question

It's more expensive to run DC wires that long than get a second charger

Since it's for an infrequent need you could make it portable and put clips on it and the rest of the time parallel it with your installed one to charge the Lifepo4's faster (unless your BMV is already at max input).

Am I understanding correctly? I mean that's how I'd do it if I was going to use it this way. But that's not a simple or elegant solution.

IF you only need the "JUMP" on the rare occassion (you don't want to regularly be using the starter for anything else), I'd just pick up a good jumppack battery (smallish power banks for just this sort of use) as that would serve the need and no work or wiring necessary and let the alternator recharge the starting bank and recharge that pocket thing back at the lifepo4's easily.

If you had to have a real charger from the house to the starter (I had a similar dilemma but went with the above) I'd run it maybe off the inverter with some lighter gauge wire and a switch for just this function - you don't want the charger too far from the battery it's charging or you'll have even more issues and problems over time!

Since the Victron chargers and your Lifepo4 can probably handle this you could get a cheap inverter sized just a bit up for your starting battery charger and tuck it out of the way for only this use. The big issue is avoiding all that long heavy gauge wire and voltage drops and output being less ideal for effectively charging the starting battery when the alternator can take over soon as the engine turns over a good spin for combustion! I mean THERE is your solution already built in!

if that isn't a solution you are using the batteries wrong and if the alternator can't charge the starter battery you'll have FAR bigger issues than this really quickly . So that is what your alternator does and is for.

So that's why I went with a pocket Jump pack (and I used my power tool's batteries and some small bits of heavy gauge wire before that). It sounds like you really are just talking about the infrequent jump start.

Right?

There's bench trickle maintenance chargers you can use the rest of the time ready available, if not. Super simple. And if needed add just a simple battery cut off marine-style switch right next to the starter battery (just need one 6" or so battery cable and the switch) and the kids can still play, no harm done!

:D