Sunday, October 31, 2010

On Batteries

Batteries are an integral part of any automotive, RV, marine or home power electrical system. Since most people are fairly familiar with automotive batteries, we will concentrate on deep-cycle power storage batteries used in home power, RV and marine applications, with brief comparisons between deep-cycle and automotive batteries.

Battery Capacity

Battery capacity is a primary concern in home power systems. The storage battery bank must have enough storage capacity to meet your power needs between charging cycles. Making sure the battery storage capacity is about double the power that would be used in a normal use day is a good minimum.
Home power (deep cycle) batteries are generally measured in "amp-hour" capacity. One amp-hour is equal to one amp of current drawn for one hour of time. Amp-hour capacity is generally given as the "20 hour rate" of the battery. Therefore, the number given as the amp-hour capacity for a deep cycle battery will be the number of amp-hours the battery can deliver over a 20 hour period at a constant draw. A 105 amp-hour battery can deliver 5.25 amps constantly over a 20 hour period before it's voltage drops below 10.5 volts, at which point the battery is discharged.
Amp-hour requirements for your home power system can be calculated with help from the System Sizing section of our System Design information pages.

Types of Batteries

Lead Acid Automotive Batteries
Automotive batteries are designed to deliver a relatively high amount of current in a short period of time, but should never be heavily discharged. An automotive battery plate is very porous (like a slice of swiss cheese), to maximize surface area and enable the sudden high current output. Because home power systems require repeated deep discharges of stored power, automotive batteries are largely useless for these applications.

Lead Acid Deep Cycle Batteries
Deep cycle batteries are designed to have a large amount of their stored current discharged between charging sessions, with very heavy non-porous battery plates to withstand repeated major discharging and charging cycles (deep cycles). They are generally useless for delivering the sudden surges of power needed from automotive batteries.
RV/Marine Batteries are usually 12 volt, and available in a variety of capacities up to 100 amp-hours. They can be found in "sealed" or standard servicable types, and are commonly used in small home power or portable power applications. RV/Marine batteries are generally small, compact and easy to handle and install. They are relatively inexpensive, and the sealed type batteries are non-spillable and safer for indoor applications.
However these batteries are not designed for very heavy cycling (as is found in a home power system), so their life-spans are often shorter than other types of deep cycle batteries. Sealed batteries are also very sensitive to overcharging, which may further shorten their useful lifespan. Also, in order to obtain more than 100 amp-hours of storage capacity, multiple batteries must be attached in parallel, which is less efficient than using a single, higher capacity battery.
Golf Cart Batteries have capacities in the 220-300 amp-hour range, and are generally 6 volt. They are well suited to small to medium home power systems. They are designed for deep discharge cycles, so they will tend to have longer lifespans and better performance in a residential alternative energy system. They are still relatively light weight, but are generally cheaper per amp-hour than RV type batteries. They are also less sensitive to mild overcharging.
However since most home power systems are 12 volt, two 6 volt batteries must be connected in series, which is a bit more complicated than connecting a single battery. Since golf cart batteries are unsealed, they need to be stored in a well ventilated area and will require periodic water replacement. Their amp-hour capacity is also too limited to be of use in a large power system.
Industrial/Stationary Batteries are normally manufactured as individual 2 volt units, which are then combined to create the necessary voltage for the power system. (Six for 12 volt systems, twelve for 24 volt systems) They're available in a wide variety of capacities, up to 3000 amp-hours. A very high amp hour capacity can be obtained with a single six cell set, so charging characteristics are very stable. Industrial batteries will have the longest average lifespan under deep cycling home power conditions.
However due to their extremely high amp-hour capacities, industrial battery sets will have a significantly higher initial cost. These batteries can also weigh up to 350 lbs. per two volt cell, so they will need to be stored in a well supported area, contained in a rigid external box, and will likely require special transporting assistance.
Nickel Alloy Batteries
Nickel Cadmium (NiCad) and Nickel Iron batteries, rather than consisting of lead plates submerged in a sulfuric acid solution, feature nickel alloy plates in an alkaline solution. They are also well suited for home power use, but are much less common and much more expensive than lead acid types.
A nickel alloy battery can have up to 50 years of useful life, compared to 20 years with a well-maintained lead acid battery. They can also sit for extended periods of time partially or fully discharged without suffering damage, unlike lead acid types. They are lower maintenance, and can be completely discharged repeatedly without suffering damage. A lead acid battery should never be completely discharged, meaning they need to be more closely monitored. Nickel alloy batteries operate better at lower temperatures, and can discharge more of their total amp-hour capacity as useful current.
Despite all these advantages, the higher initial cost of the batteries is prohibitive. Also, nickel alloy batteries are harder to dispose of when they finally become unchargeable. Their unique charging voltage range can also create compatibility problems with battery management and charging equipment.

How Batteries are Used in Home Power

A storage battery bank is what enables a home power system to deliver a constant level of power to the electrical system. Without storage batteries, the entire electrical system would be limited by the immediate output of the alternative energy generators. At night, a solar-run house would have no electrical power available to turn on interior lights. A wind-powered system would be subject to constant power fluctuations as the wind speed increased, dropped or disappeared entirely.
By running the output of renewable power generators through charge controllers and into a battery bank, power can be available 24 hours a day, regardless of weather. Solar panels or wind generators can deliver power to the battery bank regardless of current power usage, so excess power can be stored during low use times (generally the middle of the day and middle of the night) and be available during high use times (usually morning and evening).
Batteries supply DC power, so if power is needed for an AC power system or a mixed AC/DC system, the battery power will need to be run through an inverter to change 12VDC or 24VDC power into 120VAC household current.

Using Batteries in Alaska

Optimal operating temperature for a lead acid battery is 68ºF. The rated amp-hours given for a battery are calculated at 68º. If battery temperature significantly exceeds or falls below this level, charging efficiency and amp-hour capacity will drop.
Obviously, storing deep cycle batteries outside or in an unheated enclosure during an Alaskan winter would not be a practical option. The batteries would be destroyed. If the battery bank is being stored in a stand-alone power shed, the building should have some form of heating installed. An attached, heated garage would also be an appropriate storage location.
Automotive batteries should be winterized if the vehicle will be parked outside during sub-zero weather for more than an hour at a time. Battery heating pads are widely available in northern climates, either as small pads designed to be placed underneath the battery, or as fitted blankets which slip over top of the battery.

Basic Lead Acid Battery Function

Lead acid batteries are by far the most common type of power storage battery in use today. A fully charged lead acid battery undergoes a chemical reaction when attached to an electrical load, which releases stored energy from the battery. All lead acid batteries consist of the following components:
A positive plate, composed of lead dioxide (PbO2)
A negative plate, composed of "sponge" lead (Pb)
An electrolyte solution of sulfuric acid (H2SO4) and distilled water (H2O)
When the battery discharges current, the sulfate (SO4) in the electrolyte combines with lead from the plates to form lead sulfate deposits (PbSO4). After repeated or extended discharge, the sulfate content of the electrolyte becomes increasingly "bound" in the lead sulfate deposits and can no longer be used to create electric current. The battery becomes discharged when too much of the electrolyte sulfate is depleted.
Over time, in a non-sealed battery, the water content of the electrolyte solution will drop due to evaporation during discharge. This leads to excessive acid concentration, which raises the resistance of the battery. Periodic checking and refilling of the fluid level in an unsealed battery is essential to its proper functioning.
When a discharged battery is recharged, the majority of the lead sulfate is broken down and the sulfate returns to the electrolyte where it is once again available to create electricity. However, over time a sulfate residue builds up on the battery plates and begins to crystallize. As more of the sulfate becomes locked in the crystallized residue, the battery capacity and ability to be recharged declines until the battery finally "dies."
With deep cycle batteries, the sulfate crystals simply "insulate" the battery plates from the remaining weakened electrolyte, preventing the chemical reactions needed to produce current. In automotive batteries, with their thin, porous plates, crystallization will actually cause the plates to break apart, permanently destroying the battery.

Battery Charging & Maintenance

In an alternative energy system, battery charging is usually accomplished through charge controllers attached to the various power generators. A good quality charge controller will use a three stage, pulse width modulated charging system. This allows the battery to receive the highest charging current during the bulk stage of charging, with a second lower absorption level to bring the charge to maximum voltage, and a third "float" charging current to maintain the battery charge. A good quality charge controller will maximize charging efficiency and minimize lead sulfate build up, increasing the battery's useable lifespan.

Lead acid batteries will lose their charge if they are left unused for an extended period of time. If an automotive or deep cycle battery goes unused for a month or longer, it should be outfitted with a charge maintainer or "trickle charger" (if the deep-cycle battery is not attached to a three-stage charge controller). Solar panels are available for this purpose, and will deliver a low level of current to the battery while exposed to sunlight. For batteries or vehicles stored indoors, plug-in charge maintainers are also available.
Sulfate crystallization in batteries can be slowed or reversed by the use of battery pulse conditioners. Lead sulfate can be more effectively removed, and negative battery plates better maintained if battery voltage periodically reaches 2.5 volts per cell. (15v for a 12v battery, 30v for a 24v, etc.) A pulse conditioner will deliver periodic brief pulses of higher current to the battery, causing the sulfate residue to be released back into the electrolyte and maximizing the lifespan and performance of the battery.

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