Replacing battery and perhaps upgrading alternator & split charge relay?

[OldNick]

Hi, I acquired a February 1985 T3 Westfalia Joker (1.9 L petrol engine) last year. I fitted a Bluetti Charger-1 last year to charge a Jackery whilst on the move (portable for camping but also the internal lights run off the starter battery!! so this powers alternative lighting) and I attached this to the leisure battery on the assumption that the alternator would charge this. The leisure battery was old and is now dead so I need to replace it. I was thinking of putting in the Ultravolt LiFePo4 but I’m not sure the electrics are up to it??? I took multimeter readings on the starter battery (also getting a bit old but not too bad, needs the occasional jump if I leave the van too long) it hadn’t been used for a week and the engine off reading was 12.1V, when running at idle 13.9 V and under load (headlights & heater blower on) 12.9V. I’ve just charged it for a couple of hours & the readings are now 13.2/14.35/12.9-13.1 . I havn’t accounted for the potential draw from the Bluetti. The Ultravolt LiFePo4 is said to need a minimum of 14.2 V to charge so this looks a bit marginal even when not under load & I’m concerned that the draw would overload the aging alternator. Although it might help power the fridge I’m not sure that’s viable for more than a few hours without LPG anyway? So I’m thinking of just doing a like for like WET lead acid battery replacement. Should I also be thinking about upgrading the alternator to a 70A Bosch unit or do you think this is sufficient for now? ?   Someone also suggested fitting a Victron Orion-Tr Smart B2B Charger between the starter batter/alternator and the leisure battery but I’m thinking this is only necessary if I’m upgrading to LiFePo4. welcome any advice. many thanks Nick