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Article by Val Rigoli ©, free advice freely given, my personal thoughts and advice gathered from in-excess of 40 years of practical hands-on experience, learned skills, and industry knowledge.
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AGM (Absorbed Glass Matt) batteries can have many advantages over conventional deep cycle batteries.

Hopefully with all the reading and research you have already done you will have come to an understanding that deep cycle batteries are the best way to go when it comes to auxiliary batteries, but for many people AGM (Absorbed Glass Matt) batteries will have huge advantages over the conventional wet cell deep cycle battery.

Although this may appear a little long winded, it is well worth understanding the virtues of both types of batteries, and how you can put them to best use, I won’t go deeply into the technical details of these batteries, just point out some of their advantages, and why.

Basically it starts with how you intend to recharge your batteries, I am talking about your main recharging source here.

If you are going to be using any of the smart charging solutions, like solar panels and a quality smart regulator, or a smart 3 stage mains (240v) battery charger, then in most cases the conventional wet cell deep cycle battery can serve you well.

If treated correctly (and few are) conventional deep cycle batteries can outlast AGM’s, but only if they are well maintained and cycled within their design perimeters, with both correct discharging/charging and equalising (now that a whole other story) and with keeping electrolyte (water) levels up.

However if your main source of charging is from your vehicles alternator, or you need to place the batteries in places that are less than optimum for safe battery storage, keeping in mind that wet cell batteries give off hydrogen (explosive) gas as they are charging, then you should consider using AGM batteries as they are totally sealed and spill proof.

Did you know that if our good quality wet deep cycle batteries have been discharged fairly deeply, it can take 8-12 hours of continuous engine running to achieve just 70-80% charge?

A bit about Alternators.

Standard car and truck alternators are designed, for various good reasons to recharge car batteries to only 70-80%, and this works very well for normal cars and trucks, but when it comes to charging our storage batteries, where of course we need as full a charge as possible, that same 70-80% is not very helpful at all.

Really, standard alternators are designed primarily to top up quickly the surprisingly small amount of energy that is removed from the battery by the starter-motor on engine start up, and then to keep up with all your accessories, radios, head lights etc., they were never designed to recharge deeply discharged batteries, and they fall short in this area unless modified or replaced with a specialised charging alternator, and smart multi stage regulator.

Automotive battery chargers suffer from much the same less than perfect regulation as car alternators, and achieve much the same results, and take a much much longer time to do it.

As an example lets imagine you have a 100 Ah (Amp hour) battery, and you main charging source is you vehicles alternator or a standard automotive battery charger, now both of these will only charge this battery to about 75% (75 Ah), and you should not discharge your batteries below 50% (50 Ah in the case of this 100Ah battery) of their capacity (see note below) if you want them to last, so all you can really safely use of this 100 Ah battery is 25 Ah, not much eh?

 

Note. Have you heard that you can drain deep cycle batteries all the way down, and then recharge them?

Have you been told that this is what deep cycle batteries are all about?

I hear this almost everyday that people have been told this, and by some battery sales people that should know better.

This line of thinking is totally wrong.

All conventional deep cycle battery manufactures say that their batteries are designed to be cycled to 50% of their rated capacity!

So each and every time you take these batteries below this 50% you are shortening their lives, and if you do it regularly and deeply you will even more dramatically reduce their lives.

 

Ok OK !! I can hear the cry's from here "but I get many more amp hours out of my deep cycle battery than what you say I can!".

The truth is that most people discharge their batteries far too low, way down until their fridges cut out, or their lights start to dim (does this remind you of anyone ??), at this point they have discharged their batteries so low that they are doing serious damage to their batteries, and consequently won't get anywhere near the life out of their batteries that they could and should normally expect if the batteries were cycled within their design perimeters.

Now remember the 100Ah battery with only 25Ah usable? A huge gain can be had if you are using any of the smart charging solutions, like solar panels and a quality smart regulator, or a smart 3 stage mains (240v) battery charger,  you can expect close to 100% (100Ah) charge,

So 100 Ah capacity, discharge to 50% (50Ah), that’s now 50Ah usable, twice what you had before!

Now lets look at some of the direct advantages of these AGM batteries.

these batteries are totally sealed, ‘transport’ classed as spill proof, never needing topping up with water, ever!

because of this they can be mounted inside a car, caravan, motorhome etc and only need to be vented to atmosphere, they do not need to be in a sealed box vented to the outside like wet batteries, and can be mounted on their sides or ends if more practical for your needs.

because of their very low internal resistance these batteries will fully charge at a lower voltage, and accept a much larger charge current, so when charging from a standard car/truck alternator these batteries will all but fully charge, and fast too, in about 2.5 to 3 hours! (with the correct cabling and dual battery system->>click here for details<<)

they can occasionally be discharged much much deeper than conventional deep cycles without major damage.

these batteries when left unattended only self discharge at the rate of up to 3% per month, and even after 12 months sitting idle can be recharged and put back into full service without any ill effects. On the other hand a standard deep cycle battery if treated the same way will have destroyed itself, it will no longer hold a good charge, and is sadly ready for the rubbish tip and recycling, not cycling!

AGM batteries were originally developed for the military, they are very robust and will take a real pounding.

The original AGM batteries were made in the US, brands like Odyssey, Concord, Lifeline, all excellent batteries, used extensively also in the aircraft and in the motor racing fields, but hugely expensive, there are now a lot of these (AGM) types of batteries coming out of China, and some are just rubbish.

My battery supplier (ALCO (now enirgi) Battery Sales Australia) have found a couple of company's over in China that has been in production for many years, and have been selling batteries into the European market for a long time and have built up very good reputations, ALCO now import these batteries into Australia and are selling them through their own outlets and dealers.

These AGM valve regulated batteries (also called SLA or Sealed Lead Acid) offer very good value, and when compared to standard deep cycle batteries in a dollar per usable amp hour format, they come out miles ahead, plus all the added advantages that I have already outlined, but there is still more.

 

Lets look at it in dollars and cents. (please note, as battery prices are changing almost on a daily basis, please use this as rough guide only)

Using a 100 Ah deep cycle wet cell Trojan (a better wet cell battery),  priced around $325.
Usable capacity if charged with smart charging system............................ say 50Ah, cost $6.50 per Ah

Usable capacity if charged from the alternator or auto charger only........say 25Ah, cost $13.00 per Ah

Using a 100Ah AGM Battery, priced at around $250.  Usable capacity if charged with almost anything........
lets say you dive deap @ 75Ah, cost $3.33 Per Ah,
but better still using a more conservative 50Ah only, at a cost of $5 Per Ah, and with all the advantages that comes with an AGM Battery!

Now lets look at the space saving

Lets say you needed 100Ah usable capacity from your batteries, and your main charging system was from the vehicles alternator.

Now with conventional deep cycle batteries because you would only have around 25% of your battery bank usable you would need 400Ah of battery bank, now there are a few ways you could do this, but the least expensive way would be four x 225Ah 6 volt Trojan batteries at a cost of around $1,120.

Now with AGM batteries you could have just two x 100Ah giving you 200Ah total, so there is your 100 Ah with heaps in reserve and an easy life for the batteries, so only 2 batteries, at a cost of $500 all up, or much better still, you could use a single 220Ah battery for a cost of still about $460. Even a 300Ah battery these days can be bought for around $550, WOW!

So with space and money saved, and with the many other advantages of AGM batteries, they really are a very good option for most people.
Personally I will never muck about with messy, cantankerous, wet cell batteries ever again for my own needs.

   I recommend AGM batteries as the best value for money batteries at this time.
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 Please also check out my Batteries Page->>click here<<.

Please note, my comments in this article are for batteries that are to be used in recreational vehicles etc, and are mostly used for weekends away and a few weeks a year, AGM's used in this way have a design life of 5 to 8 years. All AGM batteries are deep cycle by the very nature of their build, however.....  I recommend for RV use, it is best to use dedicated DC (deep cycle) AGM batteries that have been built with extra thick lead plates, to give even better performance and service life (8-12 years). Note:- If your battery bank will be in constant daily discharge and charge cycle use all year round, then your battery bank should be sized so that your daily depth of discharge is about 20 to 25%, this applies to all types of deep cycle batteries if you want them to last for many years.

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