A.G.M. Batteries

Boatservices Australia believes that a certain type of Valve Regulated Lead Acid (VRLA) battery is the battery of choice for many boaters. The operating principles behind VRLA batteries are similar to the flooded variety that you find in your car, except that the electrolyte (a diluted sulfuric acid) is held in a gel or a glass mat instead of sloshing around in liquid form. There are some very compelling reasons to use VRLAs:

Gel and Absorbed Glass Mat (AGM) batteries can dispense charge at a higher rate than flooded cells. Deep-cycle flooded cells cannot deliver more than 25% of their rated amp-hour capacity in amps without plummeting capacity as the chemistry in the battery cannot keep up with the discharge (Peukerts Law). Flooded cell battery manufacturers recommend a 4 to 1 ratio between battery bank size and the largest load encountered on board. The construction of gel-cells and AGMs allows the use of fewer batteries to power the same load.

Virtually no gassing under normal operating conditions: Unlike flooded cells, gel cells and AGMs are hermetically sealed and operate under pressure to recombine the oxygen and hydrogen produced during the charge process back into water. You find VRLAs in the bilges of high end yachts such as Hinckley, Hans Christian, Island Packet, etc... Every boat benefits from a low center of gravity over the keel (good for righting purposes) and the minimal venting requirements make it possible.

The ability to put VRLAs in the bilges (they can operate under water should you hole yourself) also lengthens their lives: For every additional 10 degrees of heat over 25 deg C, lead acid battery life (regardless of type) is cut in half (batteries self-destruct with time, you can only slow that process). Chances are, the bilges are the coldest place on board (outside the freezer) and the keel provides protection.

VRLAs can operate in any orientation (although you may lose some capacity that way) and even if a container is broken, a VRLA will not leak. This is a feature particularly important to blue water sailors who may encounter survival storms - you don't want to coat the inside of your boat with sulfuric acid if you ever get rolled. Proper (heavy duty) battery restraints are a must, regardless of battery type.

Gel cells and AGMs require no maintenance once the charging system has been properly set up. No equalization charges (usually), no electrolyte to replenish, no specific gravity checks, no additional safety gear to carry on board in order to protect yourself. If you want to be anal retentive about VRLAs you can load test them. However, proper charge control and protection is much more important with VRLAs because once fried it is impossible to revive them.

The higher charge efficiency of AGMs allows you to recharge with less energy: Flooded cells convert 15-20% of the electrical energy into heat instead of potential power. Gel-cells lose 10-16% but AGMs as little as 4%. The higher charge efficiency of AGMs can contribute to significant savings when it comes to the use of expensive renewable energy sources (wind generators, solar panels, etc.) as your charging system can be 15% smaller (or just charge faster).

While flooded cells lose up to 1% per day due to self-discharge, VRLAs lose 1-3% per month. Why employ a solar charger to trickle-charge your battery banks if you don't have to?

High vibration resistance: The construction of AGMs allows them to be used in environments where other batteries would literally fall to pieces. This is another reason why AGMs see broad use in the aviation and the RV industry.

Absorbed Glass Mat Batteries (A.G.M’s) are still lead-acid batteries but are sealed instead of vented. The electrolyte is held captive in a fibrous glass mat that can't be spilled and therefore can be shipped without hazardous material restrictions. This glass mat also provides pockets that assist in the recombination of hydrogen and oxygen gasses (that are generated during charging) back into water.

We found their charge acceptance to be greater than flooded batteries and it requires significantly less time to recharge these AGMs. This translates into higher efficiency which means a shorter generator run time or charging times, when you find it necessary to recharge quickly. They also hold up better and at a higher voltage when heavy loads are powered by them. Like when you fire up the microwave through the inverter.

These batteries have very thick positive plates and belong in the true Deep Cycle class. They don't outgas (unless severly overcharged) and because of this, don't corrode terminals and don't need to be watered. The savings in maintenance alone will be worth the extra cost to some.

Developed initially for military applications, AGM (Absorbed Glass Mat) batteries are now widely used in 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, motorhome and boat 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 or boat 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 or boating.

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.

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.

If you have any further questions regfarding our batteries please do not hesitate to call us. We stand by our product unconditionally and believe that our batteries are the best value for money and the safest batteries available on the market today.

Rohan Lewis
Jamie Lewis