June 7, 2021

TT# 124 12 Volt RV Battery and Control Systems, Care and More! (Part 5)

By: Rob Lowe

Featured Picture

This TechTip is the fifth in a series of six, that covers the twelve-volt (12V) battery and power 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. For simplicity, this TechTip will focus on the power sources that are found in a motorhome, with two independent 12V systems, one for the engine, control and running lighting and the second for the Coach or House DC power. 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)
New products that Improve Battery Longevity and Maintenance (TechTip #123) 
The interaction between Chassis and Coach Battery Systems (TechTip #124) (This one)
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. 

Since motorhomes have two battery systems, it is important that both systems charge when the coach is being driven as well as when the coach is parked at a campground, connected to a Coach power source. Of course, newer RVs often have a solar panel system that can assist with the charging, most often when the coach is stored or parked, when not connected to an energy source. Concurrent with the charging process is that both systems must be isolated from each other, so that a current draw on one system does not affect the other system. A dead chassis battery should not follow a dead coach battery and vice-versa. 

Battery Charging: (1) On the Road
Big-Boy RelayOn most older RVs, when the RV is driven, the Chassis battery (the one that supplies power to the starter, engine computer, control, lighting and running systems) is the one that begins to be charged by the engine driven alternator. When charging begins, the chassis battery takes any excess energy not required by the running systems that is provided by the alternator. As the chassis battery becomes charged, a point is reached where the RV control system, (which has been monitoring both battery systems), brings in the coach battery for charging.  This is typically in the 13.2V to 13.4V voltage range and that voltage must be achieved for over one (1) minute. At this point, the two batteries are connected together and the normal charging voltage of 13.8V to 14.4V is applied to both batteries. Many of us are familiar with a large 200 Amp relay known as a ‘Big Boy Relay’ which connects the two battery systems together when the control system known as a Bi-Directional Isolator Relay Delay (BIRD) [gas or diesel] deems it appropriate. B.I.R.D. Isolator
This Big Boy Relay requires continuous power to keep the two battery systems connected. Some newer RVs have a ‘smart’ relay that is used to connect the two battery systems together for charging. This ‘Smart Relay’ is ‘Green’ since it consumes energy only when actually making the  connection of the batteries, then it stays connected until the control system turns off the charging. 
Big Boy for Gas RVsOn gas powered vehicles the demands on the starting system are lower, such that a smaller 100Amp relay can be used to connect the batteries. Either way, the two systems are connected when the chassis battery becomes close to fully charged and the power from the alternator can be transferred to the Coach Battery. Driving longer ensures that both batteries are fully charged by the alternator. It is noteworthy that these types of relays are used for the battery disconnects on diesel and gas chassis RV as well as the ‘Boost’ relay that connected both coach and chassis batteries together to allow the engine or generator to start, should the usual battery be low and neither engine will start. On diesel coaches these relays are often found near the batteries. On gas powered coaches, the smaller style relays are often found in the Battery Control Center (BCC), near the batteries as well. 

Battery Charging: (2) When at the RV Park
A similar procedure takes place when the RV is connected to a Park AC power source (or the generator is started), with the two batteries exchanging priorities. When 120VAC power is available and connected, the power management system first confirms that the source has all the proper characteristics, then power is supplied both to the RV’s 120VAC systems and to the charger, which begins to charge the coach battery. This charger may be part of the Inverter or it may be part of the Converter in RVs without an inverter/charger.  This charger prioritizes the Coach Battery system until it reaches the same 13.2 to 13.4 voltage range, then the same control system (B.I.R.D.) connects the chassis battery into this charging source by activating the same relay. In both charging scenarios, the object is to bring both batteries to the optimum, fully charged voltage. The Inverter/Charger has various ‘Profiles’ stored in memory (or able to be programmed) to match the required charging characteristics of wet cell (Flooded), AGM1, AGM2, or Lithium-Ion batteries. In the newest software versions, a Custom profile can be set up, that is optimized for a battery that requires different settings. These settings vary in the voltages provided and the length of time a profile maintains the voltage in each stage along with other similar characteristics to meet the characteristics of the battery, as determined by the battery maker/supplier. Generally, newer Inverter/Chargers have the capability to be programmed, older ones do not. In some cases, a ‘software’ upgrade will expand the capabilities. This may involve upgrading (replacing) the circuit board inside the inverter/charger with a newer version and replacing the remote control with a newer version that will allow the newer battery profiles to be utilized, or the customization to be programmed. Where it is possible to upgrade the Inverter/Charger, the electrical components can supply the charge, the programming is required to instruct the control what and when to adjust the charging. The control simply needs to be updated to carry through with the new programming requirements.

Battery Charging: (3) Solving Charging Issues:
Corrosion BlockThe issues that most frequently arise with charging two systems on a motorhome almost always relate to the condition of the batteries, especially the primary battery in each charging mode. For instance, if the coach battery is failing to attain the proper fully charge level when plugged in at a RV Park, then the control system will not transfer the charging power over to the Chassis battery. The RV owner does not become aware of the low chassis battery, until he/she tries to start the engine and then learns that the starting battery is dead or low in charge. The opposite occurs when the RV arrives at the RV park and in the transition from driving to being fully plugged in, the RV coach systems are not fully operational, due to a chassis battery that is almost dead, and the RV depended upon the running engine with the alternator to provide power to the chassis system while operating on the road. 

Before blaming the charging failure on the actual battery, no matter which system, it is important to check the battery connections on the Chassis and the Coach batteries at the batteries and the ground connections where the ground cables connect to the chassis frame. With all power turned off, remove the frame connections, clean the area where the cable is attached, spray some Corrosion Block® on the frame and cable connection and reinstall the mounting hardware. This same product should be sprayed onto the battery connections and the hold down hardware after neutralizing any corrosion with baking soda. Then each battery should be charged and tested to confirm its condition and state of charge. If the battery checks out OK, then it should charge when in the RV.  This battery and cable maintenance should be done every other year to ensure that dependable charging will occur. 

In addition, there is a (white) connector going to the control system circuit board, that can also buildup corrosion that cannot be seen by the naked eye. Applying Corrosion Block® to each individual connection of the connector may solve the charging issue. Check out my ‘Rob’s Blog’ for articles dated January 23,25,30, 2014 and February 7,2014 for more information to check out this issue. 


Thanks for your comment.It will be published after reviewing it.