Solis inverter pull energy from battery even with solar

Had exactly this head-scratcher on my narrowboat last summer. Solis was pulling from the battery pack even with decent solar coming in, and it drove me mad for about a week before I worked out what was happening.

The inverter has a minimum PV voltage threshold before it'll prioritise solar over the battery bank. If your panels aren't quite hitting that threshold — early morning, partial shading, anything like that — it'll quietly dip into your batteries to make up the difference rather than waiting for the panels to step up. Annoying behaviour, but it's by design apparently.

Worth checking your inverter mode settings as well. Solis units can default to "battery first" depending on firmware and how they've been configured out of the box. I'd pull up the parameter list and specifically look at:

• Grid/battery priority mode
• Minimum battery SOC cutoff
• PV input voltage thresholds

My setup runs Fogstar lithium cells with a Victron SmartShunt monitoring everything, so I could see exactly what was happening in real time — without that visibility I'd have been completely in the dark (no pun intended).

Also worth asking — what battery chemistry are you running? Some Solis models behave differently depending on whether they're talking to a BMS or just reading voltage.

Has anyone else noticed this being worse on certain firmware versions? I've seen a few people on the Solis forums mention a particular revision that seemed to make the battery draw more aggressive. Would be curious whether a firmware update actually helped anyone here or just shuffled the problem around.

@HalfAJob had almost the exact same saga on my narrowboat a couple of years back. Turned out my Solis had its battery priority threshold set too high — it was essentially treating anything under a certain voltage as "low battery assist required" even when solar was producing well.

The sneaky bit nobody tells you: the inverter reads battery voltage under load, not open circuit. Fire up the kettle and that voltage sags just enough to trigger battery draw, even mid-afternoon with panels doing their thing.

Worth checking:

• Battery discharge threshold in the Solis settings
• Priority mode — is it set to Solar First or Utility First?
• Cable sizing between solar and inverter (voltage drop can fool the logic)

Once I sorted the threshold settings to match my actual lithium bank's real working range, it stopped raiding the batteries entirely.

@HalfAJob worth checking the DC bus voltage target in the Solis parameters. On mine the default battery charge voltage was set lower than what the Fogstar cells actually needed to register as "full," so the inverter's SOC calculation was permanently pessimistic — it thought the pack was at 60% when it was actually sitting at 95%. That mismatch causes exactly the phantom discharge behaviour you're describing.

If you're running LiFePO4, make sure your charge voltage ceiling, float, and the BMS communication protocol all agree with what Solis expects. The Solis RS485/CAN comms to a Victron-adjacent BMS can be particularly fussy about which profile is selected under Battery Mode in the advanced settings — wrong profile = wrong SOC = inverter pulling battery unnecessarily even in full sun.

What BMS are you running?

@HalfAJob there's a delightful little gotcha I stumbled across when commissioning my shepherd's hut setup — the Solis has a priority mode buried about seventeen menus deep that determines whether it favours solar-to-load, solar-to-battery, or some baffling hybrid of both.

Mine was set to "battery first" from the factory, which meant it was happily draining the Fogstar cells to power the load while simultaneously charging them from solar. Thermodynamically pointless, but the inverter was technically doing exactly what it was told.

Check parameter 00 or 01 depending on your firmware revision — it'll be labelled something like Energy Management Mode. Switch it to Solar Priority and the daft behaviour stops immediately.

The manual explains this with all the clarity of a Welsh road sign translated by someone who learned English from instruction booklets. You have been warned.

Great to see a narrowboat thread getting some decent traction — welcome to the forum @HalfAJob, stick around, plenty of knowledgeable people here.

One thing nobody's mentioned yet: check your grid/generator priority order in the Solis settings. Even in off-grid mode, some firmware versions have a quirk where the inverter briefly samples alternative sources on a timed cycle, which can cause it to pull from battery momentarily even when PV is more than sufficient.

Also worth confirming your battery communication protocol is set correctly — if the Solis is talking CANbus to a BMS but the parameters don't match, it'll default to its own conservative voltage curves and essentially ignore what the battery is actually telling it. Seen this catch people out with Fogstar packs specifically.

What battery chemistry are you running?

Been there with my boat — the thing that finally cracked it for me was the battery type setting buried in the advanced parameters. Mine was left on AGM defaults even though I'm running Fogstar lithium cells, so the Solis was essentially confused about what voltage constituted "full" and kept dipping into the pack to stabilise the bus rather than trusting the solar input.

Worth pulling up the parameter list and cross-referencing it against your actual battery chemistry. The Solis documentation is... optimistic about clarity, so the Victron community forum actually has better third-party walkthroughs for Solis quirks than Solis's own support pages.

@HalfAJob what battery setup are you running? That'll narrow it down considerably.