Centaur on shore power to start battery while starting - ok?

Bit of an odd one this, and something I've been mulling over with my own setup on the Sprinter.

Running a Centaur 12/60 from shore power to top up the starter battery whilst simultaneously cranking the engine — has anyone actually done this and confirmed it's safe? My concern is what happens at the shunt during that high-current draw event.

In my case the negative from the Centaur output joins the chassis-side of a Victron SmartShunt before anything else. When the starter motor pulls 200-400A through that same negative path, I'm wondering whether the Centaur's output stage sees any of that spike or whether it's effectively isolated by its own internal protection.

The topology I've been looking at is:

• Centaur positive → starter battery positive terminal
• Centaur negative → in-board side of shunt
• Engine starter negative → also in-board side of shunt
• Shunt out-board → chassis earth

Logically the Centaur should see the battery voltage collapse during cranking and simply current-limit itself — that's what a decent charger does. But a 12/60 pushing 60A into a battery that's momentarily sitting at 9V whilst the starter hammers it... I'd want to be confident the unit handles that gracefully rather than latching a fault or, worse, damaging the output FETs.

Anyone running a similar arrangement, particularly with lithium starter batteries where the voltage drop during cranking is sharper? Does the Centaur log any fault codes after such events?

@SprinterProject the Centaur 12/60 is a battery-to-battery charger, not shore power kit — think you might be mixing it up with the Blue Smart IP67 or Phoenix charger range? Easy mistake, the Victron naming is a minefield 😅

The actual concern here is whether the charger can supply enough current during cranking. Short answer: it can't really help — cranking draws 100A+ in milliseconds, the charger's just along for the ride.

What matters is your starter battery's own state of charge before you crank. Keep that healthy and you're fine.

If you're worried about the starter battery draining over time on the boat or van, a Victron Orion-Tr Smart isolated DC-DC is the proper solution. Done exactly this on my narrowboat setup. Solid bit of kit.

@PanelGraham is right, but I'd add — even if you're thinking of the Victron Blue Smart IP67 or similar shore power unit, I'd be cautious about charging the starter battery while cranking. The voltage drop during cranking is significant and most chargers will either fault out or at minimum behave oddly trying to compensate.

Seen this cause issues on a mate's setup — charger kept throwing low voltage warnings mid-start.

What's the actual goal here @SprinterProject? If it's a flat starter battery situation, a proper DC-DC charger (like the Orion-Tr Smart) from your leisure bank to starter makes more sense as a long-term solution. Keeps the starter topped via your leisure battery rather than trying to fight the cranking load simultaneously.

What state is your starter battery normally in when this becomes a problem?

@SprinterProject the Centaur is a DC-DC charger, not a shore power unit — @PanelGraham already covered that.

But the actual question buried in your post: do not charge a starter battery while cranking. Voltage during cranking drops well below 10V, your charger will either cut out or behave erratically. Either way it's not helping.

Charge between starts, not during. If your starter battery is struggling that much, just replace it — a Yuasa or Banner from any motor factor won't break the bank.

Sort out what you're actually running first. Centaur, Blue Smart, Orion — they're completely different bits of kit for different jobs. Read the datasheets before buying anything.

@SprinterProject had exactly this situation with my motorhome last winter — Dorset, freezing mornings, dodgy starter battery that needed nursing along.

Worth knowing: even with a healthy shore power setup feeding your leisure bank, cranking draws such a savage spike that your DC-DC charger essentially sees a voltage collapse and will just back off or shut down momentarily anyway. It's not harmful, just largely pointless at that precise moment.

What actually sorted me was fitting a Fogstar Drift lithium as the starter — holds voltage under load far better than lead-acid, so the whole "charge while cranking" question becomes moot. Alternatively a simple Victron Orion-Tr Smart between banks, properly configured, does the job elegantly once the engine's running.

The cranking moment itself is fleeting. It's the recovery charge afterwards that matters.