Heavy-Duty Alternator Charge System Modification

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:

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:

• Two Class T 150 amp fuses
• Class T 150 amp fuse block
• One 12 Volt 150 Amp Continuous Duty Solenoid
• One Phillips Industries Lift Gate Dual Connector Socket
• One Phillips Industries Dual Connector Lift Gate Charging Plug
• A Length of 2 gauge wire (measure for your own specific needs)

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).

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

Summary

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.