Batterie auxilliaire 4X4 NO LITHIUM

Bit of a niche one this, but curious if anyone else has gone down the dual battery rabbit hole for an older vehicle?

Got a mate who runs a classic 80 Series Land Cruiser — similar vintage to some of the beasts I see mentioned on overlanding forums — and he's adamant about keeping lead-acid as the aux battery. Reckons lithium is "too clever for its own good" on an older alternator setup.

I get it to a point. My boat has a Victron MPPT and a Fogstar lithium bank, and yeah, you do need to think carefully about charge management when the alternator's involved. Older vehicles especially — some of those regulators just aren't happy pushing into lithium without a DC-DC charger in the mix (Victron Orion-Tr is ace for this btw).

But dismissing lithium entirely feels a bit 2010s to me 😄

The weight saving alone on something like a roof-tent overlander is massive. And cycle life vs a bog-standard AGM? Not even close.

So the question I'd throw out to this lot:

If you're running aux power off an older alternator — are you going DC-DC charger + lithium, sticking with AGM, or something else entirely?

Running 24V complicates things slightly but there are solutions. Keen to hear if anyone's tackled this on a classic 4x4 rather than a shiny new Defender with a fancy BMS already baked in.

@JimWilson oh the 80 series dual battery setup is a whole thing isn't it 😅

Done something similar on my old Defender before I moved everything onto the boat. AGM is honestly the sensible call for older vehicles — the alternator charging profile just works without faff. Victron do a decent Battery Protect that plays nicely with it all.

Main thing your mate needs to watch is the VSR (voltage sensing relay) — cheap ones are garbage, had one fry on me. Bimble Solar or 12V Planet stock decent ones for not loads of money.

Also worth checking the alternator output before throwing a big AGM at it — those old 80 series alternators weren't exactly beefy from the factory 🙄

@JimWilson this actually overlaps with my emergency backup obsession more than you'd think — the logic of keeping a dedicated reserve that never gets dragged down by the main system is identical whether you're in a Cruiser or a static setup at home.

The isolation relay question is the real crux of it. VSR versus manual switch versus a proper Victron Cyrix — they all behave differently under load and the 80 Series charging system has its own quirks around alternator output at idle.

What's his use case primarily? Deep recovery tracks where the winch will punish both banks, or more casual overlanding where it's just insurance? That changes everything about how you'd size and isolate the secondary.

@BayLisa the Defender crowd have been doing this longer than anyone — always worth raiding their forums for the older lead-acid wisdom before people started retrofitting lithium into everything.

@SolarRachel you're not wrong — my narrowboat aux bank taught me the same lesson: isolate your starter battery like it owes you money, because one day it will.

For the 80 Series specifically, a VSR (Voltage Sensitive Relay) is the old-school bulletproof option — no faffing with lithium compatibility issues, plays nicely with the original alternator, and won't confuse the vehicle's charging logic. Something like a Ring RSCDC30 dual-to-dual charger if he wants to be a bit more clever about it.

Key thing with older vehicles: the alternator isn't expecting smart battery behaviour, so keeping it old-school AGM + VSR is genuinely the right call here rather than forcing lithium into a system that'll argue with it constantly.

@JimWilson the 80 Series is actually a brilliant candidate for a proper aux setup precisely because it's old enough not to have smart alternator nonsense to contend with. Straightforward D+ signal, predictable charging profile — fit a VSR or a proper Victron Orion DC-DC (non-smart variant is fine here) and you're done.

For the aux battery itself, if lithium is off the table (and on a classic 4x4 I'd probably agree — temperature exposure under bonnet or in a wheel-arch box can be brutal), a decent AGM like a Victron AGM or even an Odyssey PC925 will handle the vibration and depth-of-discharge cycles far better than a standard lead-acid flooded unit.

Main thing I'd stress: fuse both sides of the isolator, not just the aux run. Seen too many builds that fuse the output but leave the feed unprotected. Classic fire waiting to happen.

@CliveBaker dead right about the older vehicle being easier to work with — no CANbus nonsense trying to "manage" everything into a brick.

Running something similar in my motorhome. Went Victron BatteryProtect between the banks and honestly it just works.

Quick question though — has anyone actually tested what happens when you forget to