Many Vanagon owners choose to add an auxiliary battery to run the stereo, interior lights, cell phone chargers and other 12V accessories when parked or camping without draining the van's starting battery. While it seems like it should be a pretty simple and straightforward task to run a few wires and tuck a 2nd battery into an out of the way spot, there are actually quite a few factors that need to be considered when designing and installing an isolated auxiliary battery system that will allow you run your 12V necessities for the length of your camping trip and recharge from the alternator while driving to your next destination.
I have used several different auxiliary battery charging systems in vans over the past ~20 years. Since I tend to gain just as much knowledge from "what doesn't work" as I do from "what does work" , I offer the following recommendations based on my experience, starting with a system that many people are familiar with, but I seldom recommend:
There are several commercially available "auxiliary battery kits" on the market that use a cylinder shaped solenoid (which is basically just a heavy duty electro-mechanical relay) to charge a small auxiliary battery that can be housed under the drivers seat. This type of system has been available for years and has worked (to varying degrees) for many VW campers since long before the Vanagon was ever conceived. If your power needs are very modest (ie. charging your cell phone and listening the radio for an hour or two per night), one of these kits might work for you.
Most of the solenoid based charging kits use the van's stock wiring to deliver the charging current to the auxiliary battery. The long runs of old, thin factory wire severely limits the amount of current that can be delivered to charge the battery. As a result, the battery tends to be chronically undercharged, which leads to less available power when camping and a much shorter lifespan than a properly maintained battery would offer. To help deliver more current to the auxiliary battery when using a solenoid based charging system, you can run a dedicated 10AWG (or larger) wire from the positive post of the starting battery to the input post on the solenoid.
It is also worth mentioning that the solenoid based auxiliary battery kits use the blue wire from the alternator to initiate charging. This often adds enough extra load to the blue wire exciter circuit that the alternator won't begin charging until the engine is revved to about 3,000RPM. Many people have started the van and let it idle for a while to charge the battery half way through a camping trip, only to discover later that the alternator light never went out and the battery didn't receive a charge.
While there are many ways to successfully add a second battery to your van, few are more simple to install and require less owner input or maintenance than the Voltage Sensing Automatic Charging Relay (ACR).
The ACR is installed between the positive post of the main starting battery and the positive post of the auxiliary battery. Instead of depending on a signal from the alternator to turn it on, the ACR combines or isolates the batteries based on a pre-set voltage level. There is no need for any switches, trigger wires from the alternator or key in the ignition in order to have the ACR operate.
Basic installation is as simple as running a heavy gauge (10GA minimum) wire from the positive post on the starting battery to the ACR and then another wire from the ACR to the auxiliary battery. The only remaining connection is a ground wire attaching the ACR to the chassis of the van.
The ACR monitors the voltage in both batteries and combines them when the voltage in either battery is over 13.3V. When camping and using the auxiliary battery, the ACR isolates them from each other when the voltage in either battery drops below 12.5V. This ensures that the starting battery will be charged and ready to start the van regardless of the voltage in the auxiliary battery.
The most popular ACR's for the Vanagon include the Blue Sea Systems ACR 7610, the Victron Cyrix-ct and the SurePower 1315. All of these ACR's install and operate pretty much the same way, with the main differences being the physical attachment points of the wires and the amount of amperage they can handle.
The Blue Seas 7610 has 3/8" mounting studs to accept the charging cables between the batteries. This allows the installer to choose the gauge of the wire used. In general, the thicker the wire used, the more current will pass through the ACR, allowing the battery to recharge faster. The 7610 is rated for 120A constant current and can handle 210A max for up to 5 minutes.
The Victron Cyrix-ct has 1/4" mounting studs. It is also rated for 120A constant.
The SurePower 1315 has 3/8" mounting studs. It is rated for 100A constant.
For uni-directional charging (from the alternator to the auxiliary battery only) SurePower offers the 1314 voltage sensing relay.
All of these ACR's can be found online for $50.00-$75.00. Search around and you can often find free shipping or other offers for an even better deal.
When installing the ACR, be sure to include proper circuit protection. Fuses or circuit breakers are available in a multitude of styles and amperage ratings. Install the circuit protection as close to each battery as possible to protect the wiring run between the batteries.
An added benefit of the ACR is the simplicity of adding solar or wind power to your auxiliary battery system. By connecting solar/wind power directly to your auxiliary battery, it will charge quickly to provide power for camping or tailgating. As the voltage in the auxiliary battery reaches full charge (13.3V) it triggers the ACR to combine the battery banks and begins to charge the main starting battery as well. This is great for times when the van will not be driven every day, as it keeps the batteries in top condition and may extend their lives. This same feature is also useful when camping because the combined batteries will act as a single larger battery until the voltage in either battery bank drops below 12.75V and isolates the starting battery ensuring plenty of voltage in the main battery regardless of the depth of discharge to the auxiliary battery.