Developed initially for military applications, AGM (Absorbed Glass Mat) batteries are now widely on the commercial market. They are well known in the 4WD market, where they are favoured for their physical ruggedness. Possibly for this reason, AGM battery marketing concentrates on this virtue, and also on the battery's ability to withstand deep discharges with less damage than their conventional counterparts.

For campervan and motorhome owners their major benefit is curiously overlooked. This is that they can be close to 100% charged in two/three hours by a conventional vehicle alternator/regulator. One should not underestimate this benefit. No standard vehicle charging systems can take a conventional lead-acid battery beyond 70% charge (often it's a mere 65%). As such batteries are progressively seriously damaged if discharged repeatedly below 50% this limits usable capacity to 15%-20%.

Many people do however routinely discharge conventional batteries until they are 'flat'. Even then, usable capacity is still little over 40% ('flat' still being 30% of charged). And such usage reduces battery life to a fraction of that otherwise possible.

AGM batteries, by contrast, can be routinely and quickly charged to well over 90%. They can also be safely discharged to 40% or so without damage, and to 30% without seriously shortening their life. You still don't get the full number of amp/hours at which the battery is rated - but you've got virtually double that from conventional lead-acid batteries of the same nominal capacity, plus their ability to be typically recharged in two/three hours driving. To some extent, switching to AGM batteries removes the need for the only truly other effective alternative: that of replacing the vehicle's original alternator regulator by a smart regulator.

Nothing is perfect!

These batteries may be perceived as having a few minuses. They have nothing like the theoretical life-span (in charge/discharge cycles) of top quality, well-maintained and properly charged (and properly discharged) deep-cycle lead acid batteries. But few such batteries, in motorhome use, ever are maintained and used that way! In practice, AGM batteries (and gel cell batteries) are likely to end up way ahead.

AGM batteries are about 30% larger and heavy per nominal amp/hour. But this is substantially offset by their ability to be charged more thoroughly and discharged more deeply. Their size and weight are thus comparable in terms of usable capacity.

These batteries are expensive - way beyond deep-cycle battery prices. And it's an all or none situation: AGM batteries absolutely must not be paralleled with conventional lead-acid batteries. You must change all or none - including the starter battery, unless the AGM batteries are charged from a completely separate source (which readers of my books will know that I recommend anyway).

How They Work

Conventional lead-acid batteries begin to gas once past 50%-60% charge. They typically emit 0.42 litres of hydrogen and 0.21 litres of oxygen for every amp/hour thereafter. This gas is highly explosive, necessitating the charge rate to be cut back. It also causes water to be lost. It is substantially to limit this gassing and consequent water loss, that vehicle charging systems stop charging when batteries are around 70% 'full'. (Smart regulators monitor battery condition and control the charge rate very accurately, so whilst they charge a battery close to 100% full, they do it in a controlled manner to reduce excess gassing).

To eliminate this problem, AGM batteries have their electrolyte (water/acid mix) trapped within a glass fibre matrix. Whilst charging, oxygen produced on the positive plate, aided by pressure formed within the batteries' airtight cells, recombines with hydrogen from the negative plate, forming water otherwise lost through the charging process. This recombining process enables the batteries to continue to be charged at a high rate without any water loss. These batteries are thus maintenance free.

The first so-sealed batteries, gel cells, provided these benefits, but whilst they could be quickly and adequately charged at lower than normal voltages, they were wrecked if accidentally charged at over 14.4 volts. Even those made today have this limitation. The more recently developed AGM batteries can be charged at up to 14.7 volts, but at the expense of longevity, 14.1 volts however is adequate.

The batteries are thus charged deeply and speedily by the 14.2-14.4 volts from a standard vehicle charging system. The process will be speeded by upgrading the alternator (a change that is only partially effective with a standard system unless upgrading also to a smart regulator). AGM (and gel cell) batteries may be orientated anyway except upside down, but must be housed in a well-ventilated compartment (whilst the batteries are sealed, pressure release valves open if internal pressure becomes dangerously high). Users of gel cell batteries are advised that earlier industry advice, to the effect that gel cell batteries do not need a ventilated compartment, has been rescinded. All such manufacturers now insist that adequate ventilation is imperative.

More explicit details on the care and feeding of these and other types of battery are included in my book 'Motorhome Electrics' - available directly from the CMCA or of course on this website.

My next Tech Notes article deals with Plasmatronics Regulators - mainly the commonly used PL20 series. Briefly, it has been established that many (possibly most) of these regulators have been incorrectly installed. Whilst they charge batteries correctly, an apparently widespread misunderstanding, relating to the function of the Load- terminal, commonly results in only part of the outgoing current being recorded.

Where an inverter is used in the system, it's odds-on that none of the inverter current, which is likely to be the largest draw, is being recorded. Thus users are obtaining a totally misleading (and serious under-estimation) of the energy being used - and of their batteries' state of charge. Please note this fault is due to a misunderstanding concerning installation - NOT with the regulator. It affects only those regulators fitted to motor vehicles, caravans, campervans and motorhomes - rarely domestic installations. My article is finished and is currently with Plasmatronics (for checking). It will be published in the first possible issue of 'The Wanderer', and placed on the CMCA website ASAP.