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Determination of lead-acid battery capacity via mathematical ...
Battery Backup Application Handbook - Specifying battery capacity involves a bit more than multiplying the load current by the backup time in hours. You must first de-rate the battery for capacity tolerance, temperature, and discharge rate.
Charging the lead-acid battery - It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge.
4. Proper Charge Methods 6 - The author of a paper in a battery seminar explained that charging a sealed lead acid battery using the traditional float charge techniques is like 'dancing on the head of a pin'. The battery wants to be fully charged to avoid sulfation on the negative plate, but does not want to be over-saturated which causes grid corrosion on the positive plate. In addition to grid corrosion, too high a float charge contributes to loss of electrolyte.
Sizing a Battery Bank (Gel,AGM, or flooded) - Industry recommendations for house battery bank sizing range from 3x to 4x of anticipated daily needs. This sizing also allows you to replenish the daily needs using a bulk charge in a minimum of time (batteries can be bulk charged until they are 80% full). Typical installations are also sized so that the battery does not discharge below the 50% mark that marine battery manufacturers typically use in their life-cycle estimates (the deeper you discharge a lead acid battery, the shorter its life).
Lead-Acid Battery Life & Usage - It is sometimes difficult to convey to users the tradeoffs involved in designing a lead-acid battery bank system. For that reason, some type of simplified model that is relatively easily understood may be helpful.
The Technical Side - Care and Feeding of Sealed Lead Acid Batteries -
lead acid batteries, battery history types and development the solar navigator world electric navigation challenge, nelson kruschandl - Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher. ... Even though battery capacity at high temperatures is higher, battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery,
After Gutenberg » Blog Archive » Traction Battery -
First, any typical RV storage battery you get will be a series arrangement of cells. Each lead acid cell will contribute a bit over two volts so a nominal 12 volt battery will have six cells. Note that the energy you get from a battery, its capacity, depends upon the total energy of all its cells whether connected in series or parallel. The connection arrangement just balances the current and voltage components of this energy.
While a battery is a series arrangement of cells, a bank of batteries may be arranged with both series and parallel configurations. When this is done, careful attention needs to be paid to balancing the currents through each part of the battery bank and protecting against internal currents caused by failure.
Series arrangement means that voltage in each battery is added by connecting the negative side of one battery to the positive of the next.
A parallel arrangement connects all the positive sides of each battery together and all the negative sides of each battery together. This means the voltage is that of the lowest voltage battery but the current capability is that of all of the cells added together.
Series arrangements means fewer cells which means each can contain more stuff for greater capacity or can be built more rugged in a given amount of space. A series arrangement also means that a weak cell does not sap the energy of the others and any current going through the battery will just pass through a weak cell. This fact, that any load or charge current goes through all cells equally, tends to keep all of the cells at an equal state. It also means that a failed cell does not cause internal circulating currents. The failed cell may add some resistance and, since it does not contribute any voltage, it will show as a reduced battery voltage.
A parallel arrangement means that current is shared between batteries and this means that each battery is exercised less vigorously which may increase usable capacity. Since removing any single battery will not change the voltage, there is a redundancy in batteries that can be useful in the event of failure (if that failure is detected early enough). Parallel is also useful in low voltage systems because it may be easier to find batteries at needed supply voltages. A failed cell in a parallel bank will sap energy as the other cells try to charge it. This can cause heat and loss of water in the failed cell as well as lost current capacity.
The usable current from a battery depends very much on how fast energy is taken from the battery. Slower energy draws usually mean more usable capacity from the battery. Energy draw is usually indicated by current because a battery's voltage is relatively fixed by the battery design. Energy draw is called power and is the product of voltage and current. Energy capacity is a product of power and time.
The capacity gained by reducing energy draw or current drain in a parallel configuration versus a series configuration can be determined by evaluating Peukert's Formula T = C / In where T is how long you can drain current I from a battery that has a capacity C and an internal resistance characteristic n. For the case of two six volt batteries in series versus two 12 volt batteries in parallel, the comparison is when the current changes by a factor of two (when voltage doubles, current halves and vice versa for the same amount of power). The formula would be
Tp - Ts = (C / In) - (C / (2I)n) = C(2n - 1) / (2n In) = Tp ( (2n - 1) / 2n )
{subscript p for parallel and s for serial, serial has twice the current of parallel, the LCD (2I)n which is where the 2n comes from in the Tp term to be able to subtract the fractions, percent change divides both sides by Tp and then multiplies by 100 - check the algebra yourself and let me know if you think you see an error!}
So percent change from the low current to high current times is (2n - 1) 2n . When this is calculated, the range for typical batteries means drain times will be from 71% (worst case worth purchasing where n=1.25) to 65% (best case, n=1.05) changed from parallel to serial. In other words, doubling current will reduce time of draw from 65% to 75%. For the 40 amp hours available (discharge to 80%) at a 5 amp load, the parallel configuration could provide 8 hours of usable battery while the serial configuration would provide maybe 6 hours. This would be balanced by the batteries in series probably having a bit more capacity to start with.
interpreting this calculation: The conclusion of this calculation is best stated as that the effective usable energy is maximized by using two batteries in parallel rather than by using one and then the other. When considering a parallel versus serial battery bank, two factors often cloud the issue. The first factor is that comparisons have to be made with comparable batteries in the bank. The other is that a serial configuration doubles the voltage which reduces the current for a particular power level by two. So comparing two 6v batteries in serial to two 12v batteries in parallel, the outcome will depend more upon the battery than the configuration.
Longevity or Load? - when you go looking for a battery, you need to consider the balance between maximum load and longevity. This is the deep cycle versus starting consideration. Your potential maximum load will definitely influence how you wire your battery bank and may be important in choosing a battery that is designed to provide the necessary current. Usually, a microwave through an inverter is about peak load in most RV uses and this means maybe 150 amps out of the battery for a few minutes. This is not a problem for most batteries but does mean you will need low gauge wires and good connections.
Size . The battery, including wires and connectors, will need to fit in the available compartment. Smaller batteries will fit easier but bigger batteries will contain more energy.
Weight . more capacity usually means more weight. But you need to consider the impact this weight will have on your rig depending upon where it is placed and you also need to consider your ability to maintain (lift, remove, shove around for access) the batteries.
Cost - The price of the battery is one of your primary clues about manufacturing trade-offs. For instance, Trojan sells three 6 volt batteries that are all the same size costing from $55 to $95 (see Uve's site). The higher prices get you more capacity, less internal resistance, and lower cycle ratings. The lesson in this case is that, for a given volume or physical size, higher price gets you more capacity but less life.
Availability . Many battery types are difficult to ship so you are limited by what you can find at outlets in your area or you need to add shipping charges to the cost.
Capacity . Be careful not to consider just an amp hour rating as a capacity measure. Capacity is always amps times volts times time, so multiply the amp hour rating times the battery voltage to get a comparable capacity number. Also be sure to use equivalent amp hour ratings. The twenty hour rating is usually a common specification.
Warranty . The warranty on the battery will often tell you something about what the manufacturer and the retailer think about the battery's longevity in typical service.
Charge cycle ratings . This indicates how many times you can expect a battery to be discharged to a certain point and then recharged before it starts to fail. It is a ruggedness rating more than a quality rating. Watch out to make sure you compare cycle ratings for equivalent discharge levels. 80% and 50% are perhaps most commonly used.
Battery Type . The primary classifications of concern are whether or not the battery is a sealed type that can be placed inside or whether it needs a specially vented cabinet. This gets into access for maintenance and other factors as well. The type of battery will also be a factor in charging and in usage considerations.
The batteries and how they are configured are only a part of the entire system in your RV low voltage system. A typical RV system does not push any particular extremes so the design does not need to give undue weight to any single factor. This complicates the choice because you have many different considerations to play against each other and none of them will likely have any significant impact on the total result for typical RV usage.
Cell failures are fairly uncommon with even modest care to use and charging so that difference between parallel and serial configurations is not significant in this regard. Plan on replacing your entire battery bank every five to ten years.
Current drains average fairly low and you should only use 20% to 50% of the battery capacity so the change in current between the two configurations for a capacity reason is also insignificant in most cases. If every last drop is important, you will likely already have solar cells or a generator.
Since all you are looking for is twelve volts, anything past two batteries in the bank will require some sort of parallel configuration. As you add batteries to the bank, your attention will need to be on battery matching and cabling concerns.
You are more likely to make a difference by attention to maintenance, good wiring and connections to reduce resistance losses, careful choice of loads for efficiency, a multi-stage intelligent battery charger, and a few lifestyle habits that can drastically impact energy use.
For the simple case of adding a bit of capacity to an existing RV system, the question of parallel or serial is more of convenience than anything else. The current loads are typically low and the demands do not usually stress the battery charge sufficient to make the current versus capacity consideration very important. Parallel might provide a bit more capacity and serial might be a bit more reliable but the benefits can be easily outweighed by smart battery choices.
Long term boondocking puts a premium on capacity, but those in this group also usually have means to keep their batteries at a reasonable charge point so, again, the differences between serial and parallel are minor.
Inverter driven heavy use is another matter. Here, the battery loads still average fairly low but there are high peak draws as when you warm a cup of coffee in the microwave. In this case, it is the wiring and connections that will be the major concern and not the battery bank configuration. [unless you go for a 24v or 48v inverter]
These considerations indicate that, unless you have some specific requirement to match, battery bank configuration choice is mostly one of convenience. The factors of cost, size, type, and availability will likely dominate the decision tree.
At SieraNevadaAirstreams.org Travel Trailer Owner's Guide: About power, energy, amps and volts, and understanding Energy and Power. Be sure to check the resources at the bottom of these pages for additional links. Also check the Zephyrs weblog for the latest links and discussion updates on this topic.
Constantin von Wentzel has a very good analysis of batteries for boating use. Similar considerations apply to RV use. See http://www.vonwentzel.net/Battery/index.html and note the links at the bottom of the page as well as his own pages. http://www.vonwentzel.net/Battery/01.Type/index.html - argues for AGM type batteries in boats. Charts how much charge the battery can store per unit weight and per unit volume for some common batteries. The next page ( http://www.vonwentzel.net/Battery/02.Size/index.html ) is about how to determine the size of a battery bank you need.
Northern Arizona Wind & Sun battery FAQ http://www.windsun.com/Batteries/Battery_FAQ.htm A lot of good information on this site about all sorts of alternative energy systems and equipment.
William Darden's site http://www.uuhome.de/william.darden/ with answers to common questions and buying tips.
http://www.usbr.gov/power/data/fist/fist3~6/3~6_cont.htm http://www.usbr.gov/power/data/fist/fist3~6/3~6_cont.htm
Proper care and maintenance of deep cycle batteries - http://www.usbattery.com/care.htm
Batteries:
How to keep them alive for years and years
(Dankoff)
http://www.dankoffsolar.com/cgi-bin/siteman/page.cgi?g=Detailed%2F23.html&d=1
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