Heavy-Duty Alternator Charge System Modification

Cal Willis - Outfitter Apex 8 Truck Camper and GMC 3500.

Interested in upgrading your truck’s alternator wiring to charge your camper’s batteries more quickly and efficiently? Cal Willis, who holds a degree in physics, provides us with the details of his heavy-duty charge system installation. Of all the modifications that you can make to your truck and camper, this is one of the best.

This article describes my setup for charging my truck camper batteries using my truck’s alternator. It’s not intended as instruction about how camper batteries must be charged, or anything but basic electrical theory. Mine is a basic, but large capacity 12 volt system, containing only batteries, the alternator charging circuit, and routine 12 volt only loads, including a compressor refrigerator, a furnace, a water pump, and lights. I also have a nominal 110 watt solar charging system, and a standard 110 volt AC input three-stage charger/converter (never used when camping), which are not part of this discussion. Others have expertly described much more elaborate electrical setups that include a generator, an inverter, 110 volt AC current, and many more and varied loads, both DC and AC. My setup is more than adequate for my four 12 volt 135 amp hour deep cycle AGM batteries. I bought them in 2007 when I bought my camper, and they are still going strong. To be complete, I also used these same batteries in my high-powered electric golf cart when not camping, and in that use they are on a three-stage charger, and are occasionally significantly discharged.

There are many discussions of charging systems and charging theory available online like Battery University. For our purposes here, the three electrical facts needed are these:

1. Most modern 12 volt batteries have a recommended cyclic charge voltage of 14.2 volts to 14.5 volts. That voltage correlates well with the maximum output voltage found in today’s truck alternators. The alternator in my GMC truck produces a maximum of 14.4 volts.

2. Current (I) flowing through a wire with resistance (R) causes a voltage drop (V=IR). In other words, charging voltage at camper batteries will be reduced from alternator voltage by the product of amps in a connecting wire times the resistance of the wire. Resistance for various sizes and lengths of wire can be determined, for example, from the table at this Wikipedia page. Representative data is shown below:

GaugeOhm/ftDeltaV 10/25V batDeltaV 15/50V batDeltaV 20/100V Bat

DeltaV x/y is voltage drop for x feet and y amps: e.g., DeltaV 15/50 for 2 gauge wire is .0001563 * 15 *50 = .117225. V bat is 14.4v – DeltaV x/y; e.g., V bat for 2 gauge wire at 50a for 15 feet is 14.4 – .1172 = 14.2828. Data in red may exceed NEC allowed ampacity for those gauge wire, depending on installation – precautionary consideration. Data in blue (and red) indicate voltage may be inadequate for complete charge in a ‘reasonable’ time.

The point of this section is that large gauge wire will carry more current and retain high charging voltage. Small gauge wire, like the alternator charge wiring as may be found in today’s RVs, will do neither.

3. Most people using an alternator charge circuit for camper batteries will want some method of disconnecting the circuit, like a solenoid, when the engine is not running to prevent the camper from discharging the truck’s batteries. When I first installed this type of circuit, there were several diode based, automatic disconnect devices available, some supplied as stock equipment in trucks. The problem is that the voltage drop across these devices is approximately 0.7 volts. Thus the voltage available to charge camper batteries is 0.7 volts less than alternator voltage before any current goes through a connecting wire. This means the camper batteries won’t be completely charged in any reasonable amount of time, and perhaps not at all. The best solution, in my opinion, is to use a high-capacity, continuous duty solenoid to connect and disconnect the circuit. The voltage loss across a good solenoid is essentially zero. The method of activating the solenoid is truck specific. Mine is discussed below.

How can you tell that if your truck’s alternator charge circuit is adequate or not? Simply compare the alternator output voltage to the voltage actually available at the camper’s batteries during charging. If the available charge voltage is less that 14 volts, either the disconnect device or the connecting wire or both are most likely inadequate.

The following parts were used in my installation and are recommended for yours:

My Installation

Here’s a basic schematic diagram of my alternator charge system. It uses 2 gauge wire (except for a short section of 4 gauge on the camper side of the connector because that side of the connector will only accept 4 gauge maximum).

Alternator Charge Circuit Modification - Truck Campe Adventure

And here are some pictures and a description of my alternator charge system installation:

Alternator Charge Circuit Modification - Truck Campe Adventure
Connections at the battery/alternator common positive junction. The top wire at the nut is for the charge circuit. The bottom one is for winch power at the front of the truck.
Alternator Charge Circuit Modification - Truck Campe Adventure
The solenoid (left) and fuse (right). The small red wire carries the activation signal. The fuse protects against a short in the connecting wire.
Alternator Charge Circuit Modification - Truck Campe Adventure
Mounting location of the Phillips Industries truck bed connector.
Alternator Charge Circuit Modification - Truck Campe Adventure
Closeup of the Phillips Industries truck bed connector.
Alternator Charge Circuit Modification - Truck Campe Adventure
The Phillips Industries truck camper plug connector.
Alternator Charge Circuit Modification - Truck Campe Adventure
Connections in the camper. The blue wires are from the connector: the red and black wires go to the other three batteries.
Alternator Charge Circuit Modification - Truck Campe Adventure
Here’s the schematic to my truck’s power distribution center. I used pin (cavity) C in connector C-8 (upper left corner) to activate my solenoid. I deliberately choose to activate during engine run and start so that the camper batteries would be available to help start the truck (if ever needed).
Alternator Charge Circuit Modification - Truck Campe Adventure
My truck’s power distribution center. Pin C activates the charge circuit solenoid.
Alternator Charge Circuit Modification - Truck Campe Adventure
As a bonus of having 2 gauge wire running to the truck bed, I could easily extend it to the back of the truck. Then by bypassing the fuse(s) and solenoid, I have winch power at the back of the truck if I need it.


This is a heavy-duty charge system. Some may ask how much current it actually carries. The answer is, I don’t know, because I’ve never measured it. It’s certainly less than 100 amps because that’s my maximum alternator output. Also, my battery capacity is large enough that I’ve never seen voltage go below 12.8 volts resting–which is a full charge, which means the batteries don’t need much current. So why have a 12 volt system with a 540 amp hour capacity? Because I don’t want, need, or have a generator. And I never want to run low on available energy. Why have a heavy-duty charge system like this? Because it provides a high charging voltage and recharges my camper batteries quickly. The cost for this modification is minimal, fairly simple to install, and pays huge dividends when you’re off-the-grid. The wiring is also useful for a rear mounted winch.

Profile photo of Cal Willis
About Cal Willis 1 Article
Known as “cewillis” on the truck camper forums, Cal Willis has been exploring national parks and off-road trails since the 1970s. Cal grew up in North Kansas City, MO until college, lived in California for 35 years, and now calls Tucson, Arizona his home. He holds a Bachelor’s degree in Physics, a Master’s in Applied Mathematics, and completed graduate work in Computer Science. His work included the development of synchronous satellite control software and the design of computer network systems. Cal stays physically fit by exercising an hour a day, six days a week. He’s been married to his wife Tina for 33 years.
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