There is a good way to do that, a better way and a best way. We will go over all three.
The good way is simple: run the wiring from the alternator to the HP-40, or in parallel from the factory battery to the lithium. This is the easiest, but offers no safety nets and may impact lifespan.
The better way is to use an isolator. Tie the factory/AGM battery into one set of contacts and the lithium battery to the other. The turn-on wire would be connected to the amplifier’s remote wire. We recommend this because lithium batteries charge faster than AGM. A lot faster.
While the car is on and the alternator running, the lithium is usually near completely charged, while the AGM is still slowly building a charge. Once the car is off, the AGM battery is still looking for energy. It will grab the nearest source, the lithium battery. This is because the resting voltage of the Lithium is higher than the AGM.
Without an isolator installed and the car is on, the lithium battery will charge at the same time the AGM does. Once the car is off, the isolator acts like a big electronic switch, removing the AGM battery from the circuit and avoiding any parasitic draw when the car is off. This is especially important when playing the system while the engine isn’t running.
The best way? Remove the factory battery from the vehicle. Why? No parasitic draw issues (not to mention saving some weight). Consider this: with rare exception, even the largest pair of factory diesel batteries on a non-commercial truck are in the 65Ah (Amp Hour) range, and come with slower charging speeds, lower resting voltage and lower wattage capabilities vs. a lithium battery. Replacing the factory or AGM batteries with lithium is perfectly safe to run on a daily basis, has a much larger wattage capability, will start the vehicle faster, can be stored for years longer than AGM and requires far less charging time to fill up.
Note: for battery deletes, it’s important to have a ratio of ¾ the Ah in lithium, compared to the stock AGM/factory battery. This is to make sure any idle drains (alarms, remote start, etc…) does not overly deplete the battery. The Lithium battery is NOT to be installed under the hood.