It’s easiest to describe the role of a battery in a solar system with a financial analogy.  If solar panels are your income, your loads (lights, appliances, etc.) are your expenses then your batteries are your bank accounts.  But, unlike a traditional savings account, due to battery inefficiencies and amp hour ratings, "battery accounts" pay you negative interest and have balance limits.

Entire books have been written on the subject of batteries, however, we will be discussing only the types of batteries that are currently used in modern RVs.  Lead-acid batteries are made from a mixture of lead plates and sulfuric acid. This was the first type of rechargeable battery, invented way back in 1859.  Lithium batteries on the other hand, are a much more recent invention, and have only been commercially viable since the 1980′s.  Lithium technology has become well proven and understood for powering small electronics like laptops or cordless tools, and has become increasingly common in larger applications like RVs, electric cars and off-grid homes due to their many advantages.

Lithium Batteries
Extended Lifespan
Lithium batteries cost about three times as much as AGM batteries, but the longer lifespan makes them well worth the added cost.  Laboratory tests show that you can expect to see about 2500 to 5000 cycles from a well-maintained LiFePO4 battery bank. These results also show that a lithium battery will deliver more than 75% of its capacity after 2500 cycles. In contrast, even the best deep cycle AGM batteries are typically only good for 500-1000 cycles.  Consider the longer lifespan of lithium and replacement costs of AGM batteries when evaluating the price differences.

More Usable Capacity
Unlike lead-acid batteries that are rarely discharged beyond 50%, Lithium batteries can regularly discharge 80% or more of their rated capacity.  Take for example a 100 amp hour battery – if it was lead-acid, you would have access to only 30 to 50 amp hours of charge, but with lithium you could use 80 amp hours or more.  When evaluating cost, consider that there isn't an “apples to apples” amp hour rating comparison between the battery types.  You get about twice as much charge in a similarly rated lithium battery bank as you would with AGM batteries.

No Voltage Sag
As the charge level on a lead-acid battery decreases so does the voltage.  This means that your lights will get dim and your appliances start to run rougher.  The discharge curve of lithium batteries (especially when compared to lead-acid) is essentially flat, meaning that a lithium battery at 20% will be providing nearly the same output voltage as it would at 80%.  On the flip side, once lithium batteries are fully discharged, their voltage drops off rapidly, and it is the goal of the integrated BMS (Battery Management System) to protect the batteries and prevent this from ever happening.  Completely discharging a lithium battery bank, even just once, can permanently damage it.  Keep in mind that any battery monitor or generator auto-start dependent upon detecting sagging voltage levels will likely not work on a lithium bank.  A shunt based, calibrated monitoring system will be the most effective way to detect the charge level of a lithium battery bank. 

High Current Output
Another huge advantage of lithium batteries is that Peukert’s losses are essentially non-existent, meaning that lithium batteries can deliver their full rated capacity, even at high currents. With lead-acid batteries you can see as much as a 40% loss of capacity at high loads.  This means that Lithium battery banks are much better at powering high current loads like a microwave or an induction cooktop.

Fast & Simple Charging
Unlike lead acid batteries, lithium batteries have no need for an absorption phase to charge the final 20%, which means that lithium batteries can quickly be charged to full capacity.  If your charger can provide enough amps, lithium batteries can be charged in as little as 30 minutes, which means less time running your generator.  Additionally, with lithium batteries you don’t even have to fully top off the battery bank with each charging cycle.  You will never have to deal with sulfation or a manual equalization cycle.  This simplicity gives you freedom to tap into energy sources wherever you can find them without worrying about needing to do a regular full charge.  With lithium batteries you don’t have to worry about leaving your battery bank perpetually undercharged. 

Minimal Maintenance
Compared to lead-acid batteries, lithium batteries are mostly maintenance free.  The BMS does all the thinking for you, and automatically performs a “balancing” process (similar to equalization) that uses resistors to bleed off excess charge, and ensure that all the cells in the battery bank are equally charged.  Additionally, the BMS along with high current circuitry, prevents excessive discharges.  All you have to do is simply charge your battery, or better yet, let your solar panels charge your battery.

Efficient Charge/Discharge
Lithium batteries are much more efficient than lead-acid batteries at storing power. Lithium batteries charge at nearly 100% efficiency, compared to the 85% efficiency of most lead-acid batteries.  This is especially important when charging via solar, when you are trying to squeeze as much efficiency out of every hour of sunlight as possible. In theory, with lithium, nearly every second of sunshine you’re able to collect results in power stored in your battery bank. 

Temperature Resistance
Lead-acid and lithium batteries have reduced capacities in cold environments, but lithium batteries are much more efficient at low temperatures.  At -20°C, a lithium battery that delivers a 1C current (one times its capacity), can deliver more than 80% of its energy.  In contrast, the AGM battery will only be able to deliver 30% of its capacity. For harsh environments (hot and cold), lithium is the better battery technology.

Easy Placement
Lithium batteries do not vent gas like lead-acid batteries, and do not need to be stored in a vented battery compartment.  They can also easily be assembled into a variety of shapes, which is an important advantage if you are trying to squeeze as much power storage as possible into a small compartment. This is also useful if you have an existing battery bay that is limited in size, but you want more capacity than lead-acid batteries are able to provide.

Weight Reduction
Lithium batteries can store about 3.6 times more usable Amp hours than an AGM battery bank of the same weight.  Your vehicle will handle better and get a better MPG.

Tax Incentives
Consider the implications of the 30% federal tax rebate which will be available through 2016.  When combined with a solar installation, this rebate can cover a substantial portion of the cost of your battery bank upgrade.

Lithium Battery Safety
As you might recall from news stories a few years ago, lithium ion batteries have earned a reputation for catching fire in a very dramatic fashion. One of the early, commonly used, lithium ion battery formulations had been Lithium-Cobalt-Oxide (LiCoO2).  This battery chemistry is prone to thermal runaway if the battery is accidentally overcharged. Overcharging could lead to the battery setting itself on fire, and lithium fires burn hot and fast.  This is one of the reasons that up until recently, lithium was rarely used to create large battery banks.  But, in 1996 a new formula for mixing lithium batteries was developed - Lithium Iron Phosphate. Known as LiFePO4 or LFP, these batteries have a slightly lower energy density, but are intrinsically non-combustible, and thus, vastly safer than Lithium-Cobalt-Oxide. Once you consider the advantages, Lithium batteries become exceedingly tempting.

Lead-Acid Batteries
Flooded vs. AGM (expanding bullet point)
Flooded batteries, that require the owner to regularly add water, have been made obsolete by sealed, lead-acid AGM (Absorbed Glass Mat) batteries.  They are sold under the name of "Lifeline" for mobile applications and are made by Concorde (the same company that supplies the batteries for Military Aircraft).  We have been using Lifeline AGM batteries with a successful track record for over a decade.

AGM batteries cost about twice as much as flooded batteries but are well worth the higher price considering their special benefits.  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.

AGM batteries typically have a greater charge acceptance than flooded batteries and require significantly less time to recharge. This translates into higher efficiency, which means a shorter generator runtime when you find it necessary to recharge quickly. AGM batteries also hold up better and stay at a higher voltage when powering heavy loads like a microwave oven.

AGM batteries have very thick positive plates and belong in the true Deep Cycle class. They don't outgas (unless severely overcharged); and because of this, they don't corrode terminals and do not need to be watered.  The savings in maintenance alone could be worth the extra cost.

It is our opinion that the "gel-cell" type batteries have proven to be inadequate in deep cycle environments and we do not recommend them.

Classifications of Lead-Acid Batteries
Starting or 'Cranking'
This type of battery is used to start a motor, but is not practical for storing solar power to operate appliances in an RV.  This type of battery is designed with many, very thin plates, which create a large surface area of lead exposed to the electrolyte. This design allows the battery to output a large amount of power in a short period of time, and it also enables the battery to be recharged rapidly with a heavy current. The starter motor on a big V-8 engine requires this kind of short duration, high power. That being said, starting batteries are a very poor choice when used as 'house' or 'domestic' batteries, since their closely spaced plates can only withstand about 15 to 30 deep cycles before excessive sulfation causes them to stop operating effectively.

True Deep Cycle (available in Flooded and AGM styles)
This type of battery is commonly used as a “house battery” in an RV to power appliances with stored energy from the solar panels.  These batteries are designed with relatively few, thick plates. This design allows for 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 commonly used in 'house' or 'domestic' applications, since they can withstand 700 to 800 deep cycles (more depending on size and type) before they need to be replaced. 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

What is a Deep Cycle?  A deep cycle is when you start out with a fully charged battery, use 50% 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. 

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. The attributes of both battery types were needed.  This type of battery can withstand around 300 to 400 deep cycles before they need to be replaced. RV/Marine batteries are adequate as 'house' or 'domestic' batteries, but are not nearly as good or long lived as True Deep Cycle batteries.  Modern motorhomes have separate battery banks; one for 'cranking', and one for 'domestic' use.  As a result, 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.

How Lead-Acid 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 layman’s terms, when a battery is discharging, the sulfur leaves the solution and attaches to the lead plates. When in contact with the plates, the sulfur gives up an electron.  Moving electrons are electricity.  When a battery is being recharged the accumulated sulfur on the plates is being driven back into the electrolyte solution. 

What Affects the Longevity of Lead-Acid Batteries?
-Overcharging batteries on a regular basis causes water to be 'boiled' out of the electrolyte.  The heat associated with overcharging will eventually warp and corrode the plates.
-Failing to completely recharge a battery will leave some of the sulfur still attached to the plates which is called “plate sulfation”.  This diminishes the storage capacity of the battery.
-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 lifespan.

The Operating Nature of Lead-Acid Batteries
All lead-acid batteries begin gassing (“boiling”) between 14.1 volts and 14.4 volts when the batteries are at 25°C (77°F).  (Note:  Some batteries want to be pushed up to 14.6 volts or 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 cooler.  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 in 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 into the battery than it can accept, the “extra” amps begin to split the water molecules in the electrolyte.  Water molecules (H2O) split into hydrogen (H2) and oxygen (O2) gas, both of which are extremely flammable. 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'. Because of this, it is important to have the charge controller 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 sulfate crystals, which will eventually grow to cover a substantial portion of the lead plate. Once this happens, the sulfated portion 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 between charges.  An equalization charge can drive some of this sulfate back into the electrolyte solution if it hasn't yet hardened into a crystal. 

What is an "Equalization" Charge?
An equalization charge is an intentional overcharge. As discussed above, overcharging (equalizing) a battery is not good, but then neither is allowing sulphate crystals to accumulate on the plates.  Equalization is a lessor of the two evils.  Occasionally pushing the batteries up to 14.8 volts (or even as high as 15.5 volts) for three to six hours will cook off the rock-hard sulfation and allow the weaker cells to come up to a full charge.  If you replace the water that 'boiled' off during this planned overcharge, you can extend the life of previously sulfated batteries.

Lead-Acid Charging Parameters
Lead-Acid batteries require that all of your charging sources be properly tuned. This includes your Inverter/Charger (or Converter), Solar Charge Controller, and Vehicle Alternator.  Most vehicle alternators regulate between 13.5 volts and 14.5 volts while running down the road, so this 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 15.0 volts to 15.5 volts. Don't overdo this as it can force your AGM battery to outgas, and with AGM batteries there is no way to put water back into the electrolyte. 

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