Batt. protect with reverse charge from power bank?

Interesting one this — had a similar head-scratcher when I was sorting out EV charging integration on my setup in the marshes.

The core question I keep coming back to: if you've got a power bank (DJI or otherwise) that can bidirectionally push current back into your starter battery, does a standard battery protect actually play nicely with that reverse flow? Most Victron BatteryProtects are load-side only and will actively block or get confused by any back-feed current. That's not a minor quirk — it can cause some genuinely weird behaviour or even damage the unit.

A few things worth thinking through:

• Direction awareness — does your specific BP model support bidirectional current, or is it unidirectional by design?
• Voltage thresholds — the power bank may push voltage high enough during reverse charge to confuse low-voltage disconnect logic
• Cable ratings — DJI's proprietary cables aren't always rated for sustained high-amp draw in both directions over long periods

My instinct would be to look at a proper DC-DC charger (Victron Orion-Tr Smart being the obvious go-to here) rather than relying on a battery protect to manage bidirectional flow it was never really designed for. Keeps things cleaner and you get proper charge profiling rather than just a dumb pass-through.

Has anyone actually run a setup like this long-term? Curious whether the power bank's BMS does the heavy lifting and makes the BP largely redundant in practice, or whether you still need it for protection on the van electrics side when the bank's depleted.

Would love to hear from anyone who's stress-tested this properly.

@MarshChild the Victron BatteryProtect is load-side only — it'll have a meltdown if you try pushing charge back through it, it's literally in the manual in bold with tiny crying symbols (probably).

For reverse-current scenarios you want a proper DC-DC charger like the Orion-Tr Smart — handles bidirectional current properly and isolates your banks cleanly.

Learnt this the slightly expensive way during my T5 build when a Fogstar battery met an incorrectly wired BatteryProtect. The BP won that argument unfortunately.

@T5Project is spot on about the BP — learned that the hard way on my narrowboat before I knew better.

What sorted it for me was putting a proper DC-DC charger (running a Victron Orion-Tr Smart now) between the power bank and the house bank rather than fighting the BatteryProtect's one-way logic. The Orion handles the isolation elegantly and your BMS stays happy.

The BatteryProtect is essentially a disconnect relay with brains — brilliant at what it does, rubbish at anything outside that lane.

If budget's tight, a simple diode isolator on the charge input side keeps reverse current honest, though you'll lose a bit of voltage. Fogstar cells are forgiving enough that the drop rarely matters in practice on my static caravan setup.

What's the power bank voltage you're working with? That'll change the answer considerably.

@DerekDixon curious what damage you actually saw on the narrowboat — the BP or something downstream?

The way I sorted a similar headache on my motorhome was ditching the BatteryProtect altogether for that particular circuit and running a proper Victron Orion-Tr Smart DC-DC charger instead. It handles bidirectional current flow properly and has the isolation built in, so you're not fighting the hardware.

The BatteryProtect is genuinely brilliant for what it's designed for — protecting loads from low voltage — but the moment you start introducing charge sources from unexpected directions, it's the wrong tool entirely.

If budget's tight, the Fogstar forum has some threads on using a simple P-channel MOSFET arrangement as a cheaper alternative, though I'll be honest, I sleep better knowing Victron's handling mine.

@DerekDixon @OffGridTerry — worth adding that the BP's MOSFET architecture is the root cause here; it's simply not rated for bidirectional current. If you need proper bidirectional protection with a power bank in the circuit, look at a Victron Orion-Tr Smart DC-DC charger on the input side — it'll isolate the power bank from your house bank properly and handle the charge logic cleanly.

On my boat I've got a similar arrangement where I needed charge to flow in from an external source without it backfeeding through protection devices. The Orion sorts it neatly and the Victron Connect app gives you decent visibility over what's happening. Far cleaner than trying to bodge the BP into a role it wasn't designed for.