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Deep Cycle Batteries for RVsEntire books have been written on the subject of batteries, however, we will be discussing only the types of batteries currently used in modern RVs. And to narrow it down even further, this page will only be covering Flooded, Lead-Acid Batteries in detail. It is our opinion that the "gel-cell" type of batteries have proven inadequate in deep cycle environments and we don't recommend them.

There is a new type of sealed, lead-acid battery called an Absorbed Glass Mat (AGM) that has proven itself to be worthy of consideration. We've been using Lifeline AGM batteries for the last decade and have a report at the bottom of this page.

Now on to the discussion of Flooded, Lead-Acid Batteries.

How Batteries Work

   Fully charged batteries have negative plates consisting of sponge lead (Pb) and positive plates consisting of Lead Dioxide (PbO2). These plates are submerged in an electrolyte solution of Sulfuric Acid (H2SO4). In laymans terms, when you are discharging a battery, the sulfur leaves the solution and moves over to the lead plates. When it gets there, it gives up an electron and moving electrons are electricity. When you are recharging a battery, you are driving that sulfur back into the solution.

What Affects the Longevity of Batteries?

   Overcharging batteries on a regular basis causes water to be 'boiled' out of the electrolyte and will eventually warp the plates from over heating and corrode away the positive plates,

   Failing to completely recharge a battery will leave some of the sulfur behind on the plates creating plate sulfation which causes the storage capacity of the battery to fall off,

   Adding anything other than pure distilled water to the electrolyte will introduce impurities that will cause adverse chemical reactions and interfere with the normal functioning of the battery.

   Cycling your batteries deeply (80 to 100%) on a daily basis shortens their life. Using only 25 to 50% of the storage capacity of your batteries will put less strain on them and increase their useful life.

What is a Deep Cycle?

   A deep cycle is when you start out with a fully charged battery, use 80% of its rated capacity, and then fully recharge it. Depending on the type, all batteries have a certain amount of deep cycles they can withstand before they need to be replaced.

Types of Batteries

   Starting or 'Cranking'
   This type of battery is designed with many, very thin plates so that there is a lot of surface area exposed to the electrolyte. This design allows the battery to give up a large amount of power in a short period of time and also allows it to be recharged rapidly at a heavy rate. Cranking a big V-8 engine requires this kind of power. It is, however, a very poor choice when used as a 'house' or 'domestic' battery as it can only withstand 15 to 30 deep cycles before it needs replacing.

   True Deep Cycle (available in Flooded and AGM styles)
   These batteries are designed with relatively few, thick plates. This design allows a small amount of power to be used over a long period of time and requires a slow to moderate recharge rate. These are the batteries to be used in 'house' or 'domestic' applications as they can withstand 700 to 800 deep cycles (or more depending on size and type) before they need replacing. Generally speaking, 6 volt Golf Cart batteries are true deep cycle types. (note: not all batteries labeled "Deep Cycle" are TRUE Deep Cycle batteries. See RV/Marine below)

   RV/Marine "Deep Cycle"
   These batteries are basically a 'hybrid' between a Cranking battery and a True Deep Cycle battery. They were originally designed for the marine industry where there was only one battery used for both cranking the engine and for running the lights and electronics when the boat was at anchor. They needed a little of both kinds of batteries to meet that need. This type of battery can withstand around 300 to 400 deep cycles before they need replacing. They are adequate as 'house' or 'domestic' batteries but are not nearly as good or long lived as True Deep Cycle types. Modern motorhomes have seperate battery banks now, one for 'cranking' and one for 'domestic' use so there is no need for this type of battery anymore. Your money will be better spent buying True Deep Cycle batteries for 'house' or 'domestic' applications.

The Operating Nature of Batteries

   All lead-acid batteries begin gassing ('boiling') between 14.1 and 14.4 volts when the batteries are at 25'C (77'F) degrees Farenheit. (note: some batteries want to be pushed to 14.6 to 14.8 volts). It is important to note that this gassing threshold changes as the temperature of the battery changes. The gassing threshold is reached at a lower voltage when the batteries are hotter, and it is reached at a higher voltage when the batteries are colder. It is also important to realize that RV batteries are exposed to widely varying temperatures as they are not in climate controlled, insulated compartments. This dictates the need to include temperature compensation into the charging strategy.

   Gassing begins when the batteries have reached a state of charge where they can only accept a small amount of amperage. If you are trying to push more amps in than the battery can accept, the 'extra' amps have nothing better to do than go around splitting water molecules (H2O) into hydrogen (H2) and oxygen (O2) gas. This is what causes the bubbling or 'boiling'. It also causes the battery to heat up which further lowers the gassing threshold and causes more and more violent 'boiling'. So it is important to taper off the charging amperage as the batteries reach their gassing threshold.

   On the other hand, if you don't push the batteries to their gassing threshold, you will leave some of the sulfur behind on the plates. This sulfur will begin to form a sulfate crystal which will eventually grow to cover a substantial part of the lead plate. Once this happens, that part of the plate can no longer interact with the electrolyte solution and the capacity of your battery diminishes. You will know your batteries are sulfated when you find that they don't last as long as they used to before they need recharging. An equalization charge can drive some of this sulfate back into solution if it hasn't hardened into a rock like crystal yet.

What is an "Equalization" Charge?

   An equalization charge is a planned overcharge. As discussed above, overcharging (equalizing) a battery is not good but then neither is allowing sulphate crystals to grow. Equalization, however, is a lessor evil than sulfation. Pushing the batteries up to 14.8 to 15.5 volts for 3 to 6 hours occasionally will help knock loose hard rock sulfation and allow weaker cells to come up to a full charge. If you recognize that you will have to replace the water that 'boiled' off during this planned overcharge, you can extend the life of previously sulfated batteries.

Absorbed Glass Mat Batteries Discussion

There is a "new" battery out there that has proven to be a valid option to the traditional Flooded Lead-Acid Deep Cycle Batteries. They cost about twice as much but may be worth it given their special benefits. I have been using these batteries over the past decade and find myself appreciating them more and more. They are sold under the name of "Lifeline" for mobile applications and are made by Concourde (the same company that supplies the batteries for our Military Aircraft).

They are still lead-acid batteries but are sealed instead of vented. The electrolyte is held captive in a fibrous glass mat that can't be spilled and therefore can be shipped without hazardous material restrictions. This glass mat also provides pockets that assist in the recombination of hydrogen and oxygen gasses (that are generated during charging) back into water.

I found their charge acceptance to be greater than flooded batteries and it requires significantly less time to recharge these AGMs. This translates into higher efficiency which means a shorter generator run time during those times when you find it necessary to recharge quickly. They also hold up better and at a higher voltage when heavy loads are powered by them. Like when you fire up the microwave through the inverter.

These batteries have very thick positive plates and belong in the true Deep Cycle class. They don't outgas (unless severly overcharged) and because of this they don't corrode terminals and don't need to be watered. The savings in maintenance alone will be worth the extra cost to some.

Charging Parameters

   These batteries do require that all of your charging sources be set properly. This includes your Inverter/Charger (or Converter), Solar Charge Controller, and Vehicle Alternator. Most vehicle alternators regulate between 13.5 and 14.5 volts while running down the road so shouldn't be a big concern.

   The AGM batteries don't like to be pushed over 14.6 volts during "Bulk" charging and want to be held between 13.2 to 13.4 volts during "Float" charging. They can withstand an occasional short duration "Equalization" charge as long as it is less than 15 to 15.5 volts. Don't overdo this as it can be forced to outgas and there is no way to add water back in

Technical Manual for Lifeline Batteries

For more technical information about the Lifeline AGM Batteries, click here.

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