Put a fuse right at each battery cable to prevent either side from shorting out. Try to get oxygen free cable (OFC). I’d also want to put a bi-directional shunt current measurement system in, and a volt meter referenced at the car battery.... both of these I’d want viewable from the drivers seat. Another nice feature would be a diode (or “ideal diode”) to ensure current only flows out of the vehicle. Another upgrade would be a relay, solid state switch, FET, etc. to provide a manual toggle on/off or something automatic where it only engages once the vehicle is started. It’s also a good idea to check the maximum charge voltage of the 4Runner (highest in vehicles is typically when things are cold) and ensure it’s compatible with your lithium battery. While most lithium batteries have over voltage protection, it would be good to know if there are conditions which the vehicle wouldn’t charge the pack.
At low RPM (idling) don’t expect much out of the alternator.... this is something that people into car audio have to deal with.
Another approach, albeit more expensive, is to install a large inverter in the 4Runner and then run 120V AC to your trailer and then in your trailer have a battery charger. The inverter which comes stock in the 4Runner wouldn’t be very good because it’s only 100W when driving. Doing this mod has the benefit of providing one with a high power outlet in the 4Runner, and then there is already likely a battery charger in the trailer. This setup also is better at controlling the battery charge profile. This approach is the method I’ll be doing.... the question yet to be answered is if the 4Runner can support an inverter which supplies 120V at 15A steady state when traveling at highway RPMs with the vehicle electrical system loaded down (headlights on, etc.)
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