RV Modifications Part 6- Chassis Battery Charger

 October 22, 2021

RV Modifications Part 6- Chassis Battery Charger

Most class C RV's have 2 separate 12V electrical systems.  There is a chassis 12V battery which is used to start the motor and operate all the chassis 12V electrical equipment including the headlights, entertainment system and anything else normally associated with an automotive cab.  There is a second electrical system that serves the coach part of the RV which provides power for the refrigerator, lighting, furnace, fans, TV and other accessories.  The coach electrical system includes a pair of deep cycle batteries for energy storage and shore power transfer switch to operate AC appliances and the 12V charger and inverter while plugged into shore power, generator and solar power systems for providing AC power when not plugged in at a campground.  All of the batteries, whether chassis or coach, are subject to continuous loss of charge due to electrical loads, even when apparently sitting idle.  Many of these "ghost" loads come from electronic devices like engine control computers, entertainment systems, USB power outlets, carbon monoxide and smoke detectors.  Over time, without charging, these batteries will slowly discharge and end up dead.  

The solar charger in the coach is very helpful in this circumstance.  Any time the RV is exposed to daylight, the solar charger generates electricity which recharges the coach batteries and provides power to run the refrigerator and other loads.  Unfortunately, the solar charger in the coach does not provide power to recharge the chassis battery, which is responsible for starting the motor.  

Our Winnebago View 24D did not come with any sort of charging system for the chassis battery.  For long term storage, most people would use a clip on battery maintainer to keep the battery properly charged.  One popular option in RV Forums was a device called the TRIK-L-Start which was basically a battery combiner which would connect between the coach and chassis batteries.  When a charging voltage was sensed on the coach battery (over 13.3V), a relay would then connect the two batteries.  This would allow the shore power or solar system that was recharging the coach batteries to also charge the chassis battery.  However, the device inexplicably became unavailable and some internet sites were really gouging the price, selling it for over $100 when it was originally a $35 product.  There are many other battery combiners available for RV and Marine use, but these are also expensive.  

My solution was to use a battery charger/maintainer that I had on hand (Deltran Battery Tender Plus) plugged into one of the coach AC outlets, and connected to the positive battery terminal via a simple 12V SPST normally closed automotive relay triggered by the ignition circuit.  The Deltran Battery Tender Plus is a smart battery maintainer which senses and adjust charge rates to prevent overcharging, and can safely be left connected for float charging indefinitely.

There is a fuse box on the outside of the driver seat pedestal that has circuits that are direct wired to the battery and also connected through the ignition switch, so it was easy to route the Battery Tender Plus's charge directly to the battery, and to tap into the ignition circuit to operate the relay.  

I used a self tapping screw to provide a common ground behind the driver seat and mounted the Battery Tender under the cab-over cushions so that the charge status could be easily seen from inside both the cab and coach.

When the chassis ignition is turned on, the relay disconnects the charger from the battery, preventing any potential problems with the chassis charging system when the ignition system is on.  Under most circumstances, the AC outlets will only be energized when the chassis is parked and either plugged into shore power or when the power inverter is turned on to operate AC appliances in the coach.  But there may be instances when the inverter is left on while the RV is underway, so having the relay disconnect the charger from the battery is the safest way to configure this.  Cost-wise, the relay was only $2.40 on eBay (5 for $11.99 with free shipping).  The Deltran Battery Tender Plus was $40 at Costco.  

With this system installed, we shouldn't have to worry about ever having a dead starter battery so long as there is sunlight, shore power, or LP for the generator available.