What does a responsible overland vehicle owner do when they realize that they have more accessory current draw than can be maintained by the alternator and battery? Well, they can stop adding accessories (yeah, right), or upgrade the vehicle's charging system. This can be accomplished in several ways, but first let's look at the major components in a charging circuit.
The alternator is the charging, or supply portion of your system, and needs to not only be in proper working order, but properly sized. If you are fortunate enough to own a late-model mid or full-sized SUV, then it was likely equipped with an excellent electrical system from the factory to power all the luxury appointments and computers that run the vehicle. However, if you own a 4x4 from before 2005, then you may have to address the alternator when preparing your pinnacle overland vehicle.
It is quite possible that you may be able to source a larger alternator that will bolt into your overland vehicle without much trouble. An example of this would be to install a 130 amp alternator from a 1996 Grand Cherokee or Dodge Dakota V8 into your Jeep YJ Wrangler. This type of simple upgrade is possible with many 20th century trucks. But, if you need more than even what a factory alternator can produce, you will have to go to the aftermarket for a specialty high-output alternator.
First, a word on replacement alternators- Don't necessarily rely on an auto parts store alternator as a high performance replacement. Most of these alternators are "remanufactured" which means the parts are all interchanged at a refurbishing center, and you may not be purchasing an alternator that actually produces the claimed amount of amperes. Either have the alternator bench tested, or insist on a manufacturer test sheet when you buy it. Reputable manufacturers will provide such a "proof of life".
When looking at replacement and high-output aftermarket alternators, you should keep in mind that stated peak output is not idle output. One of the reasons for spending money on a specialty alternator is for higher idle output. This is important for a 4x4 that sees significant trail driving. Highway driving will maintain alternator outputs, but slow driving on technical trails for hours at a time may not even replenish vehicle battery charge losses. Look at what the idle output is for the alternator you are considering buying as well as the nominal and peak output. If the retailer can't tell you anything but the rated peak output, run away.
If possible, try to upgrade with a factory style replacement alternator if you must. This will allow you to easily replace that alternator if it ever fails on you on a journey. Specialty aftermarket alternators can cost $400 to $1200, and often require maintenance and replacement more often than an OEM type. This is due to the power output and has nothing to do with poor workmanship or materials.
The modern overlander has many more options for batteries than even 10 years ago. The two most common batteries available are the flooded lead/acid battery and the Absorbed Glass Mat (AGM). There are also some capacitor and lithium-ion automotive batteries out there, but they are not only astronomical in price, but not yet well suited for overland use. The battery of choice for the serious overland vehicle is the AGM battery. Many OEM new vehicles are now coming equipped with these latest generation batteries.
The traditional flooded battery has lead plate that are suspended in liquid sulfuric acid in order to chemically store their charge. AGM batteries are similar, but there is a fiberglass matrix that houses a thicker version of the acid between the plates. AGM batteries are also not as prone to hydrogen outgassing when in a recharge cycle. AGM batteries can often be mounted in non-vertical orientations without spilling or draining. These characteristics make them ideal for off highway and extreme duty use.
Cost is the typical reason that many recreational overlanders shy away from AGM batteries. While a standard flooded battery can usually be easily purchased for under $100, AGM batteries run anywhere from $150 to $250. As with most products, not all AGM batteries are created equal. You need to know what you are buying. Some AGM batteries are sold as "starting", "deep cycle", and "marine". This can mean much more than what kind of terminals it has. Some AGM batteries are combination types that can handle various operating parameters- but are priced to reflect this. The "starting" batteries may not be able to handle much more than a minimal discharge, and may not be able to fully recharge without special attention after a discharge cycling. Research any AGM battery before you buy it.
If you plan to utilize a specific use type of battery, and you have the ability to use dual batteries, it is recommended that you install one starting and one deep cycle AGM in parallel. If you are only going to install one AGM battery in your overland vehicle, a deep cycle or combination type battery is recommended. Overland vehicles will often cycle the vehicle battery further than a typical automobile and will benefit from the recharging characteristics of a deep cycle or combination type. Flatwater Overland highly recommends Odyssey batteries from personal experience for overland use.