Charging AGM batteries
Recharge AGM batteries correctly
Did you charge AGM batteries correctly? AGM (Absorber Glass Matt) batteries are currently used as a standard battery, next to the extremely expensive LiFePo4 battery.
I can also advise you to use these batteries when you are planning your camper or when the old batteries are worn out. If you keep a few points in mind, AGM batteries also last a long time.
Here I want to tell you what you should pay attention to, what size your AGM battery should be for your campervan and what little helpers there are.
Basics about recharging AGM batteries
In order approach the subject properly, we need to look at a few small aspects. We want to do this that your AGM battery is not only charged correctly, but also its service life is extended through correct handling.
I have already written some things in my post on batteries. To understand everything, you should definitely take a look at the post.
Capacity
A lead-acid battery, which also includes AGM technology, should have a max. Remove 20 – 40% of the total amount in order to achieve the highest possible number of cycles. Quite simply, with a total capacity of 100 Ah you shouldn’t take more than 20 Ah to 40 Ah before you recharge your battery.
If your solar system recharges every day, the whole thing will look different of course.
I have already put together some information for you on the subject of solar systems. {english version coming soon}
Installation of an AGM battery – not upside down
The installation of an AGM battery is only limited by its own dimensions. Otherwise you can install them in any position. The only thing to note is that you should not stack the batteries on top of each other.
Temperatures for charging the AGM battery
If you store and use the motorhome battery at around 20 degrees, you will never have problems. Most chargers also have temperature sensors that adapt the charges to the ambient temperature.
Cold is of course not optimal, but AGM batteries can cope with cold best, which is also reflected in the cold start properties.
But you should be careful at temperatures above 50 degrees. The batteries do not tolerate this well and you should make sure to get them out of the heat.
Charging procedure for AGM batteries
Fortunately, today we are at a state of the art technology which is very forgiving. Most chargers have the correct charging characteristics saved for the different battery types.
IU characteristics is standard when using an AGM battery.
If there is IUoU characteristics, it is better.
The end-of-charge voltage at which the batteries begin to outgas is also important. With an AGM battery, you should make sure that the charging voltage never exceeds 14.8 V. Check this with your voltmeter directly at the poles of the battery. This way you can quickly find out whether the charger you have purchased is working properly.
IU characteristic for AGM battery
The IU characteristics or the CCCV (constant current constant voltage) method charges the battery with a constant current and a slowly increasing voltage during the charging phase.
The charger now continuously measures the current and voltage values during the charging process. When the end-of-charge voltage is reached, the current drops. The charging continues with a low current and a constant voltage.
A IU characteristic charges during the main phase with a constant current and a slowly increasing voltage that remains constant after a certain charging time. After reaching the charging voltage, the current drops and a low current is used to recharge.
IUoU characteristic for AGM batteries
IUoU characteristic works in the first steps in the same way as the IU characteristic described above. Here, however, when the nominal charging voltage is reached, a switch is made to trickle charging, which is intended to counteract the self-discharge of the battery.
The trickle charge takes place with 13.8 V and a lower current. Since the self-discharge of a battery strongly depends on the temperature, chargers with a temperature sensor are recommended.
If the voltage deviates from the nominal charging voltage, the chargers will normally start over.
Charge your AGM motorhome battery via the solar system
Most solar charge controllers have now also saved all the necessary characteristics. Since the characteristics are also implemented sensibly, you need a certain current or sufficient voltage.
Since what your controller can deliver depends very much on the weather with solar, we can only recommend an MPPT charge controller. This ensures sufficient voltage even when there is little sunlight and ensures that the battery is fully charged. Here I can only advise against cheap equipment.
Charge your AGM battery with the alternator
That is also possible and even useful. While driving, the alternator runs and electricity is generated. Often more than the vehicle’s starter battery requires. Here you will find cheap cut-off relays on the market – the starters – and your AGM supply battery will also charge while you are driving.
If you are standing, the relay disconnects and you only draw power from the AGM supply battery.
Sounds reasonable at first. Unfortunately, this solution is not exactly ideal – the alternator usually has the task of charging your starter battery and that’s what it’s designed for.
Thus it delivers a certain voltage and a certain current. An AGM needs e.g. but a completely different charge voltage than a lead-acid battery.
We can also assume that you draw more current from the supply battery than the starter battery every day – but if it is full while driving, the voltage and current drop.
The result is clear – the second battery will not be full. Your AGM battery would literally starve to death.
In order to properly charge an auxiliary battery with the alternator, you need a 12V charger or a so-called B2B charger. Here you can connect several batteries and also select the right battery types.
Recommended charging booster for the battery via the alternator:
In this way, correct charging curves are used and all your batteries are charged correctly and, above all, fully.
4 tips for charging AGM batteries
- Use the correct battery charger settings for your battery type and observe the installation instructions.
- Do not use more than 20% to 40% of the total capacity of your batteries.
- Measure with your voltmeter whether the end-of-charge voltage is not exceeded and thus an outgassing is prevented.
- Use IU characteristics, even better IUoU characteristics.
If you follow these four rules, you will be happy with your AGM motorhome batteries for a long time. And even if they lose their capacity after a few years, that is no reason to replace the battery immediately. Keep using it until you can’t. This way you have optimally used the battery and are also kind to the environment.
