How to determine whether a argofet can be used

Been pondering this for the static van setup actually — whether an Argo FET isolator makes sense versus just running a proper DC-DC charger like the Victron Orion.

The core question with any FET isolator is really about your alternator's voltage sensing. If the alternator has an external voltage sense wire, a FET isolator can work reasonably well because the alternator can compensate for the voltage drop across the diodes. If it's relying on internal sensing only, you'll likely end up with undercharged batteries — the alternator sees a higher voltage than what's actually hitting the bank.

Few things worth checking before committing:

• Does your alternator have a separate D+ / sense terminal? If yes, you can wire sense directly to the output side of the isolator
• What's your alternator's max output vs the FET isolator's rating — don't run it near its limit continuously
• Are both banks the same chemistry? FET isolators don't care what voltage each bank wants, they just split current — can cause issues mixing AGM and lithium for example

Personally for the van I just went Victron Orion DC-DC isolated. More control, handles the Fogstar lithium properly, and I don't have to worry about what the alternator's sensing is doing.

Anyone else gone down the FET route with a modern smart alternator? Curious whether it's caused headaches or been fine in practice.

@HY_OffGrid the key thing I'd check first is whether your alternator is "smart" (variable voltage) or a conventional fixed-voltage unit. If it's a modern Euro 6 engine with a smart alternator, the Argo FET is basically useless — the voltage drops too low under load and you'll get next to no charge into your leisure bank.

For my setup I went back and forth on this exact question. The Orion-Tr Smart wins out for smart alternators every time, but if you've got an older van with a dumb alternator pushing a steady 14.4V, the FET isolator is a simpler, cheaper solution with no power conversion losses.

What engine/year are you running? That'll probably answer the question straight away.

@T6Project makes a good point — and worth adding that even with a conventional fixed-voltage alternator, you'll still lose around 0.7V across the FET diodes. Sounds minor but it genuinely affects your absorption voltage reaching the cells properly.

For my van build I went Orion TR Smart in the end precisely because I wanted proper charge profiles rather than just "voltage minus a bit." If your battery is lithium (Fogstar Drift cells in my case), that voltage drop matters more than people realise.

The other thing to check: what's your alternator's rated output? FET isolators work fine if you're just topping off a small leisure bank, but the moment you're pulling serious amps the Orion's 30A controlled draw is actually kinder to the alternator long-term.

Really useful thread this. To add to what @T6Project and @RayPowell are getting at — another consideration for a static van setup specifically is that you're likely not moving it daily, so the alternator charging opportunity is limited anyway. That actually shifts the balance further toward a DC-DC charger in my view, since the Orion's ability to properly profile the charge (especially if you're running lithium) matters more when you've only got a short run to work with. The Argo FET really shines when you're racking up consistent drive time. Also worth checking your van's cable runs — FET isolators need decent gauge cabling between batteries to avoid voltage drop eating into whatever you've already lost across the diodes.

Really interested in this thread as I've been weighing up the same question for my narrowboat setup.

One thing I haven't seen mentioned yet — what's the actual battery chemistry you're charging into? I'm running lithium (Fogstar Drift cells) and from what I understand, an Argo FET won't give you proper charge profile control, which matters quite a bit with LiFePO4.

Is that a concern for your static van @HY_OffGrid, or are you sticking with lead-acid? That might be the deciding factor for a lot of us reading this — if you're on lithium, does the Argo FET essentially become a non-starter regardless of alternator type?