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Message: <blockquote data-quote="SyKo13" data-source="post: 2405096" data-attributes="member: 564109">Available Capacity vs Total Capacity Since batteries depend on a chemical reaction to produce electricity, their Available Capacity depends in part on how quickly you attempt to charge or discharge them relative to their Total Capacity. The Total Capacity is frequently abbreviated to C and is a measure of how much energy the battery can store. Available Capacity is always less than Total Capacity.Typically, the amp-hour capacity of a battery is measured at a rate of discharge that will leave it empty in 20 hours (a.k.a. the C/20 rate). If you attempt to discharge a battery faster than the C/20 rate, you will have less available capacity and so on. The more extreme the deviation from the C/20 rate, the greater the available (as opposed to total) capacity difference.However, this effect is non-linear. The available capacity at the C/100 rate (100 hours to discharge) is typically only 10% more than at the C/20 rate. Conversely, a 10% reduction in available capacity is achieved just by going to a C/8 rate (on average). Thus, you are most likely to notice this effect with engine starts and other high-current applications like inverters, windlasses, desalination, or air conditioning systems.For example, the starter in an engine will typically quickly outstrip the capacity of the battery to keep cranking it for any length of time. Hence the tip from mechanics to wait some time between engine start attempts. Not only does it allow the engine starter to cool down, it also allows the chemistry in the battery to "catch up". As the battery comes to a new equilibrium, its available capacity increases. A very elegant equation developed in 1897 by a scientist called Peukert describes the charging and discharging behavior of batteries. As you can see below, the Peukert equation consists of several factors.Peukerts Equation: [T/(R*C)]*[i/(C/R)]^n=C (Ty <a href="http:////forum/member.php?u=30525" target="_blank">Thnking</a> for clearing it up //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif)Where R equals the battery amp hour rating ( ie, 20 or 8 or etc)• l is the current (usually measured in amperes)• T is time (usually measured in hours)• n is the Peukert number / exponent• C is the theoretical storage capacity of the battery (usually measured in amp-hours). Use the C/100 capacity or add 10% to the storage capacity at the C/20 rate.As you can see, the available current is dependent on the rate of discharge and the Peukert exponent for the battery. The closer the exponent is to 1 (one), the less the available capacity of a battery will be affected by fast discharges. Peukerts numbers are derived empirically and are usually available from manufacturers. They range from about 2 for some flooded batteries down to 1.05 for some AGM cells. The average peukerts exponent is 1.2 though the exact number depends on the battery construction and chemistry.(Hope what you just read made sense lol)Reserve MinutesReserve Minutes are a measure of how long your battery can sustain a load before it's available capacity has been completely used up. This measure is especially useful for folks who want to run inverters, fridges, and other large loads.Conversion EfficiencyThe conversion efficiency denotes how well a battery converts an electrical charge into chemical energy and back again. The higher this factor, the less energy is converted into heat and the faster a battery can be charged without overheating (all other things being equal). The lower the internal resistance of a battery, the better its conversion efficiency.One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a. NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive battery technologies.Battery LifeBattery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge (for example, a cell has shorted) or when the available battery capacity is 80% or less than what the battery was rated for. The life of Lead Acid batteries is usually limited by several factors:• Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out. The greater the average depth-of-discharge, the shorter the cycle life.• Age also affects batteries as the chemistry inside them attacks the lead plates. The healthier the "living conditions" of the batteries, the longer they will serve you. Lead-Acid batteries like to be kept at a full charge in a cool place. Only buy recently manufactured batteries, so learn to decipher the date code stamped on every battery... (inquire w/manufacturer). The longer the battery has sat in a store, the less time it will serve you! Since lead-acid batteries will not freeze if fully charged, you can store them in the cold during winter to maximize their life.• Sulphation is a constant threat to batteries that are not fully re-charged. A layer of lead sulphate can form in these cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries. Many batteries can be saved from the recycling heap if they are EqualizedEqualizationSulphation layers form barrier coats on the lead plates in batteries that inhibit their ability to store and dispense energy. The equalization step is a last resort to break up the Sulphate layers using a controlled overcharge. The process will cause the battery electrolyte to boil and gas, so it should be only done under strict supervision and with the proper precautions.GassingBatteries start to gas when you attempt to charge them faster than they can absorb the energy. The excess energy is turned into heat, which then causes the electrolyte to boil and evaporate. The evaporated electrolyte can be replenished in batteries with removable caps such as most flooded deep cycle batteries. Many car batteries are sealed and thus need to be replaced when their electrolyte evaporates over time.Since AGM and Gel cells are always sealed, it is very important to guarantee they are not overcharged. The only way to ensure this is to use a temperature-compensated charging system. Such chargers use a temperature probe on the battery to ensure that the battery does not get too hot. As the battery heats up, the charging current is reduced to prevent thermal runaway, a very dangerous condition.Thermal RunawayThis is a very dangerous condition that can occur if batteries are charged too fast. One of the byproducts of Gassing are Oxygen and Hydrogen. As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode.Self DischargeThe self-discharge rate is a measure of how much batteries discharge on their own. The Self-Discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside.For instance, flooded cells typically use lead alloyed with Antimony to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to 8-40% per month. This is why flooded lead-acid batteries should be in use often or left on a trickle-charger.The lead found in Gel and AGM batteries does not require a lot of mechanical strength since it is immobilized by the gel or fiberglass. Thus, it is typically alloyed with Calcium to reduce Gassing and Self-Discharge. The self-discharge of Gel and AGM batteries is only 2-10% per month and thus these batteries need less maintenance to keep them happy.Battery Group SizeTo further complicate matters, manufacturers for marine batteries make them in all sorts of sizes and voltages. Battery case sizes are typically denoted by a "Group Size" which has nothing to do with the actual size of the battery. For example, Group 8D batteries are much larger than Group 31 batteries.Table of Battery Group Sizes, Voltages, and Approximate Exterior Dimensions<img src="http://i67.photobucket.com/albums/h303/SyKo760/battgroups.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" />The group size will merely indicate the approximate exterior dimensions (including terminals) and voltage of the battery in question. However, the exact dimensions can only be directly obtained from each manufacturer.Nickel Cadmium CellsSeveral people have inquired about NiCad cells for Marine environments. I don't like them due to their high toxicity and low power efficiency.There you go peoples //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif</blockquote>

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