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Basic AGM battery management information

Battery Basics explained

A battery is a storage vessel for energy, so whatever you take out, you must put back, preferably as quickly as possible. The worst thing you can do to a battery is, extracting all the energy and then leave it flat for a long period of time. That could wreck the battery completely in just a couple of weeks.

We have seen many motor-home owners that complain because their batteries are playing up, usually they do not have a solar system or a proper charge system that tops up the batteries correctly, and mostly they have no, or poor quality battery monitoring systems in place and they have no idea what damage they are doing to their house batteries.

How do I know what the condition of my RV or Motor-home house Batteries are?

It is, however, possible to tell the state of charge of the batteries by the voltage on a digital display alone:
For example, with a 12-volt system, 11.9 volt is a dead flat battery and 12.7 volts is a fully charged battery at rest. The difference is only 0.8 volt. That is why a digital display is required. Then you must also be aware that such a small voltage difference can be lost in resistance through the wiring of the motor-home. If the voltage measures 12.7 at the battery, it could easily measure less than 12 volts at the point where the energy is being consumed, especially if the cables are undersized. So the question is: What is your correct battery voltage? Where have you measured it? What else is going on in the system? Are your cables able to carry the load? What is the condition of your cables and connections?

How do I know the batteries are charging properly?

Assuming all is well with your system, you can rely on the following measures: If you read a battery voltage at 13.7, with a good digital meter, it means that the battery is being charged, through solar, alternator, or a 240-volt charger. Disconnect all charge systems, and the voltage should go down to 12.7 when at rest (nothing going in or out), a battery voltage lower than 12.7 means the battery is not charged properly yet or is damaged.

A voltage of 14.9 during the daytime is usually just the solar system on its final stage of the charge. This happens for about 90 minutes per day. If the voltage of 14.9 persists for more than 90 minutes, the system is faulty and your batteries may get damaged by overcharging.

So if you suspect something is wrong with your batteries, disconnect the solar panel, or put a sheet of cardboard over the panel before checking the batteries. We have seen many "so-called experts" test the house batteries with the solar system still connected. The solar system will skew the results of the test, and this gives good readings on a bad battery.

What sort of Solar System would I need for my RV, Motor-home or Caravan?

A customer once told me that he had the perfect solar system and that he never ran out of power, naturally, I was very interested because I have not found the perfect system yet. I asked him exactly what his system was made up of and how he was using it.
First thing he said was, I have an 80-watt solar panel, and that is more than plenty. I can go weeks without plugging in. Well, how is that possible? a small panel like that can only produce about 5 or 6 amps at 12 volts in perfect conditions for a maximum of say 6 hours per day. He was watching TV, running lights, pump etc and must have been using more energy than the panel can produce, I questioned him a little more.

He had the following in his motorhome:
  1. 80-watt solar panel and controller
  2. 240 AC volt charger
  3. Engine alternator, size unknown
  4. 800 amp/hrs of battery capacity
The key factor here is the size of the batteries, what really was going on here was that the batteries were storing all the energy when he plugged the motor-home in at home, the system would be slowly running down even though the solar panel was trickle charging the batteries, in reality, he was using mostly stored energy, not solar energy. The storage capacity of the house batteries was extremely big and heavy (weighing about a quarter ton) and costing about $2800.

The ideal system is balanced with sufficient solar power to produce enough energy, even in poor weather conditions.

A good comparison is to imagine that your batteries are like the rainwater tanks at home and that the roof is like the solar system. If you have a large roof and small tanks, you cannot benefit from the large surface area of the roof because you do not have enough storage capacity in your tanks, but the tank will fill up fast when it does rain, however, the tank will not serve you well during long periods of dry weather.

If you have large tanks with a small roof then the tank will take forever to fill up, and might never become full.

Solar Systems in relation to Solar Fridges

The solar system should have enough capacity to be able to recharge the batteries run the fridge all day long, even in poor weather conditions. At night the fridge runs on the battery which is then recharged the following day. If it is not possible to have large solar panels, then alternative charge systems should be in place, or the battery capacity should be increased. Batteries need to be charged properly at all times. It is good practice to maintain the battery state of charge above 80%. Poorly managed batteries may only last 12 months, whereas well maintained batteries can last 5 years or longer.
The less your deep cycle battery is discharged before recharging again, the longer it will last.

The cycling ability of an AGM battery:

Below is an example of the typical life cycle of a battery based on the amount it is discharged daily.

  1. 10% discharge before recharging on a daily basis = 3800 cycles = approx 10 years
  2. 25% discharge before recharging on a daily basis = 1500 cycles = approx 4 years
  3. 50% discharge before recharging on a daily basis = 650 cycles = less than 2 years
  4. 100% discharge before recharging on a daily basis = 300 cycles = less than 1 year

It is good to always good to avoid discharging your batteries no more than 20%

If by now you have decided that it is time to replace your batteries and you need to decide what batteries are best, then you might consider buying Lithium batteries instead of AGM batteries. Lithium batteries can be discharged almost completely unlike AGM batteries, they also last much longer and are designed to be cycled much harder than AGM batteries

We hope this has been helpful for you

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