The aircraft industry developed the A.G.M. battery during the 1980s as an alternative to the very expensive Ni-Cad batteries, which were used in naval helicopters and fighter jets at the time. The A.G.M. battery was originally designed to be resistant to warm weather and vibration.
A.G.M. is the abbreviation for absorption glass mat. It was given this name because these batteries consist of mats, in which glass fibers are woven to increase the surface area.
These glass fibre mats are designed to hold the electrolyte in place and prevent it from spilling out when the battery is tipped over. Although A.G.M. batteries are typically rectangular in shape, the plates inside can be changed to any shape, including flat or cupped.
Advantages of an A.G.M. Battery
- A.G.M. batteries are unspillable, meaning that if you turn the battery upside down, the electrolyte will not spill out.
- You will never need to add water to an A.G.M. battery, and they are considered to be maintenance free.
- When being charged, A.G.M batteries do not give off any gas (hydrogen), so there is less concern about ventilation. This allows them to be used safely inside a motorhome.
- A.G.M batteries can stand very low temperatures without freezing.
- These batteries are also very resistant to vibration, due to their sandwich construction
Disadvantages of an A.G.M. Battery
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, in theory, the lead acid battery can run appliances for a longer time on a single charge.
- The A.G.M battery is more expensive than the lead acid battery.
- The deep cycle A.G.M batteries can only be discharged to 50%. On the other hand, the lead acid batteries can be discharged to 80%. Consequently, the lead acid battery can run appliances for a longer time on a single charge.
- You should not overcharge an A.G.M battery, as it will reduce its lifespan or damage the battery immediately.
Agm deepcycle battery are a great investment for your recreational vehicle, but there are a few things you should know before you replace your lead acid batteries with them. AGM batteries require a higher voltage to charge, so you’ll need to replace your original motorhome charger with a programmable one. Even if your alternator is able to charge the battery at the required voltage, you’ll need to fit a device that will increase the charge voltage. This way, you can charge your leisure batteries while travelling.
Is it the battery or is it the charger?
We frequently get asked to check the batteries when we’re servicing a motorhome. The owner will usually tell us that the leisure batteries don’t last very long before they go flat, and they only use one light while watching TV for a few hours. More often than not, the problem is with the battery charger.
Many American motorhomes are left plugged into an external power source continuously, so the manufacturers fit a charger that won’t allow the acid in the battery to evaporate. This also means that no topping up is required.
If you unplug your American R.V. from its power source and drive it to your holiday destination, then plug it back into another external power supply, you won’t be able to tell if your batteries are being damaged. It is only when or if you go camping without an external power supply (wild camping), that you will begin to notice how quickly your batteries discharge.
We will explain some of the charge and voltages that relate to a 12 volt battery. A 12 volt battery is considered fully discharged at 10.5 volts at a temperature of 77 degrees Fahrenheit. The table below shows the state of charge in relation to voltage.
Bulk Charge
This is the first stage of charging and it is designed to show what a fully discharged battery will go through. The battery charger that is 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 in the battery rises to 80 – 90% full charge.
Absorption Charge
This is the second stage of the battery charging process. At this stage, the voltage remains constant and the current gradually tapers off as internal resistance increases during charging. This is when the charger puts out maximum voltage. Voltages at this stage are usually between 14.2 and 15.5 volts.
Float Charge
This is the final step in the charging process. After the batteries have reached full charge, the charging level is reduced to a lower level (typically 12.8 to 13.2 volts). This helps to prevent gassing and prolongs battery life. This stage of charging is often referred to as a maintenance or trickle charge, since its main purpose is to keep an already charged battery from discharging.
