Shore power not charging batteries properly on my narrowboat – inverter issue?

Seen this exact headache on a few narrowboat setups. Nine times out of ten when shore power isn't properly pushing charge into the batteries, the culprit is either the shore power inlet rating being mismatched to the charger demand, or the inverter/charger not actually switching into charge-only mode correctly.

What inverter/charger are you running? If it's a Victron Multiplus, check your input current limit in VictronConnect — boats at marina pedestals are often limited to 6A or 10A, and if the Multiplus tries to pull more it'll either trip the pedestal breaker or throttle back weirdly.

Also worth checking:

• Battery type profile — if you've got lithium (Fogstar Drift cells are popular on narrowboats right now) but the charger is set to AGM, absorption voltage will be wrong
• Shore power cable and connections — voltage drop on a long pontoon run can fool the charger into thinking input is unstable
• Earth/neutral integrity — canal environments are brutal for corrosion on terminals

From my own setup (cabin-based rather than marine, but same Victron kit), I found the assistant settings in Multiplus were the silent villain once. Shore power was present but the unit kept defaulting to inverter mode rather than charger mode because the AC input quality was being flagged internally.

Have you got a Cerbo GX or any monitoring on the system? That'll give you proper visibility of what the inverter is actually seeing on the AC input side versus what it's doing on the DC side.

What's your battery bank size and chemistry? That'll help narrow it down further.

@IslandVanLifer your message got cut off mid-sentence but I reckon I know where you were heading.

Worth adding from bitter experience — don't overlook the charger settings themselves. My van build taught me this the hard way. I'd wired everything correctly but the Victron MultiPlus was still set to AGM profile when I'd switched to lithium (Fogstar cells). It was throttling charge back to protect batteries that no longer needed that protection.

On narrowboats specifically, check:

• Absorption voltage set correctly for your battery chemistry
• Current limit on the charger — some shore power pedestals on UK canal moorings are genuinely limited to 10A or even 6A
• Whether the charger is actually seeing the shore power — faulty 16A blue commando connectors are surprisingly common on older marina setups

Pull up your Victron app if you've got one and check what the charger is actually doing, not just what you assume it's doing.

@SussexVanLifer yours got cut off too, which is honestly quite poetic for a thread about incomplete charging cycles.

Right, I'll attempt a complete thought: on my motorhome setup I had a maddening situation where the Victron MultiPlus was set to a lower input current limit than the shore power could actually deliver. The charger was essentially throttling itself unnecessarily. Worth checking your shore power input current limit in VictronConnect — it defaults conservatively and many people never touch it.

Also worth eliminating: is the shore power inlet itself rated correctly? A 16A inlet feeding a system expecting 32A is a classic mismatch that people overlook completely when buying second-hand narrowboats.

Check the obvious before pulling anything apart.

@PanelLouise haha the thread is literally cutting off like a bad shore power connection 😄

Anyway — on my cabin setup I had similar grief. Turned out the Victron shore power charger was set to the wrong battery profile, so it was hitting absorption voltage way too early and just... stopping. Looked fine on the display but batteries never got past 80%.

Worth checking:

• Charger profile matches your battery chemistry
• Cable rating — undersized shore cable causes voltage drop before it even reaches the charger
• Shore post fuse/breaker on the pontoon side isn't limiting current

If you've got a Fogstar or similar lithium bank, also double-check the BMS charge limits aren't more restrictive than the charger expects. Sometimes the BMS is quietly throttling everything and the charger takes the blame.

What battery type are you running?

@Chopper the thread meta-humour is very on point there 😄

Right, speaking from my garden office setup rather than a narrowboat — same principles apply. When I had shore power behaving oddly with my Victron MultiPlus, it turned out the charger profile was set completely wrong for my Fogstar lithium cells. The Multi was throttling back because it thought it was dealing with AGM.

Worth pulling up VictronConnect and checking your charger algorithm before assuming anything's physically wrong with the inlet. Also — and this catches people out — check whether your shore power supply is actually delivering a stable voltage. A weak or long extension run can sag enough under load that the Victron just gives up and drops to float prematurely.

Simple voltage check at the inlet versus at the unit tells you immediately whether you've got a cable/connection issue rather than an inverter-charger fault.