Issue with Charge Current Overshoot in Parallel MultiPlus-II System

Anyone else had their parallel MultiPlus-II setup decide it wants to aggressively charge beyond the configured current limit during the bulk phase?

Running two 48V/5000 units in parallel on the narrowboat — configured through VE.Bus System Configurator, limits set sensibly — and occasionally the combined charge current overshoots by a frankly alarming margin before settling down. Not a catastrophic figure, but enough to make my Fogstar Drift LiFePO4 cells rather unhappy and the BMS a bit twitchy.

From what I can tell it's most pronounced when:

• The bank is at a fairly low state of charge
• Both units have been idle for a while (overnight, typically)
• Shore power kicks in cold

My suspicion is there's a synchronisation lag between the two units during the initial ramp-up — one charges ahead before the other has properly joined the party, and the "master" overcorrects briefly. But that's speculation dressed up as engineering knowledge, so take it with appropriate salt.

Firmware is current. Yes I've checked. Yes I've power-cycled. Yes I've stared at the VRM graphs looking for patterns like a man slowly losing his grip on sanity.

Victron's official line seems to be "configure your BMS to handle it" which is spectacularly unhelpful when you've already done that and you're still watching 140A appear where 100A should be.

Curious whether anyone's resolved this with a ChargeCurrentBoostFactor tweak in VEConfigure or whether that's just another rabbit hole. Also wondering if the issue is more common on 48V systems specifically or whether our 24V friends are quietly smug about something for once.

Hey @48VQueen, yes, seen this a fair bit with parallel setups. A couple of things worth checking:

First, make sure both units are running identical firmware - even a minor version mismatch can cause synchronisation issues where one unit essentially "fights" the other during bulk, resulting in overshoot.

Second, check your VE.Bus System Configurator settings - specifically whether the shared current limit is correctly distributed across both inverters rather than each one applying the full limit independently.

Also, what cable are you using between the units? A dodgy or undersized RJ45 interconnect can cause comms dropouts mid-charge cycle, which sometimes manifests exactly as you're describing.

What firmware version are you currently on? And are you monitoring via VRM or locally through VE.Configure? That'll help narrow it down considerably.

Seen this exact behaviour with my cabin setup — took me an embarrassing amount of time to diagnose.

Worth checking whether your VE.Bus system frequency is drifting between the two units during bulk. If the master/slave negotiation hiccups even briefly, the slave can temporarily act autonomously and both units essentially "race" to charge simultaneously without proper current sharing coordination.

The fix that sorted mine was ensuring both units had identical firmware versions — even a minor revision difference caused instability in the shared current limit calculation. Flash both through VictronConnect and re-run the parallel configuration wizard from scratch rather than keeping the old settings.

Also, what cable lengths are connecting your positive busbars? Asymmetric resistance between the two inverters will cause one unit to persistently overshoot whilst the other undershoots, even when firmware is matched.

Something I noticed on my narrowboat setup that nobody's mentioned yet — firmware consistency between the two units.

Had a similar overshoot issue after I updated one MultiPlus-II but forgot the second. The units were technically talking to each other but their charge algorithm behaviour was slightly out of sync, which caused one to essentially "chase" the other during bulk.

Pulled both into VE.Config, confirmed they were on mismatched firmware, updated the laggard, re-ran the parallel assistant on both fresh, and the overshoot disappeared almost immediately.

Also worth checking your shared DC cable lengths are genuinely equal — unbalanced resistance between units in a parallel arrangement means they don't share current cleanly, which can look exactly like overshoot when really one unit is carrying a disproportionate load.

@BatteryCol and @CaddyCamper have likely covered the obvious, but these two catch people out regularly.

Good shout from @DerekDixon on the firmware point — that one catches people out more than you'd think.

One thing I'd add that hasn't come up yet: check your battery capacity setting in VEConfigure. If it's set too low relative to your actual bank, the adaptive charging algorithm can get a bit confused and effectively "borrow" headroom it shouldn't have during bulk.

Also worth having a look at the charge current knob position on the physical units themselves — in a parallel setup those still interact with the software limits, and if they're not identical between both inverters it can cause one unit to pull harder than the other, making the combined current overshoot what you'd expect.

What's your actual battery bank — lithium or lead? Makes a difference to where I'd point you next.

Great points from @DerekDixon and @TerryBurns — firmware mismatch is the classic culprit, but if you've already ruled that out, check your VE.Bus system configuration in VEConfigure; the charge current limit you set is per unit, not system-wide, so two units in parallel will happily double whatever you've told them individually.

On my narrowboat I caught myself wondering why my Fogstar Drift cells were sweating — turns out I'd configured 70A thinking that was the total, not realising I'd essentially set 140A system-wide. 🤦

Key things to verify:

• Shared VE.Bus cable properly terminated
• Phase master correctly assigned
• Charge current set accounting for parallel multiplication

The Victron parallel configuration guide is worth a re-read — it's saved my bacon more than once.