Automatic charging relays (ACRs) are sort of like smart battery isolators. They sense the battery voltage in both the cranking and auxiliary circuits and only combine the two for charging when the voltage of the cranking battery’s circuit rises to about 13 volts and stays there for a couple of minutes.

Typically, this only happens when your engine is running and your alternator is charging the cranking battery.

When the cranking circuit drops below about 12.75 volts for around 30 seconds the relay senses that you’ve shut off the engine and it opens, isolating the circuits so your accessories can’t draw the cranking battery down.

Some ACRs have an under-voltage lockout feature that prevents your cranking battery from being combined with an auxiliary battery circuit that has dangerously low voltage.

Batteries tend to seek the same level when combined and a completely dead auxiliary battery circuit could drag your cranking battery down too low to crank your engine or keep it running. This feature also prevents combining your cranking battery with an auxiliary circuit that is shorted or otherwise faulty.

Your pickup’s electrical system includes expensive components that can be damaged when fed the wrong number of volts!

Auxiliary Battery Systems

The big advantage of a separate auxiliary battery system is the ability to wire additional batteries in parallel for a virtually unlimited supply of accessory power.

If you need to mount more than two big deep-cycle batteries, however, you might check with your dealership’s service manager or a local electrical service shop to ensure you have enough alternator to handle the load.

A single extra battery may fit under your hood, but multiple batteries will probably have to be mounted in the truck’s bed or under the body.

Installing them in securely anchored marine battery boxes contains any electrolyte spillage, provides safe ventilation and keeps an insulating layer of plastic between the batteries’ posts and your truck’s metal body.

Battery Types

Batteries come in three types suitable for work truck use. The first and oldest design is the flooded-cell, full-maintenance type.

Many AGM batteries, like this Deka Intimidator model, have both threaded and clamp-on posts, making it easier to wire up more accessory power cables with either type of terminal connector.

These have removable caps that allow you to check the electrolyte level in each cell and replenish it as necessary by adding distilled water. This antiquated design remains popular because it continues to provide more amp capacity per pound and per dollar than other lead-acid battery types.

The chief downside to these flooded, full-maintenance models is that they must be checked and serviced regularly, including cleaning their tops and posts as necessary to prevent a buildup of crud heavy enough to leak voltage between their positive and negative posts.