Charging Lifepo4 house and AGM Starter Batteries

Been wrestling with this one on my motorhome build and curious if others have hit the same wall.

Just finished swapping out my AGM house bank for a 200Ah Fogstar Drift LifePo4 setup. Loving it so far but now I've got the awkward situation of trying to charge both the new lithium house bank and the original AGM starter battery from the same alternator/solar setup.

The problem is obvious — LifePo4 and AGM want completely different charge profiles. My Victron SmartSolar is sorted for the house bank via MPPT, but what's the cleanest way to handle the starter battery without either undercharging the AGM or cooking it?

A few things I'm mulling over:

• Using a Victron Orion-Tr Smart DC-DC charger to keep the starter topped up from the house bank
• Running a separate dedicated solar circuit just for the AGM (feels like overkill?)
• Some sort of VSR/relay setup, though I've heard these play badly with lithium

Anyone actually running a dual-chemistry setup like this? Specifically wondering:

1. Does the Orion-Tr genuinely protect the AGM properly with its own charge curve?
2. Is there any risk to the LifePo4 bank when the BMS cuts out and the DC-DC is still connected?
3. Worth fitting a Renogy battery monitor on the AGM separately or just leave it?

Feel like I'm overcomplicating this but I'd rather get it right now than rewire the whole van again in six months. Seen some conflicting advice online and none of it seems aimed at UK builds specifically.

What are people actually doing out there?

Running both chemistries off the same alternator is basically asking two people with completely different diets to share a plate — someone's going home hungry.

My setup uses a Victron Orion-Tr Smart DC-DC charger to isolate the LiFePO4 from the AGM starter battery entirely — charges both properly without either chemistry throwing a tantrum about voltage profiles. Worth every penny of the ~£120 it'll cost you.

Key thing most people miss: your AGM starter wants to see ~14.4-14.7V absorption, while the Fogstar Drift is perfectly happy at 14.2V — close enough to seem fine until one day it really isn't.

@Chopper54 😄 fair analogy tbh

The key thing nobody mentions — your alternator doesn't care about chemistry, your charge source does.

Most folks fit a Victron Orion-Tr Smart DC-DC isolator between the two banks. It takes whatever the alternator chucks at the AGM starter, then

@EcoFlowMaster this is exactly the scenario I sorted on my van build last year.

The solution that's worked flawlessly for me is a Victron Orion-Tr Smart DC-DC charger between your alternator/starter battery and the LiFePO4 house bank. It isolates the two chemistries completely — your AGM starter battery sees normal alternator voltage, while the Orion delivers a proper lithium charge profile to the Fogstar.

Key practical point: the Orion also protects your starter battery from being dragged down by the house bank, which a basic VSR simply won't do with lithium.

I ran 30A non-isolated on mine — plenty for a 200Ah bank during normal driving. If your alternator is older or smaller, the engine-running detection feature means it won't hammer it at idle either.

Worth every penny compared to the headaches of mismatched charging.

@EcoFlowMaster done exactly this on my boat — two different chemistries, one alternator.

The cleanest solution is a DC-DC charger (battery-to-battery charger) between the two banks. Victron Orion-Tr Smart is what I'm running. Alternator charges your AGM starter at its native voltage, DC-DC isolates and steps up correctly for the LiFePO4. Both batteries get what they actually need.

Key things to check:

• Size the Orion for your alternator's spare capacity, not just the LiFePO4 bank
• Make sure your Fogstar BMS can handle the charge current the Orion delivers
• Engine-on/off sensing keeps things tidy

Avoid a standard VSR/split charge relay — it'll just smash your LiFePO4 at AGM voltages and your BMS will kick it out constantly.

Really important point that's been glossed over here — your BMS will aggressively disconnect under high charge current if the cells hit absorption voltage, and that sudden open-circuit dumps the alternator's full voltage spike back into your AGM starter battery and any sensitive electronics. Seen this kill regulators.

If you're running a Victron Orion-Tr Smart DC-DC between the two banks, make sure you've got the input voltage lockout set sensibly (around 13.2V) so it only pulls from the alternator when the engine's properly running, not slowly draining your starter battery at idle.

On my shepherd's hut setup I replicated this same isolation logic using a Victron Cyrix-Li-ct — it's chemistry-aware and handles the disconnect scenario without the voltage spike problem @CE_Builds and @MarinePhil are alluding to.