematurely and may even become dangerous due to overheating.

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