Sierra Nevada Airstreams -|- Owners Guide

Enjoyment of the whispering winds, the zephyrs, the airstreams of the Sierra Nevada and Great Basin areas of the United States in a recreational vehicle.

Battery Myths, Deep Cycle Edition

When trying to find a new battery for your RV it can be difficult to sort through the many claims about what you need. There is marketing hype to wade through, teaching fictions people use to try to understand batteries that may be out of date, and some very strongly held viewpoints about what is and is not. Here are a few ideas that can help you sort things out. To show them in use, the 'deep cycle means thick plates' idea is examined.

  1. Differences are more a matter of degree than of kind. Lead acid battery design balances cost, capacity, and ruggedness goals for various needs. It is a mature technology and well optimized so the variances between the batteries commonly available are not that large.

  2. It takes a difference of twenty percent or more between battery measures to have significance. Individual batteries can vary by ten percent or more in actual use due to temperature, age, cycle to cycle variance, or how they are used and maintained.

  3. Deep cycling greatly shortens the life of all lead acid batteries. This makes measures and assertions based on taking a battery down to 20% or less of its capacity (a deep cycle) not directly applicable to typical RV battery use.

  4. Measures that are directly connected to what you plan to do are better than those whose impact is only asserted or assumed.

Note that the context is important. The Battery FAQ (BFAQ) says it “assumes 12-volt, six cell, negative grounded, lead-acid car and deep cycle batteries at 80° F (26.7° C) with capacities from 5 Amp Hours (AH) to 250 AH.” That applies to this discussion, too, except neither voltage nor wiring method for chassis ground is considered important.

The context here is lead acid battery banks with no more than about 10 kilowatt hours of energy capacity made up of batteries a single person can move as commonly found at retail.

What do you need to know?

When you buy a battery, you want to be assured that it will have the capacity you expect and that it will have a good service life when you use it.

Capacity is reliably measured and a common specification for batteries. It is always related to the current drawn because the higher the current, the less total energy the battery can deliver. Common capacity measures include the energy you can expect over a twenty hour duty cycle and how long you can pull 25 amps from the battery. The 20 hour rating is a power draw of about 60 watts (5 amps at 12 volts) while the 25 amp draw is 300 watts for a typical group 27 sized battery. (see Peukert about current's influence on capacity)

Service life has no 'retail level' measure other than warranty. That is because how long a battery will last depends mostly upon how it is used and maintained. The Arizona Wind Sun FAQ (NAWS) provides some numbers with a qualification. “These are some typical (minimum - maximum) typical expectations for batteries if used in deep cycle service. There are so many variables, such as depth of discharge, maintenance, temperature, how often and how deep cycled, etc. that it is almost impossible to give a fixed number.” If you look at the numbers, what should strike you is the range. Expect a battery for your RV to last from 1 to 8 years. The fact that it is so difficult to predict how long a battery will last is an indication that measures and assertions about batteries related to their lifespan should not be given much weight in your purchase considerations. How long your batteries will last is mostly up to you, not the battery.

Deep cycle, what is it and what does it do?

A battery cycle starts with a fully charged battery that is then used to provide energy and then recharged back to its original fully charged state which ends the cycle. The voltage and current available drops as the battery supplies energy. When a 12 volt lead acid battery gets down below 11 volts or so with no load, it is usually considered fully discharged. A 'deep cycle' usually refers to taking 80% of the available energy out of a battery before recharging it. That means the resting voltage gets down to a bit under 12.0 volts. (see NAWS for a chart)

The energy storage of a battery is a matter of converting back and forth between lead suflate and lead and sulfuric acid. Discharging a battery forms lead sulfate that coats the plates. Recharging the battery breaks the lead sulfate back into lead and sulfuric acid. This isn't a cost free operation. There are efficiency losses as the current to charge and discharge the battery goes through resistances. The sulfate will harden if left alone for too long and be very difficult to break back up into lead and sulfuric acid. Every cycle leaves a little bit of sulfate that doesn't get reprocessed during charging. Deeper cycles provide more time and more sulfate so they tend to build up the residue faster. That is why deep cycling causes batteries to live shorter lives as it is sulfation that degrades the battery performance over time.

Since the plates of batteries are a paste on a grid and not solid metal, the sulfate forms not only on the outside but also on all the exposed surface. That means the plates are like a sponge absorbing water. This can cause some change in volume that can be pushed to an extreme and if the battery isn't designed for this, it can cause problems.

Batteries are sets of cells and there are differences between cells. It is possible to discharge a battery far enough so that one cell goes below zero and starts charging backwards from the current provided by the other cells. This is another reason why you do not want your batteries to go totally absolutely flat.

These are the reasons for the 50% rule. SmartGauge has a good explanation of this. “Discharging deep cycle batteries to 50% results in the most economical use of the batteries in terms of battery life and monetary outlay.“ – what that says is that most talk about deep cycles is not relevant to the effective use of lead acid batteries in an RV.

Battery construction (thick plates)

To imagine how a battery could be more rugged, people envision thick plates for strength and plenty of space to allow corrosion products to pile up without shorting out the battery. NAWS describes battery plates as being about a tenth of an inch thick varying plus or minus by half that. Some assert that you can measure 'deep cycleness' by taking apart a battery and measuring its plate thickness. You can see this at wikipedia and NAWS and many other places.

Sometimes there is a qualifier that one way to tell a true deep cycle battery from other types is that it has solid lead plates instead of the Faure pasted-plate construction. You won't find solid lead plate batteries in the context of this discussion. A reason is that lead inside a plate cannot react with the electrolyte so it only contributes weight, not energy capability. That does not make for efficient portable energy storage, especially since lead acid batteries are inherently rather weight and space inefficient. Solid plates are also prone to shedding while modern plate design can minimize this.

The strength of the lead in batteries isn't due to how thick it is because lead isn't all that great of a structural material. Instead, it is in the mixture of lead with other materials and the design of the structures that provides strength. Antimony is the usual element added to lead to increase strength but it also increases the battery's self discharge rate and outgassing during charging. Sealed batteries use calcium instead of antimony to reduce gassing but that makes them less rugged. It is all about the trade-offs necessary to optimize a battery for a particular use.

As for corrosion causing cell shorts, this is not a common failure mode for modern lead acid batteries (e.g. Battery University). Sulfation due to age appears to be the primary mode of failure if the battery is not mistreated. In Eliminating Battery Failure – Two New Leading Indicators of Battery Health – A Case Study (PDF - Salloux, McHardy. 2007, IEEE) the authors say “We have devoted less effort to grid corrosion because a battery that reaches the end of life solely by this failure mode has generally exceeded its design life.”

One should also note that these ideas (of thickness and space) about how to make a rugged battery would be reflected in the size and weight energy densities of the battery. You'll find that wet cells all run about 22 watt hours per pound or 1.7 watt hours per cubic inch with only minor differences. AGM's often have a lower energy weight density probably due mostly to their use of glass mats and starved electrolyte design. These measures indicate no significant weight or space differences between batteries intended for different uses.

These are some of the reasons that the idea that 'deep cycle' means thicker plates or other such distinctive physical design characteristics serve no useful purpose. The design adjustments to favor ruggedness include many factors in modern lead acid batteries and manufacturers tend to adjust them to fill out their line to allow customers to choose what is best for what they want to do between all three goals (cost, capacity, ruggedness). The overall differences are barely more than what you can find in a single battery during its lifespan.

Service Life

NAWS says “If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%.” Note that it can take 12 hours to fully charge a battery so daily deep cycling is an 'industrial' activity that very few Rvers or even solar enthusiasts accomplish. BFAQ says “A good quality wet deep cycle (or "leisure") battery will cost between $50 and $300 and, if properly maintained and used, will give you at least 200 deep discharge-charge cycles.” There are 260 weeks in 5 years. That means you could get a 5 year battery life on this measure by doing a weekly deep cycle or by two half discharge cycles every week or by moderate discharges every day. Any of these scenarios is much more activity than most RV's encounter. Cycles, deep or not, just aren't that significant an issue in practical terms.

There is also the problem of defining just when the battery is no longer serviceable. Aging does not usually cause sudden death (see Battery Life and Death). Most cycle tests terminate when capacity falls to 80% of the capacity of the first cycle. Additional performance degradation is often tolerated in practice because most RV battery banks have significant extra capacity for normal use.

There are some key things you can do to gain optimum battery life.

  1. Avoid deep discharging. Make sure you have enough battery so you don't usually take your battery below 12.0 volts measured after at least a half hour's rest with no significant charging or discharging.

  2. Do discharge your batteries below 12.4 volts as proper exercise helps shake things up and works like a deep breath to get all of the battery involved.

  3. Charge promptly, properly, and fully with an intelligent battery charger that will create some bubbling on initial charging but not overcharge the battery. It can take 8 to 12 hours to fully charge a battery so give it time.

  4. Keep your battery out of the heat.

  5. Maintain your battery between uses with a proper maintenance device that will keep a full charge and provide a sulfation inhibiting technique.

Making Battery Decisions

Making good decisions effectively means putting your efforts into what is significant and important, When it comes to choosing a battery for your RV, there is an awful lot out there that is not significant or important and can be distracting or misleading. You can get lost browsing through all of the information that is at your fingertips. If what you need is “to be assured that it will have the capacity you expect and that it will have a good service life when you use it” then the information presented here will help you determine what is important.

Capacity is a balance between what you need and what you can carry. Lead acid batteries don't provide much energy capacity so you get what will fit your RV and adapt consumption of battery energy as need be.

Battery service life isn't so much the battery as how you use and maintain it. The first expense should therefore not be battery but rather the equipment you use to charge and maintain it.

There is a lesson here as well. The 'deep cycle means thick plates' idea is prevalent in many battery explanations. As shown here, that idea serves no significant usable purpose in the context of typical retail battery purchases. Even when many authorities assert something, you should always maintain a bit of skepticism. Understand the measures and what they will do for you before you act on somebody's say-so.

Do get a battery intended for folks who use them like you do. Buy from a reputable retailer who will stand behind what he sells. Take care of your batteries. You can keep it simple and still get good results.

Sierra Nevada Airstreams -> Travel Trailers Owner's Guide -> Understanding

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