March 5, 2021

TT #121 12 Volt RV Battery and Control Systems, Care and More! (Part 2)

By: Rob Lowe

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This TechTip is the second of a series of six, that cover the twelve-volt (12V) power systems used in modern RVs. As time rolls on, it is not beyond the realm of possibility that the low voltage systems on a RV will move to 24,36 or even 48 volts. The benefits of higher voltage need to be considered against the lack of components designed for these higher voltages in the near term. A few years ago, the automobile industry planned to move to 36V systems, using alternators designed to provide a 42V charge, however when many consumer electronic systems moved to 5V USB power sources, the industry decided to stay with 12V. Once again as the industry prepares for an EV revolution and the impending switch over to electric powertrains running 200, 400 or 800V power systems, it is a relatively simple procedure to power low voltage systems, currently at 12V or to move up. It is possible that mobile electronic applications will continue at 12V, since the inverters have already been developed to transform the higher voltages to usable 12V power for instrumentation and control systems and charging conventional 12V batteries used by these systems.

All RVs depend upon a reliable 12V power source, however that venerable 12V system has been changing under the ever-increasing demands placed on it. The next few TechTips will focus on some of these advances in RV Technology. These TechTips will focus on the systems that are found in a motorhome consisting of two independent 12V systems, one for the engine and the ‘on-the-road’ systems and the second for the ‘Coach’ or ‘house control’ system. Trailers have a single ‘Coach’ power system, lacking the complexity of having two independent, co-existing systems, as in a motorhome.
The Topics covered in this group of TechTips are:
The impact of the use of Computerized Control (Multiplex) Systems (TechTip #120)
Battery Types: The Significance of Battery Type on usability (TechTip #121) (This one)
Inverter/Charger Systems and Settings (TechTip #122)
New Products that Improve Battery Longevity and Maintenance (TechTip #123) 
The Interaction between the Chassis and Coach Battery Systems (TechTip #124)
An Alternative Battery Maintenance Approach for a Coach in Storage (TechTip #125)

Questions arise from a lack of understanding about how these systems operate, as expressed by new owners, as well as recent significant changes in the systems themselves, often overlooked by experienced RVers as they exchange their RV. This TechTip series will assist both in understanding the battery, the control and inverter systems. 

Battery Types:
There are three common battery types and configurations found in most motorhomes. 

Flooded Lead-Acid: (FLA)
Flooded Lead Acid (FLA) BatteryThis original RV power source is known as a Flooded, Lead-Acid (FLA) battery, which has removable cell caps that can be used to check the fluid level and (if necessary) add distilled water to each cell when required. In lower priced RVs, this power source is still the most common for Coach applications. There may be one or more 12 Volt batteries connected in parallel to provide the required capacity, or one or more pairs of 6 Volt batteries connected in a series-parallel arrangement. Frequently two GC2, 6 Volt ‘Golf Cart’ batteries are wired in series (a battery cable joins the negative of one battery to the positive terminal of another) to give 12 Volts between the two ‘unconnected’ terminals on both batteries. Typical Series-Parallel FLA Battery configurationTo increase battery capacity, that pair of batteries is connected in parallel (the open positive on one pair to the open positive on the other pair) with the negative posts similarly connected. This ‘series-parallel’ configuration doubles up the amp-hour capacity of the battery pack, while providing 12 Volts to the RV electrical system. The benefit of this arrangement is that it is a lower cost option that has good durability and longevity. All charging systems are designed by default to provide the optimal battery charging algorithm for this type of battery. The drawbacks are that each battery requires routine maintenance, both to add water and to reduce or eliminate corrosion that naturally occurs, since corrosive sulfuric acid gas is emitted when the battery is being charged. This acid attacks the mounting hardware and terminal connections, requiring routine corrosion cleaning and protection. These batteries also should not be discharged more than 50% of their rated capacity and must be in a vented compartment. In addition, the RVer should equalize these batteries regularly to ensure that each battery is in the optimum condition. This procedure is highlighted in the upcoming Tech-Tip #122.

Absorbed Glass-Mat: (AGM) 
AGM BatteryThe second type of battery configuration is a Lead-Acid battery known as an Absorbed Glass-Mat (AGM). These are comprised of lead plates, and advanced fiberglass mat suspended in an acid solution. AGM Installed in RV as Coach battery packEach plate is wrapped in a separator to avoid short circuits, then a piece of glass matting is put on each side of the plate, another plate is added, then the fiberglass matting is installed again. The sulfuric acid is absorbed by the fine fiberglass mat making the battery spill proof. This construction provides a faster charging battery that can deep cycle longer, all in a sealed and maintenance-free battery, that is safer to handle than a conventional FLA style. AGM Batteries in componentsIt does not let the naturally occurring gases escape when charged with the proper charging profile. This battery is far less prone to corrosion and because it degrades at a slower rate it should last longer in RV applications. It can also be discharged down to about 30% of its rated capacity with minimal degradation. This type of battery is the most common in middle and high-end RVs. It is often connected to an inverter to supply 120VAC to a refrigerator or other RV household components when park power is not available. 
Lithium-Ion-Phosphate: (Li-ion)
Lithium Ion BatteryThe Lithium-Ion battery is starting to appear in RVs. The nominal voltage of these batteries is 3.2 volts per cell or 12.8 Volts in 4 cells as compared to 2.0V per cell in 6 cells (12.0V) for other battery types. The Li-ion battery is sealed, being composed of hundreds of small cells connected in groups. There is a Battery Management system inside each battery to manage both the charging and discharging of that battery and to manage the battery when used in a group. They batteries are much lighter than either FLA or AGM batteries (approximately 28 pounds for 100AH compared to 66 pounds for a FLH or AGM) of the same capacity. They can be discharged (the internal control system prevents them from becoming ‘fully’ discharged) and they can be cycled through thousands of charge-discharge cycles with little degradation. They do not self discharge when not in use, whereas conventional Lead Acid batteries lose about 3% per month in storage. Their discharge profile is very flat, providing constant voltage until just before the built-in control system disconnects the battery to preserve it. Typical configurations use about 60% of the FLA or AGM amp-hour capacity, because of the steady output voltage and ability to be recharged so many times. Many engineered systems attempt to match the size of a comparable FLA or AGM system, however my experience is that is overkill in RV applications, given the capabilities of these batteries. A lithium-Ion battery can be quickly recharged, often taking around 1.5 hours to reach full charge in RV applications. These batteries do require a unique battery charge profile that has only recently been included in newer inverter chargers. (more about that in the next TechTip). They do not perform as well in cold temperatures and some engineered systems include a warming mat to maintain the temperature above a safe minimum. Pricing has been dropping, however the initial cost to install them is still more than an AGM system. One offset to the initial cost is that installing them will likely result in never needing to replace the batteries again.
Lithium Ion Battery Installed in aRV

For more detail on these batteries in actual RV usage consult the following TechTips on our website: TechTip #82, 83, 84, 94, 109 as well as my Blog article dated March 17,2015.


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