Just another view on this - in my experience a DC-DC charger is not strictly necessary to connect an additional battery. I've connected a 100ah LiFePo4 (
this one here) directly in parallel to the stock 12V system of a car ( a 2016 BMW X5). Then with thick short wires (something like 0 AWG, 1ft) connected to the Lifepo battery is a 2kw cont / 4kw peak inverter. I've had this setup now for 2 years / 50k miles, and it works really well. I can both operate fairly heavy tools via the inverter (air compressor, angle grinders, heat guns, full size vacuum cleaners, etc), as well as use it in the car for very extended periods of time to power lower smaller loads (Starlink, laptop, heating blank for winter camping, etc). The majority of the power for the inverter is coming from the lifepo, but the stock battery is helping some.
Another side benefit is that the lifepo battery also reduces strain on the stock battery e.g. when starting the engine etc.
The BMS of at least this particular Lifepo4 battery seems to be both very solid as well as forgiving. It allows for up to 150A charge / discharge for limited time (maybe < 1min), and cuts out at overvoltage / undervoltage etc. Also this is probably obvious - but these Lifepo batteries are galaxies better at providing continuous power to say an inverter than the lead/acid dinosaurs. They keep voltage much better and you can draw power out of it for much longer.
I don't think the wire gauge connecting the two battery systems is safety critical as long as the wire is long enough - which it will be if you connect a rear lifepo to a front stock battery/alternator. This is because if the voltage drop over the wire is too large, the charging current will automatically drop as well, limiting the total heat transferred into the wire to reasonable levels. E.g. empirically I'm observing not more than 50A charging current on the lifepo if the voltage is not exceeding 13V. With say a 14V voltage from the alternator, you could have a 1V voltage drop from using 10ft of 10AWG wire and would be putting 50W of heat into the wire - not exactly ideal but also not so much that it would melt the wire. This is probably a worst case scenario heat wise - if you use even thinner wires, the main thing happening is that charging rate will be very low. If you use thicker wires, charging will be faster and you will have fewer losses.
Main disadvantage of connecting directly are:
- need to be careful not to empty the starter battery
- if you use this a lot in freezing / deep freezing conditions, may damage the lifepo battery due to charging it at too low temperatures
- I don't know if the older alternator designs in the 80 series could have overload issues if too much charging current is drawn into the lifepo battery? On modern cars the alternators seem to be smart and current limited, not sure how it's in the older ones.
