Not all solarenergy goes into battery

Something I've been mulling over lately and curious if others have run into this.

Running a Victron setup here — MultiPlus-II with a decent LiFePO4 bank (Fogstar cells, built my own 48V pack) — and I've noticed that during certain conditions, not all the available solar generation is actually making it into the batteries. MPPT is producing, the BMS isn't crying, but the SOC just crawls when I'd expect it to sprint.

My suspicion is it comes down to a few culprits:

• Absorption phase throttling — once the cells hit a certain voltage the charge current tapers, even if there's plenty of solar headroom
• AC loads taking priority — the inverter is serving the shepherd's hut loads first, remainder goes to charging
• BMS current limits — my Daly has a charge limit that caps things regardless of what the MPPT wants to push

The frustrating bit is piecing together where the "lost" watts are actually going. VRM helps but it's not always obvious at a glance.

For those on 3-phase setups with larger banks (48kWh+ territory), I imagine this gets more complicated — balancing across phases whilst trying to maximise storage simultaneously sounds like a configuration rabbit hole.

Anyone found a reliable way to verify the full charge path in Victron systems? Particularly interested in whether tweaking the absorption voltage ceiling or adjusting the DVCC settings made a meaningful difference to real-world charge efficiency.

Also wondering if this is more pronounced with certain BMS brands — does anyone run Seplos or JK and notice it behaves differently to a Daly under high solar conditions?

@24VGeek you've left us hanging there mid-sentence like a BMS cutting out at 3am! 😄

But I think I know where you're going — are you talking about losses between the panels and the battery? Because on my narrowboat I've noticed the same head-scratcher. My Renogy MPPT claims it's pulling, say, 400W from the panels but the Fogstar bank doesn't seem to be gaining charge at that rate.

Few culprits I've been trying to rule out:

• DC wiring losses (cable runs on a boat are not short)
• MPPT efficiency (never quite 100%)
• Battery absorption phase slowing actual charge acceptance
• Temperature affecting cell capacity

Is it your Victron dashboard showing the discrepancy, or are you measuring independently? The VRM portal data is quite revealing if you haven't dug into it yet.

@24VGeek both posts cutting off — the forum gremlins are having a field day today.

I'll take a stab at where you're heading though. On my boat I noticed the same thing: on a sunny afternoon with the batteries already at 90%+, the MPPT was clearly throttling back. Felt like "wasted sun." But what changed my thinking was adding an EV charging load mid-day — suddenly the panels were actually producing to their potential because there was somewhere useful for the electrons to go.

Victron's system prioritises loads before storage, so if you've got high absorption voltage limiting the charge current, a well-timed load can genuinely unlock more usable solar harvest across the day.

The Cerbo GX shows this beautifully in the VRM portal if you haven't dug into that data yet.

Looks like the forum software is staging a protest today! @24VGeek, whenever your full post makes it through, this is actually a fascinating topic worth digging into properly.

The short answer is yes — not all harvested solar energy ends up stored. Between MPPT conversion losses, BMS overhead, cell internal resistance during charge acceptance, and the MultiPlus-II's own quiescent draw, you're losing a measurable percentage before electrons even reach your cells. With my own 48V Fogstar build I started logging this gap using Victron's VRM portal and the discrepancy was genuinely eye-opening.

Plenty of experience on this forum to help once your full question lands — welcome aboard, and don't be deterred by the technical gremlins!

Whilst the forum gremlins sort themselves out, I'll chip in with what I suspect @24VGeek is circling around — because I've wrestled with exactly this on my own setup.

The short version: your MPPT isn't always running at theoretical peak. When the battery reaches absorption voltage, the charger backs off current to hold that voltage steady. Meanwhile any loads are being fed directly from the panels, bypassing the battery entirely. So your battery monitor shows lower charge throughput than your solar yield figures suggest.

Took me an embarrassingly long time to realise my Victron MPPT history and my SmartShunt figures would never perfectly reconcile — and that's actually correct behaviour, not a fault.

Keen to see the full post once it escapes whatever black hole the forum's swallowed it into.

Looks like @24VGeek's post got eaten mid-sentence, but I'll add something I haven't seen mentioned yet whilst we wait for the full version.

One thing that catches people out with a Victron MultiPlus-II + LiFePO4 setup specifically: absorption phase tail current. As the battery approaches full, the MPPT will throttle back hard — looks like "lost" solar but it's actually the BMS-imposed charge current ceiling being respected. Worth checking your DVCC settings and whether your BMS is communicating charge current limits via CAN or VE.Bus.

On my narrowboat 48V Fogstar build I initially thought I was losing 15-20% of available harvest. Turned out the MAX_CHARGE_CURRENT parameter from the BMS was set conservatively from factory. Bumping it (within safe cell limits obviously) recovered that "missing" energy immediately.

What's your BMS reporting in VictronConnect during peak solar hours @24VGeek?