This TechTip is the third in a series of six, that cover the twelve-volt (12V) battery and power control systems used in modern RVs. All RVs depend upon a reliable twelve-volt (12V) power source, however that venerable 12-volt system has been changing under the ever-increasing demands being placed on it. These TechTips will focus on the power sources 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. Unlike motorhomes, trailers have a single Coach power system, lacking the complexity of having two independent co-existing systems.
The Topics covered in this group of TechTips are:
•The impact of the use of Computerized Control (Multiplex) Systems (TechTip #120)
•Battery Types and the Significance of Battery Type on usability (TechTip #121)
•Inverter/Charger systems and settings (TechTip #122) (This one)
•New products that Improve Battery Longevity and Maintenance (TechTip #123)
•The interaction between Chassis and Coach Battery Systems (TechTip #124)
•An alternative battery maintenance approach while a Coach is 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.
The Inverter Systems & Power Consumption:
Inverters are electronic components that provide 120 Volts of AC (VAC) power. Inverters use electronic components to convert 12V DC from the Coach Batteries to 120 VAC. Typical inverters designed for RV use have wattage ratings of 2,000 2,500, 2,800 or 3,000 watts. Older models provided a modified sine wave (MSW) output, which is often known as a square wave, while newer ones provide pure sine wave (PSW) output which replicates the shape of the alternating current wave that is provided by electrical utilities.
In most motorhome configurations, inverters also include a three (3) stage battery charger and an integrated transfer switch. Their output power is connected to the RV’s 120VAC electrical system, often through a subpanel containing circuit breakers in the RV’s main electric circuit breaker panel, to give the user control over those specific circuits that can be powered by Park Power or in its absence (with the inverter turned on) by the inverter. There is often a resettable push button circuit breaker, (see Yellow Arrows in photo) on the side of the inverter (mounted upside down in photo) that will pop out, should the load on the AC circuits be greater than the inverter can provide.
The transfer switch inside the inverter handles the transition from line cord power to battery inverter power, seamlessly. Most inverter/chargers have a remote, wall or cabinet mounted, control panel that provides both control of the inverter and charger, as well as the ability to adjust the operation of the inverter/charger to the RV’s battery and electrical systems. These panels are connected by a flat network cable, appearing like the telephone cables most of us are familiar with. The connections on each end, while similar are different, telephone and network cables are not interchangeable.
There are some basic principals that, once understood, make inverter/chargers much easier to use and manage. Inverters are about eighty-five percent efficient in their conversion of the 12V DC to the 120VAC. For our purposes it is enough to know that there is a mathematical relationship between the voltage (V), the current (I) and the Power in watts (W). It is that the power is the product of the Voltage multiplied by the Current (P=VI). One also should understand that there is about a fifteen Percent (15%) loss of efficiency in the conversion process using an inverter. Thus, of the power that goes into the inverter, only about 85% comes out, with the conversion process absorbing the balance.
What this means is that running any item that requires 120VAC on an inverter, takes more energy than that shown on the power rating of the item. For instance, a modern Flat Screen TV uses about 1 amp of power when connected to the 120V AC system or (120 Watts). The energy consumed by the TV using a 12 V DC battery source is P/V=I or 120/12 = 10 amps of 12V DC power. When the inverter produces that power then there is an energy draw off the 12 Volt batteries of 10 amps, plus the additional energy consumed in the inverting process of about 15% or 1.5 amps. Therefore, that TV will use draw about 11.5 amps to operate using the inverter as its power source.
Another factor that needs to be factored into our calculation, is the fact that as the current draw increases, most batteries will present a decrease in voltage, therefore an increase in current is needed to compensate. Batteries of all chemistries react to increasing power draws in different ways. A Lithium-Ion battery provides the most stable voltage, due to their characteristic of maintaining their voltage until they are almost ‘dead’. They have a very flat power curve. When you add the Satellite Receiver, the Home Theatre and perhaps multiple TVs being on at the same time, in addition to the lights, a refrigerator and other battery powered equipment, you can see that the batteries can be overwhelmed with all these loads over a period of time.
Let us assume that you decide to start the generator (can plug in to a park power source) so that the charger portion of the inverter/charger can begin recharging the battery bank. If the batteries are under eighty percent (80%) of full charge, then the charger will enter the Bulk charge phase which means it will provide as much amperage as is possible (within the capabilities of the charger), to bring the battery voltage up. Once the voltage crosses the Absorb charge threshold, (often 14.4V to 14.8V), the charger enters the Absorb phase and the battery will be between eighty and ninety percent (80 and 90%) of a full charge. During this phase, the voltage is maintained at the programmed amount and the amperage used to charge the battery, reduces as the battery approaches a full charge. Once the voltage reaches a pre-set level, or a certain length of time has passed, then the charger enters the Float charge phase. Voltage will drop to a pre-set level and at this stage the battery is considered fully charged. Some chargers will cut back and ‘check in’ on the batteries regularly to ensure they stay at the fully charged level.
There is another charge phase that some Chargers provide, which must be user engaged occasionally. It is known as the Equalization phase, and is only used on wet cell, lead-acid batteries. This phase increases the voltage above the usual charge voltage and is used with all loads disconnected. It begins a higher voltage charging process that aims to bring all cells in the batteries in coach battery pack up to the same voltage and state of charge, "cleaning them up internally” by reversing some of the effects of the acid on the lead plates. Running the equalization phase occasionally will extend the life of the batteries. This phase is not part of the preprogrammed automatic charging process.
All modern inverter chargers have specific profile built in by the inverter maker to properly charge Lead-Acid, Wet Cell Batteries, AGM1 or AGM2 for Absorbed Glass Mat Batteries. Recently, an additional capability has been included that provides a Lithium-Ion battery profile. In other cases, there is the ability to enter a Custom profile that can be set up to properly charge Lithium-Ion Batteries. In the setup process, the type of battery must be selected, then the amp-hour capacity of the battery bank selected, (the size of the battery bank) and numerous other parameters that can be adjusted, are entered to optimize the charging for the existing (or new) battery pack. If replacement batteries are similar in electrical specifications to the old battery pack, then there should be no need to adjust them. All battery manufacturers can provide the requirements, many have them shown on their website, so that the settings can be properly configured. If you are uncertain about how to make the adjustments, have a knowledgeable RV Technician adjust the settings to optimize the charging and thereby extend the life of the batteries.