Argofet on a stand alone solar system

Been mulling this one over after a similar head-scratcher on the narrowboat last winter.

I've got two separate 24V banks — one older Battle Born bank I've had for a couple of years, and a newer Fogstar Drift lithium set I added last season. Didn't want to parallel them for obvious reasons (mixed ages, different internal resistance), so I started looking at how to charge both independently from the same solar array.

The Argofet route seemed elegant on paper. Victron's isolators are essentially diode-based splitters, which means no relay clicking, no complex wiring logic — just passive isolation. Dead simple. The problem I kept reading about (and eventually experienced myself) is the voltage drop across the diodes — typically 0.5–0.7V — which means your MPPT controller never quite sees the true battery voltage. On a 24V system that can confuse things, particularly with lithium where the charge curve is already unforgiving.

My workaround was running a voltage sense wire from the Victron SmartSolar back to one of the banks (the one doing heavier duty), which helped enormously. But it does mean the other bank is getting slightly less accurate charge termination.

Has anyone here run an Argofet between a single MPPT and two mismatched banks and found a cleaner solution? I've wondered about running two separate, smaller MPPT controllers instead — one per bank — but that feels like overcomplicating it when the array is already shared.

Would love to know what others have landed on, especially if you're dealing with different battery chemistries rather than just different ages.

@Ozzy8 the Argofet won't work the way you're hoping here — it's a diode-based isolator so you'll get a voltage drop (~0.5V) on each output. Fine for lead acid where you just bump the charge voltage, but with LFP that precision matters. Your Fogstar cells especially will be fussy about seeing the right absorption voltage.

What's your MPPT? If it's Victron you're better off using a BatteryProtect on each bank and letting the BMS do the heavy lifting. Or honestly just run two separate charge sources if the array size allows it.

The voltage drop issue caught me out too — had weird cell imbalance on my own 24V setup until I traced it back to exactly this kind of thing.

@LiFePO4Fan is right about the diode drop being a pain, but worth adding — on a solar-only system you've also got the issue that your MPPT won't know what either bank is actually doing. It's just seeing a blended load.

Had something similar when I tried combining my van bank with the static caravan hookup. Ended up ditching the Argofet entirely and running a Victron Orion-Tr Smart between the two banks instead. Isolates them properly, lets each bank charge on its own terms, and you can set different charge profiles for the older vs newer chemistry if needed.

Two separate MPPTs feeding each bank independently would be the cleaner solution tbh — more kit but your Fogstar will thank you for not being dragged down by the older Battle Born.

@Ozzy8 had almost exactly this setup in the motorhome two seasons back — older Fogstar bank plus a spare emergency 24V block I kept isolated for breakdown cover.

The voltage drop @LiFePO4Fan mentions isn't just annoying, it'll genuinely confuse your MPPT. Your Victron controller reads battery voltage to decide when it's done charging — shove a diode in the way and it's essentially lying to itself.

What actually sorted mine was a Cyrix-Li-Ct instead. Proper voltage-sensing relay, no diode nonsense, plays nicely with Victron kit. Keeps the banks separate when you want them separate, combines intelligently when you don't.

The Argofet is brilliant on a vehicle with an alternator. Solar-only? It's solving the wrong problem. Don't bodge it.

@Ozzy8 worth considering a Victron Orion-Tr Smart DC-DC charger between the two banks instead — isolates them properly, handles the voltage differences between an aged and a newer bank, and you can configure charge profiles independently via Bluetooth. I ran a similar dual-bank arrangement on my narrowboat last winter and it solved exactly this problem. The Argofet is really designed with alternator or shore power setups in mind; on solar-only you're fighting against it from the start as the others have noted.

The Orion non-isolated version would likely do the job at lower cost if the banks share a common negative, which on a 24V system they often do.