All about the RV-battery

THE RV-BATTERY – YOUR RV ELECTRICAL UTILITY

Whether you are a RV-camper, truck-camper or motorhome owner the subject of RV-battery is going to confront you. The demands that are now being placed on the average RV vehicle electrical system have generally outgrown that of battery technology with the rapid advances in electrical and electronics systems.

The new power thirsty gear list is topped by the AC inverter, which extracts a heavy price in electrical power for the comforts it delivers.

While some technical developments such as improved high efficiency inverters have eased the strain along with energy efficient lighting, future changes will see the wider implementation of 24 and 36 (42) volt power systems as a way to improve available power.

The RV-battery will still be central to the primary RV DC power system whatever the voltage used, and the basic power balance assessment and capacity calculations have taken on a new importance.

The bottom line on RV-battery power calculations has risen significantly, and consequentially so has the battery power required to support it.

The search for a bullet proof and fault tolerant RV-battery bank is as real as ever for the off-road RV camper. The flooded cell lead acid battery has always filled the role and it's position is under siege from technology newcomers, with comparison testing being performed under various controlled test conditions between all the RV-battery types. The performance claims and counterclaims are often confusing as are the various numbers are worked out, and each successive innovation claims greater cycle abilities and life expectancies than previous.

ABOUT THE FLOODED CELL RV-BATTERY

To start with it is necessary to revisit a few RV-battery basics. The fundamental theory of the battery is that a voltage is generated between two electrodes of dissimilar metal when they are immersed in an electrolyte. In the typical lead-acid flooded cell the generated voltage is nominally 2.1 volts. The typical 12-volt battery consists of 6 cells, which are internally connected in series to make up the battery. Each cell consists of the Positive plates, which are made from Lead Dioxide (PbO2), the Negative plates which are made from Sponge Lead (Pb), and they are immersed in the electrolyte made of Sulfuric Acid (H2SO4).

Current flow and discharging of the battery occurs when an external load is connected across the positive and negative terminals, and a chemical reaction takes place between the two plate materials and the electrolyte. During this discharge reaction, the plates interact with the electrolyte to form lead sulfate and water, which dilutes the electrolyte, reducing the electrolyte density. As both plates become similar in composition, the cell loses the ability to generate a voltage. Re-charging of a cell reverses this reaction and the water decomposes to release hydrogen and oxygen, with the two plate materials being reconstituted to the original material.

When the plates are fully restored, and the electrolyte is returned to the nominal density the battery is completely recharged. Recommended densities are normally obtainable from RV-battery manufacturers, and can vary a little between battery. In warm tropical locations it is common for RV-battery to have a reduced electrolyte density, which does not cause separator and grid deterioration as fast as temperate climate density electrolytes. Deionized and distilled water is the preferred for topping up cells however many use rainwater or straight out of the RV-park water faucet, which introduces impurities and degrades the plates.

ALL ABOUT RV-BATTERY SULFATION

Sulfation is the single greatest cause of flooded cell RV-battery failure and the causes are relatively simple. During discharge, the chemical reaction causes both plates to convert to lead sulfate, and if recharging is not carried out promptly the lead sulfate starts to harden and crystallise. This is characterised by the formation of white crystals on the typically brown plates and is almost non-reversible. The immediate effect of sulfation is partial and permanent loss of capacity as the quantity of active material is reduced. Electrolyte density also partially decreases, as the chemical reaction during charging cannot be fully reversed.

This sulfated material also introduces higher resistances within the cell and inhibits charging, and as the level of sulfated material increases, the cell's ability to retain a charge is reduced and the RV-battery ultimately fails. The deep cycle RV-battery has unfairly gained a bad reputation for sulfation, however the battery is not the cause, improper and incomplete charging is the real cause. The process of regular equalizing charging assists in reducing sulfation, and there are a few battery additives that also claim to reduce or reverse the effects. The other principal cause of failure is lack of maintenance, and a failure to monitor cell electrolyte levels and top them up with distilled water.

The charge and discharge cycle releases water, along with natural evaporation due to ambient air temperatures, and the exposed plates are seriously damaged leading to premature failure. The deep cycle RV-battery does not tolerate inactivity and battery left without charging have very high self discharge rates of up to 6% a month, and on many RV’s which sit in the driveway or in an RV-park unattended for long periods, the effects are serious. Installation factors have a high level of importance for flooded cell battery. Flooded cell battery generate potentially hazardous hydrogen gas so they must be well ventilated.

WHICH RV-BATTERY?

Flooded or wet cell lead acid RV-battery are by far the most popular battery types in service, and this is generally based on economic considerations. The lead acid RV-battery open cell arrangements first progressed to sealed lead acid units which are safety vented and have built in hydrometers taking the human out of the equation on the maintenance issue.

The battery decision making process also depends on the load types, such as start or housepower, and for deep cycle battery it is about discharge rates, charge acceptance rates, projected cycle life etc and for start battery it is their high cranking current ability and the deep cycle battery for house power duty. In most RV applications inefficient charging is a major cause of shortened life and performance and fast charge smart regulator systems are essential if the full capacity is to be realized. The disadvantages appear daunting however flooded cell battery are bullet proof in many respects. They are tolerant to overcharging and the use of smart alternators overcomes the lower charge acceptance rates.

There are many innovations and performance enhancing design battery features being implemented by the various manufacturers to improve reliability and performance. This may include a greater number of and thicker plates as this improves current flow, along with improved grid designs. Separator design and material is also an area of development, with some using a fiberglass matting that is bonded to the plates to reduce the shedding of active material, while there are others that now use carbon fiber. This is conductive and is bonded to the plates to stop plate material shedding and improve current flow, and the encapsulation of the plate is claimed to eliminate sulfation.

The costs are much lower than other newer battery types, and if the maintenance and charging are kept up they will have long service lives of several years. Quality deep cycle battery subject to frequent service such as the Rolls, Trojan, Deka etc, and which are properly maintained, will probably offer the best value for money and life expectancy. It is important to remember that just one serious flattening episode can ruin the battery or severely curtail its life.

ABOUT THE GEL CELL BATTERY

Sealed battery have been steadily evolving for some time, and the gel electrolyte battery was the first innovative alternative to flooded cells. These battery are a recombinant battery, in that the oxygen generated from the positive plate recombines with the hydrogen given off by the negative plate to form water, subsequently no electrolyte replenishment is required.

They are also known as Sealed Valve Regulated (SVR) battery as the oxygen is retained in the cell by sealing vents, which maintain positive internal pressure and this is essential to the recombination process. The valve also has a safety function to vent any excess pressure that arises during the charging process, otherwise serious damage would occur. Unlike normal lead-acid flooded cells with liquid acid electrolytes the gel cell has a solidified thixotropic gel, which is locked into each group of plates, and one feature of thixotropic gels is that they possess a reduced viscosity under stress.

Gel electrolytes have a high viscosity, and during cycles of charge and discharge voids and cracks can develop in the gel. The effect is a resistance to charging and a loss in capacity. Liquification of the gel occurs (thixotropic action) during charging due to this gas shearing effect, and it can take more than an hour to solidify again after charging ceases. The gel is manufactured from a mixture comprising sulfuric acid, fumed silica, pure water and phosphoric acid.

The construction of gel cells is different, as the plates are reinforced with calcium, and not with antimony, which results in a reduction in battery self-discharge rates, typically around only 1% per month. The newer battery types use phosphoric acid to assist in retarding the plate sulfation hardening rates, and grid designs are also undergoing change and improvement using copper calcium lead alloys. The plates are relatively thin, which is to facilitate the gel diffusion into them, and this results in higher charge acceptance rates than flooded cells, and so a more rapid charge rate is achievable.

The optimum life and performance requires constant potential, voltage regulated charging in the range 13.8 volts to a maximum of 14.1 volts at 68°F. As the open circuit voltage of a fully charged battery is 12.8 volts the charge voltage must exceed 13.8 volts to overcome internal resistance. Fast charge regulators that go over this corrected for ambient temperature cannot be used. The relatively thick separators that are used also increase the distance between the plates, which reduces the high current transfer rates.

Gel electrolytes also have lower densities, which also reduces the charging voltages with the low temperature performance is also better than flooded cells. There are downsides, and while self discharge rates are very low, the charge acceptance rates high and no maintenance requirements are all positive advantages, unlike the flooded cell battery the gel cell is intolerant to high charging voltages such as in regulator failures, and this includes the application of equalising charges, and either will seriously damage the battery. In addition the cycling capability compared to quality deep cycles is less, along with considerably higher capital costs.

The expected lifespan of the gel cell in comparison to a quality deep cycle flooded cell lead acid battery is in the range of 800-1000 cycles of charge and discharge to 50%, where a quality deep cycle has a life of up to approximately 2500 cycles. Gel cells however do have a much greater cycling capability than normal starting battery, and in many applications they are ideal. If safety valves malfunction the cells are also easily damaged by oxygen contamination, although in quality Gel cells this is uncommon.

ABOUT THE ABSORBED GLASS MAT BATTERY

AGM battery like Gel cells are also classed as Sealed Valve Regulated (SVR) battery. The electrolyte is held within a very fine microporous (boron-silicate) glass matting that is placed between the plates, which absorbs and immobilizes the acid while still allowing rapid plate and acid interaction. Another term used for AGM battery is starved electrolyte battery, and this is because the glass matting is only 95% soaked in electrolyte.

In a normal lead-acid battery, water loss will occur when it is electrically broken down into oxygen and hydrogen near the end of charging. In a battery during charging, oxygen will evolve at the positive plate at approximately 75% of full charge level, and Hydrogen evolves at the negative plate at approximately 90% of full charge. In normal battery, the evolved gases disperse to atmosphere, resulting in electrolyte loss and periodic water replacement, and these are the bubbles seen in the cells during charging.

During charging the current causes decomposition of the water, and oxygen is evolves on the positive plate. The oxygen then migrates through the unfilled pores of the separator matting to react with the negative plate and form lead oxide, lead sulfate and water. The charge current reduces and does not generate hydrogen. The low maintenance recombinational battery has different characteristics. The plates and separators are held under pressure.

During charging, the evolved oxygen is only able to move through the separator pores from positive to negative, reacting with the lead plate to recombine. The negative plate charge is then effectively maintained below 90% so inhibiting hydrogen generation. They emit less than 2% hydrogen gas during severe overcharge (4.1% is flammable level). The operational principle is called the recombinant gas absorbed electrolyte, as the generated gases recombine within the battery and significantly reduce hydrogen emissions.

They emit less than 2% hydrogen gas during severe overcharge (4.1% is flammable level). This recombination process reduces water loss by over 98% in comparison to wet cell battery, so the elimination of maintenance is obvious. The recombination process is different to a gel cell and takes place within the separator in a molecular state, with the cells being sealed and the relief valves provide a safe positive pressure during charging.

There are variations to traditional flat plate manufacturing techniques, and the Optima® AGM battery have a spiral cell, and dual plate construction. Another important claimed feature is a greater shock and vibration resistance than gel cell or flooded RV-battery. They also have extremely high CCA values of up to 800 amps at 0ºF, however the one drawback that stopped me installing these was the limitation in rating sizes, which is only 56 Ah. They are however a very good option for smaller RV’s.

Charging of AGM cells have few limitations, and no special charge settings are required to smart fast charge regulators. The batteries have a very low internal resistance and during heavy charge and discharge there are no heating effects. As they have a high charge acceptance rate they can be bulk charged at very high currents, typically by a factor of 5 over flooded cells, and a factor of 10 over gel battery. They also allow 30% deeper discharges and recharge 20% faster than gel RV-battery and good recovery performance from full discharge conditions.

Self-discharge rates are only 1%-3% per month at 77°F, which is improved over rates in flooded cells and gel cells. Water loss can occur in battery, and this is caused by decreases in the efficiency of recombination, excessive charge voltages in particular float charging, corrosion of grids, transpiration through the cell casings (so keep battery as cool as possible), and self discharge, which also increases with higher temperatures. If you are a long term off-road RV-camper, that does limited motoring periods, or leave the RV unattended for long periods, the AGM RV-battery is a viable proposition, as it has very low self-discharge rates, and very high recovery rates from deep discharges.

As charge acceptance rates are very high and charging is in the range 14.4 to 14.6 volts a fast charge regulator has the capacity to burn out alternators. Many undersized alternators run at full output for considerable time periods and overheat. There is a good case for installing high output alternators to maximize charging, and the added load on the engine is a bonus. I have recently installed an AGM RV-battery so will be watching the overall performance closely.