Advantages & Disadvantages A.G.M. Batteries

AGM RV Battery UK
During the 1980s the aircraft industry developed the A.G.M. battery. This was an alternative to the then very expensive Ni – Cad batteries, which were used in naval helicopters and fighter jets. The A.G.M. battery was originally designed to be warm weather and vibration resistant.

A.G.M. is the abbreviation of absorption glass mat. It was given this name because these batteries are made up of mats, in which the glass fibres are woven to increase the surface area. This then allows the A.G.M. battery to work like a lead acid battery, although the electrolyte in the A.G.M. battery will not freely flood the plates, as the glass mats will prevent this happening.

These glass fibre mats do not absorb the electrolyte, but simply hold the electrolyte preventing it from spilling when the battery is tipped over. Although the A.G.M. battery is seen as a rectangular shape case, the plates themselves can be changed into any shape, including flat or even cupped.

As manufacturing processes have become cheaper, these Deep Cycle Batteries (Leisure Batteries) are now used in Motorhomes.

Advantages of an A.G.M. Battery

AGM Battery
1. Unspillable. This means if you turn the battery upside down, the electrolyte will not spill out of an A.G.M. battery.
2. An A.G.M. battery never needs any water, and is considered to be maintenance free.
3. A.G.M. batteries do not give off any gas (hydrogen) when being charged, so there is less concern about ventilation when charging them.  This allows them to be used safely inside a motorhome.
4. A.G.M. batteries can stand very low temperatures without freezing.
5. These batteries are also very resistant to vibration, due to their sandwich construction.

Disadvantages of an A.G.M. Battery

1. The A.G.M. battery is considerably more expensive.
2. The deep cycle A.G.M. batteries can only be discharged to 50%, whereas the lead acid battery can be discharged to 80%. This means that the lead acid battery can run appliances longer each single charge.
3. An A.G.M battery should not be overcharged, as it will shorten its life or can immediately kill the battery.

Before you all go out and replace your leisure batteries with A.G.M. ones, there are a few other things you should know.

The charging of A.G.M. batteries is different to that of the lead acid battery. The voltage required to charge them is greater, therefore you will need to replace the original motor home charger if it is not programmable. Even  if this is the case, you will then need to fit a devise that will increase the charge voltage from your engines alternator. This would then enable you to charge the leisure batteries whilst travelling at the required greater voltage.

State of Charge12 Volt BatteryVolts per Cell
100%12.72.12
90%12.52.08
80%12.422.07
70%12.322.05
60%12.202.03
50%12.062.01
40%11.91.98
30%11.751.96
20%11.581.93
10%11.311.89
0%10.51.75

Is it the battery or is it the charger?

Frequently when LAS are servicing a motorhome, the owner will ask us to check the batteries.  This is then followed by “The leisure batteries don’t last very long before they go flat, we only watch the T.V. for a few hours with one light on!”  The problem more often than not is the battery charger.

Many American Motorhomes in America are left plugged into an external power source continuously, therefore the manufacturers fit a charger that will not allow the acid in the battery to evaporate, and this also means that no topping up is required.

If you unplug your American R.V. drive it to your holiday destination, then plug it back into another external power supply, you will never know if your batteries are suffering. It is only when or if you do wild camping (this is without an external power supply), that you will begin to notice the short amount of time to discharge your batteries.

We will set out to explain some of the charge and voltages that relate to a 12 volt battery. A 12 volt battery is deemed fully discharged at 10.5 volts at a temperature of 77 degrees F. Below is a table of the state of charge, in relation to the voltage.

At the bottom of the table you can see the state of charge 0, and the voltage of 10.5 as is stated above.  If you find that your battery is below 10.5, it may require a recovery charge or replacing.

Many American motorhome chargers will only charge at 12.5 volts, which is suitable for running12 volt bulbs, but as you can see from the chart this will only give you 80% of the charge when allowing for the potential difference. This shows that an American motorhome requires a good charger to look after its batteries.

The charger needs to be able to carry out a bulk charge, an absorption charge, and also a float charge. These three charge methods require performing at the correct voltage, or the process will be deemed useless.

Bulk Charge

This is the first stage of charging and shows what a fully discharged battery will go through. The battery charger fitted to your R. V. should have a maximum charge rate of 10% of the total battery, or the batteries capacity.  4 x 100 amp hour batteries will require a 40 amp charger.

If we use the above as our example the charger will charge batteries at 40 amps current, until the voltage rises in the battery to 80 – 90% fully charged.  As you can see from the table, the voltage will be between 12.42 and 12.5. To allow the battery to reach this voltage, the charge voltage will be around 15 volts. There is no correct voltage for bulk charging, that is why the 10% of total battery capacity is important.  It is essential not to supply the battery with too greater current and damage it.

Absorption Charge

This is the second stage of the battery charging. At this stage the voltage remains constant and the current gradually tapers off, as internal resistance increases during the charging process. It is during this stage that the charger puts out maximum voltage.  Voltages at this stage are typically around 14.2 to 15.5 volts.

Float Charge

This is the final stage of the charging. After the batteries reach full charge, the charging level is reduced to a lower level (typically 12.8 to 13.2 volts).  This reduces gassing and prolongs the battery life. This stage of charging if often referred to as a maintenance or trickle charge, since its main purpose is to keep an already charged battery from discharging.

If your American motorhome has a good charger fitted, you should get 4 to 6 years battery life, but often you will find that the poor battery chargers will require you to change the batteries about once a year.

As batteries age, their maintenance requirements also change. This means longer charging times and/or higher finish rate (higher ampage at the end of the charge).  Usually older batteries need to be watered more often, and their capacity decreases.

If you have any questions on batteries or battery chargers, do not hesitate to contact LAS Motorhomes on 01604 861999.