Maximum Amps from Battery

How much current the system is able to supply can vary quite a bit depending on which of the car's own electrical things are operating. The car's DC/DC converter is rated to supply 120 amps maximum. They did not gratuitously oversize that, so it's fairly well mated to what the total draw of the car itself can be when lots of the power accessories (HVAC, defogger, lights, seat heaters, etc.) are in use, so at those times, it hasn't got much capacity left for added uses.

On the other hand, a lot of the time, not many of those things are operating, so there's plenty of capacity left for you. At such times, 40 A would be no problem, or even 80. Probably best not to plan on exceeding 80, as the car's basic operations still amount to 40 amps or so even with most optional stuff not in use.

The car's a hybrid, so telling the charger when the engine is running wouldn't really be something the charger needs. What you would want is to know when the car is in READY mode (whether the engine is running or not), because that's when the car's DC/DC converter is supplying power.

There aren't really convenient circuits to tap in the car that identify READY mode for you. Any "IG" circuits you find will be powered either in READY or in IG ON mode, and that's definitely not what you want, because in IG ON the car does not produce power.

On the other hand, in any mode except READY, the voltage at the battery will just be the voltage of the battery (generally something below 13), and in READY it will be the voltage from the DC/DC converter (generally above 13.5). So an easy way to tell when you are in READY is to buy an off-the-shelf gizmo called a "low-voltage disconnect" that recognizes that voltage difference, or build an equivalent yourself.

 

I suspect Renogy has some kind of BMS in their lifepo4 135ah battery... And a low voltage disconnect is another battery management tool that would be wise to have for existing 12v.

However, I'm skeptical that a DC to DC charger is the way the to go. Why not instead take the charging burden off the car and use a 120V AC to 12V DC charger you plug into your house electricity to charge the lifepo4 135ah battery? Then simply add a 1K or 2K DC to AC inverter into the system when you can't plug your charger into an outlet?

Of course, you've not said what you need so much 12v battery power for yet? Perhaps that would clarify how to design the best system?