Chassis Battery Chargers

By Mark Quasius, F333630

June 2014

Ready to hit the road after another pleasant camping experience, you turn the key in your motorhome’s ignition and . . . pffft. Dead battery. It can happen if you’re boondocking or if you’re plugged in to a campground pedestal. But it can be avoided, too.

A motorhome has two separate battery systems. The house batteries provide 12-volt-DC power to operate non-engine electrical accessories, such as interior lighting, the water pump, and vent fans. House batteries are charged via either a converter or an inverter/charger; both require AC power, which is available when a motorhome is plugged in to a campground pedestal or when the generator is running.

The chassis batteries, on the other hand, start the engine and supply power to driving-related accessories, such as windshield wipers and vehicle lighting. Chassis batteries are charged by the engine’s alternator when the engine is running, just as in any other vehicle.

Motorhomes go a step further by incorporating a charge solenoid or battery isolator to connect both battery banks. The solenoid is energized whenever the ignition key is on, so that the engine’s alternator can charge both battery banks as the motorhome is being driven. Once you arrive at a campsite and switch off the engine, the solenoid opens to separate the two battery banks. When dry camping, this prevents the chassis batteries from being drawn down while the house batteries are being used to run various electrical accessories.

However, the chassis batteries aren’t totally out of service. Some accessories, such as the radio or power steps, may still tap those batteries. Also, phantom loads that supply power to engine control modules (ECMs) and the radio account for a small but continuous draw. Those factors, plus self-discharge, which is the natural reduction of power when batteries go unused, can eventually weaken the battery enough that it can’t start the engine.

Fortunately, accessories are available to maintain chassis batteries at their maximum charge whenever AC power is available. Two basic system designs exist. One uses AC power to keep the batteries charged; such battery chargers are designed to provide fairly small amounts of DC power on a continuous basis without overcharging or damaging the batteries. The other design uses an intelligent relay to sense when the house batteries are being charged, and then “steals” some of that current and sends it to the chassis batteries.

A “smart” AC maintenance charger is similar to a high-output charger except that it does not provide the amperage needed to fast-charge a battery bank or to jump-start an engine with dead batteries. And unlike older-technology trickle chargers, maintenance chargers can stay connected to the battery indefinitely without damaging it. Various models, such as the popular BatteryMINDer series, provide a smaller float charge to safely maintain a battery. A 1.5-amp or 2-amp current is regulated so a battery is not overcharged. Models such as the BatteryMINDer 1510 are powered by plugging in to an electrical outlet. The DC output leads have a quick-release connector that can be attached either to a set of permanent ring terminals or alligator clips for portable use. The charger can be permanently mounted to an electrical outlet’s faceplate.

The BatteryMINDer incorporates pulse technology as a means of dissolving battery sulfation. Lead sulfate crystals build up on deep-cycle batteries that are not consistently kept at or near full charge, which can lead to problems that include shorter battery life and loss of cranking power. One remedy, called equalization, uses high voltage from the inverter/charger or the converter to remove sulfate from the battery plates. Pulse technology uses high frequency instead of high voltage to accomplish the same thing.

Pulse technology is fully automatic in the BatteryMINDer line of products. The process desulfates on a continuous basis without damaging batteries, which can happen with high-voltage equalizing. Pulse technology in some cases is claimed to quadruple the life of new batteries, while also bringing back older, weak batteries that are heavily sulfated but not yet seriously damaged.

The BatteryMINDer 1510 provides a 1.5-amp charge and monitors battery temperature to ensure the battery won’t be overcharged or undercharged. The BatteryMINDer 2012 series features 2-amp chargers that can be used on either AGM or flooded batteries. Such chargers are a good choice for maintaining and desulfating the house batteries for extended non-use periods, providing that AC power is available. The 2012 series has eight LED indicators that provide information on the batteries’ condition and state of charge.

Keep in mind that most RVs do not use AGM chassis batteries, so that option is probably not a concern for most people. However, such batteries are becoming popular replacements for the original chassis battery because of their high cranking power, slow rate of self-discharge, and longer life.

The AC charging method has its pros and cons. Pulse technology is a plus. On the downside, an AC connection is needed. Keep in mind that if you store your unit without AC power available, you must unplug the DC leads from the charger; otherwise, AC chargers such as the BatteryMINDer will draw power from the batteries while in storage. Regardless,  batteries will self-discharge, especially if stored in areas with temperatures above 80 degrees.

DC-powered chargers really aren’t chargers at all, in the sense that they don’t actually produce a charge. Instead they “steal” it from the house battery system, which is why they sometimes are referred to as battery thieves. But they are nice thieves, because, like Robin Hood, they rob from the rich and give to the poor. They really aren’t stealing from the house batteries. Rather, a portion of the incoming charge headed to the house batteries is diverted to the chassis batteries.

Most DC chargers, such as Xantrex’s Echo-Charge and LSL Products’ Trik-L-Start and Amp-L-Start, utilize a three-wire connection. One wire goes to the chassis batteries, another goes to the house batteries, and the third wire goes to ground. In most cases, the two battery wires are connected to each side of the charge solenoid for ease in installation.

The chargers use intelligent circuitry to monitor voltages. When the converter or inverter/charger is running, the voltage will be higher than when the battery is at rest. The DC charger senses this and connects the two battery banks so that the incoming charging current can be delivered to both. Once AC power is removed from the motorhome, the house battery voltage will drop to an at-rest voltage, typically 12.6 volts. The DC charger senses this and breaks the circuit between the two battery banks to isolate the chassis batteries.

The Trik-L-Start is a 5-amp charger with a series of LEDs that indicate charging status. Its big brother, the Amp-L-Start, has 15-amp capacity and a few other features, such as over-voltage protection. The charger also works when solar panels are connected to the house batteries. The Echo-Charge is also a 15-amp system, similar in operation to the Amp-L-Start.

Another DC-powered charger, Intellitec’s Bi-Directional Isolator Relay Device (BIRD), is similar to the LSL Products and Xantrex offerings in that it senses when the house or chassis batteries are receiving a charge from a running engine or an operating generator. But current doesn’t pass through the BIRD, as is the case with the aforementioned products. Instead, the BIRD operates the charge solenoid, which passes the power directly to the house and chassis batteries. And because the charge solenoid can handle higher current loads, the 5-amp or 15-amp limit imposed by the other DC chargers is eliminated. The BIRD utilizes four wires; the additional wire connects to the ignition circuit. The BIRD also has preprogrammed delays to prevent rapid cycling of the charge relay during brief dips or spikes in voltage.

Like their AC counterparts, DC systems have pros and cons. Pulse technology is not available to desulfate the batteries, but there are no AC power requirements, either. So, it’s an easy task to add a “battery thief” without concerns about disconnecting the DC lines during storage. Nor is there any potential high-voltage shock hazard.

Many chassis battery charger systems, either AC or DC, now are being installed as original equipment in motorhomes. For those without, it’s a good idea to add one. The simple three-wire DC units can be an easy do-it-yourself project or can be a job for any RV dealership or service center. Installing the AC units might be a bit more involved, unless you have an available AC outlet near your charge solenoid. Either system will help ensure that your chassis batteries remain charged and ready for use when you need them.

1485 Jacobs Road

DeLand, FL 32724

(800) 251-2408

(386) 738-7307

5804 Babcock Road #512

San Antonio, TX 78240

(877) 257-4655

VDC Electronics

155 W. Carver St., Suite 2

Huntington, NY 11743

(800) 379-5579

541 Roske Drive, Suite A

Elkhart, IN 46516

(800) 670-0707

*FMCA commercial member