When it comes to adding a bit of electricity storage to our RV, the question needing a good answer is how to arrange a bank of batteries. There are many factors at work that play against each other. A good engineering decision means finding an effective balance between these factors.
Question: I want a bit more juice - should I add another 12 v battery in parallel or use two 6 volt batteries in series?
Quick Answer: For most folks, the matters of convenience, budget, and battery type are more important than the factors that differentiate between two 12v in parallel or two 6v in series.
What follows is a summary of the factors involved and where you can find more information.
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 together 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. The battery bank voltage is the sum of all the individual batteries in the bank. The current through each individual battery is the same as the current of the load.
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 the same for all the batteries but the current is divided between them.
The critical difference is that series batteries add voltages together at a common current while parallel batteries add currents together at a common voltage. This is important because lead acid battery energy capacity is significantly influenced by the current drawn from them and not by their voltage.
Series arrangements may mean fewer cells which may allow for more stuff for greater capacity or they 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 as 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 (which does sap energy, but only when a load is applied) and, since it does not contribute any voltage, it will show as a reduced battery bank 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 (all the time, which adds up in sapping energy). This can cause heat and loss of water in the failed cell that run a battery down by itself.
There are a number of myths which are very strongly held by some individuals about RV batteries, what is best, and how to configure small battery banks. These often involve an almost religious belief in a phantom 6v golf cart true deep cycle battery that is superior in every way to any other battery in existence. One of the keys to watch for in any discussion is when the certitude of opinion does not match the accuracy of measurement in the issue. In the case of battery configuration, a simple examination of the specification sheet for a line of 6v and 12v RV batteries will tell you quite clearly that there is no one single best solution for all circumstances. It is not accurate to say one is best.
The idea of a 'true deep cycle' battery, like many myths, is probably a hold-over from days long gone. The idea is that a 'true deep cycle' battery uses stouter construction and can handle more abuse for a longer time than 'regular' batteries. Modern batteries have done a lot to improve the ruggedness in design as can be seen in battery warranties going out to 5 years or more. In practical terms, typical RV use does not place significant 'deep cycle' stress on batteries in any regular way that would advantage a stouter battery than is readily available as an RV/Marine battery.
Battery capacity is more a function of weight than anything else. The voltage of the battery is nearly insignificant in comparison. Heavier also usually means bigger (more volume to put lead and acid) but it is the weight of lead and acid in the battery, in the proper proportion, that provides energy capacity. If you need more capacity then you need to either reduce loads, increase frequency of charging, or increase the size and weight of your battery bank with either more batteries or bigger and heavier batteries.
The myth that a serial configuration is inherently better has no foundation in fact or in practicality. The facts are that capacity is primarily weight, that the current sharing in a parallel configuration that a serial configuration does not have can have a significant impact on capacity, and a low voltage bank means that serial options are highly limited.
Watch out for the myths and don't be mislead by preachers. You want observable measure here, not religion. Look for measure and not anecdote (e.g. witness of a true believer). Make sure you understand both the precision and accuracy behind assertions made before you take them as is.
Usage - when you go looking for a battery, consider the balance between maximum load and intended depth of discharge. This is related to 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 energy capacity means more weight. 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. See Understanding Battery Capacity for a discussion of factors influencing the available energy capacity from a battery. It appears that this topic is another one of those that really gets people going on the forums.
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, the wiring and connections are important and a parallel configuration might provide an advantage in reduced battery resistance.
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.
Batteries:
How to keep them alive for years and years (Dankoff)
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