A Detailed Guide to AGM Batteries

Introduction

With the recent explosion in Campervan, Motorhome and Caravan holidays, the demand for high quality leisure batteries has soared. With this has come the ever increasing need for using batteries off-grid for wild camping trips and more frequent use. Customers disappointed with their old wet lead acid batteries have been looking to get more juice from their battery bank with a zero maintenance and low gas option that can withstand regular discharge and recharge.

What Does AGM Mean?

AGM stands for Absorbent Glass Matt. AGM batteries are still fundamentally lead acid batteries, but the electrolyte (acid) is soaked on absorbent fibreglass mats between the lead plates inside, rather than traditional liquid acid. You will also notice that AGM batteries are rated over three capacity discharge times C10, C20 & C100. This relates to the power available in the battery over 10 hours, 20 hours and 100 hours. Designed for multi-application use the power available will very much depend on how quickly you discharge the battery. From this common base AGM batteries come in three different forms:

AGM Type 1 – European

These batteries are used in multi-application environments such as modern stop-start vehicles and deep cycle applications. Great for starting engines and off grid use with a typical cycle life of around 300 – 350 cycles. It’s worth pointing out that ‘life cycles’ don’t mean the battery expires and needs replacing, rather it refers to the optimum power of the battery life. AGM 1 batteries are limited in size but are popular for the low height fitment demands in Campervans, Caravans and Motorhomes. The Expedition 105AH Platinum AGM is a good example of an AGM 1 battery.

AGM Type 2 – Eastern

AGM 2 batteries are the more cyclic type with a typical max cycle life of between 500 – 700 cycles. They are generally recommended for regular off grid use as they have a higher cyclic ability to discharge and recharge over a long term period. In addition, they are less prone to sulfation and charge much more quickly than traditional batteries – if you charge them correctly, more on that later! A good example of an AGM 2 battery would be the Expedition 130AH Plus AGM.

Lead Carbon AGM

By adding carbon graphene to the traditional AGM Type 2 battery you get Lead Carbon AGM. This adds further improvements to charging times and a typical cycle life of 1200 – 2000 cycles depending on the model. Lead Carbon AGM are becoming more and more popular and are often the last step along the AGM range before looking into Lithium based solutions. The Leoch 115AH Carbon is one example of a Lead Carbon AGM battery.

Charging AGM Batteries

Despite what you may have been told, all 12v batteries are not the same! Wet Lead Acid, AGM, Carbon AGM and GEL batteries all charge and behave differently. To get the full benefits from an AGM battery it takes a little bit of research and time to check your existing charging method each step of the way.

Most Caravans and Motorhomes up until 2017 come factory fitted with non-AGM charging systems and to make things worse some manufacturers will state that they ‘may be suitable’ for charging AGM batteries – they simply aren’t, but why? If you take a look at the datasheets and technical specifications for AGM batteries on our website, you will see that we publish specific charge settings and this ultimately centres around voltage. AGM batteries are designed to require 14.6v – 14.8v when performing a full charge from a state of discharge, whereas standard lead acid batteries need 13.8v – 14.4v, hence this is what most onboard chargers are designed to deliver – not the 14.6v – 14.8v that AGM requires.

Let’s go into a little more detail and break-down the most common charging options on Campervans, Motorhomes, Caravans and Boats and point out what you need to look for:

Mains Chargers

Onboard you will, in most cases, have a mains powered battery charger / power management system. These are designed to be used when you have access to mains power to replenish the state of charge in your batteries or maintain them when not in use. Given that these chargers are generally the main and best method of charging, you need to make sure that you know what it can and can’t do when it comes to charging AGM.

You’ll need to check the make and mode of your onboard charger – if you have the owner’s manual check it as it should tell you in there – you are looking for specific charge programmes for different 12v battery technologies. If it says nothing and the charger has no selectable charge options, then you can assume it is designed for traditional wet lead acid batteries not AGM.

Furthermore, if your manual states ‘may be suitable with AGM check with your battery supplier’ and has a max charge voltage of 14.4v then this isn’t suitable for AGM either. We are aware that most Sargent onboard chargers and PSU’s state this – please take our advice, they are not suitable for AGM in the long term. Good quality modern onboard chargers have selectable charge profiles that support AGM and it’s important to check this first before investing in quality AGM batteries as battery warranties do not cover incorrect charging profiles.

My charger doesn’t support AGM but I want the benefits from AGM technology. What can I do?

All isn’t lost here. You can still use the onboard charger but you will need to supplement the undercharge from the non-AGM charger with a secondary mains charger at least once a month, as your non-AGM charger will only recharge the battery to around 85% and this will cause a memory effect in the battery reducing its life. The cost to replace the onboard power system for an AGM one will be costly, so using a secondary charger is more cost effective and will prolong the life of the AGM battery by getting the battery up to the all important 14.6v – 14.8v it needs.

Secondary chargers are great, you can use them anywhere when you have access to mains power and are a sure fire way to ensure your battery is getting the correct charge it needs. We recommend Victron Energy smart chargers which have dedicated charge profiles for AGM batteries – they are fully automatic and do everything that the battery needs. The size you will require depends on the size of your battery bank but Victron offer a range of solutions such as 7A, 10A, 15A.

Alternator Charging / Split Charging

Similar rules apply to split charging AGM batteries and standard alternator split charging by common durite systems are not optimised for AGM – they simply divide the preset wet lead acid starter battery voltage and apply it to the secondary leisure battery or bank. Again, like mains chargers this will be a typical voltage of around 13.8v – 14.4v and over time this will cause under charge to the AGM battery.

We are big advocates of battery to battery chargers also known as DC/DC chargers when it comes to split charging AGM batteries as they have specific AGM charge profiles and are simple to fit. Once programmed for AGM it takes the standard alternator charge voltage from the starter battery and converts the settings to suit an AGM battery ensuring a true and even charge. Some even come with solar charge controllers built in and can act as a single charging hub for your system – effectively acting as a wild camping mains charger when you have no mains power! Some great examples here are the Ring RSCS30 and Victron Orion.

Similarly as with mains charging, if your system doesn’t support AGM then it’s not the end of the world, you can use your existing set up, just make sure you compensate for the undercharging via an AGM optimised mains charger or solar charger if you have enough watts.

Solar Charging

Solar has become an ever popular method of charging batteries though we would never advise relying on solar alone as your main charge method, though they do offer benefits when used for maintenance or top charging. AGM batteries work well with solar and are widely used on solar systems. Most if not all solar charge controllers support AGM however the terminology varies when it comes to solar controllers and they may refer to AGM as ‘sealed’ – always check your manual and ensure that the AGM or sealed charge profile gets up to that magic 14.6v – 14.8v parameter.

Something to note – we have seen instances were charge controllers on AGM settings utilise whats known as equalisation mode. We would advise removing this setting on solar charge programmes as it can over charge the battery at more than 15v! Not good. Solar charging can be utilised to compensate for undercharging from your other, non-AGM optimised, methods of charging. If your standard split replay or mains charger is designed for wet lead acid up to 14.4v then you can let your solar panel get the AGM battery up to the 14.6v – 14.8v it needs.

Maintaining AGM Batteries

As with all batteries correct maintenance is important – particularly if it’s unlikely to be used for an extended period of time. With that in mind there are a few core points to follow:

1. Ensure the battery is fully charged before being left. A fully charged AGM battery should read 13.00v or above. Ideally you need to ensure the final charge is done via an AGM optimised charging profile.

2. Never leave a battery on permanent charge (including trickle!) – despite having no free flowing acid inside, long term charging will cause the battery to dry out. Once fully charged and stored just ensure you charge the battery once a month overnight instead.

3. Try to keep your max discharge level to 50% – this is 12.1v on AGM batteries – as our cycle life data is based on a 50% discharge level. You can technically take AGM batteries lower than that, but the cycle life will reduce accordingly. The opposite is also true – discharging above 50% will increase the cycle life.

Note: Remember, the cycle life is a guide on the optimum life span of the battery – it doesn’t mean the battery will pack up and stop working – but you will notice considerably less running time once the cycle life has been achieved. Battery warranty does not cover loss of capacity as this is subjective to the usage, the frequency the battery is under and whether it’s been correctly charged.

If you are looking for true deep cycle batteries designed for regular discharge and recharge then AGM products can offer all the benefits you require. No topping up with water, non-spill, quicker charging and long life. If you can invest some time in getting your charging right, then you will see much better performance over traditional wet lead acid types – something we have first hand experience of over many years – low return rates and minimal warranty issues speak volumes for AGM batteries.