Is it bad for the charge controller when the battery is full and solar is still coming in strongly so it has to shut off?

Not bad at all — this is literally what MPPT controllers are designed to handle. The controller doesn't just "shut off", it throttles back the power draw from the panels through PWM switching. Victron calls this the absorption/float cycle. The panels aren't damaged, the controller isn't stressed.

The slight caveat: if you've got a genuinely oversized array hammering a small controller at or near its rated current constantly, that's where you'd start seeing thermal wear over years. But a controller hitting float on a sunny afternoon? Completely fine, happens on my narrowboat every clear day in summer.

Worth distinguishing between MPPT and PWM controllers here though — they handle the excess differently under the hood. MPPT is more elegant about it.

One thing that does matter: make sure your battery voltage settings are correct. If the absorption voltage is set too high and your controller keeps pushing into a full LiFePO4 bank, that's where you can cause actual harm — but that's a settings issue, not an inherent flaw in the process.

What controller are you running? And is it LiFePO4 or lead-acid? Makes a difference to the conversation.

Just to add a small clarification to what @OakSpirit said — worth noting that the panels themselves don't suffer either. People sometimes worry that having panels "blocked" from delivering power causes some kind of back-pressure damage, but PV panels are perfectly happy sitting at open-circuit voltage when the controller isn't drawing current. They just warm up slightly, which marginally reduces efficiency, but causes no harm whatsoever.

The only scenario where I'd say keep an eye on things is if you're consistently hitting float for hours every sunny day — might be worth asking whether your battery bank is undersized for your array, as you're leaving free energy on the table. But from a hardware longevity standpoint, no concerns at all.

What @Davo79 said about the panels is spot on, and it actually took me a while to wrap my head around that when I first set up my boat system.

The bit that clicked it for me was realising the panels aren't being "held back" — the controller is simply not drawing current from them, so they sit at open-circuit voltage quite happily. Nothing fighting against nothing.

On a hot August afternoon when my Victron SmartSolar is floating my Fogstar batteries, I've watched the whole system just quietly ticking over at absorption — panels baking away, controller barely breaking sweat. It's genuinely one of the most elegant parts of a well-matched system.

The only caveat I'd offer: make sure your float voltage is properly calibrated for your battery chemistry, that's where people sometimes cause long-term damage without realising it.

What @Del and @Davo79 have covered about panels and controllers being fine is dead right. The one thing I'd add from bitter personal experience: that "throttled back" state is actually where your MPPT earns its keep thermally. My Victron SmartSolar runs noticeably cooler at absorption/float than during the morning bulk charge ramp-up — the switching losses at partial duty cycles are lower. The only gotcha I've seen people miss is that some cheaper no-name controllers handle the float regulation less gracefully and can cause minor voltage hunting, which isn't dangerous but isn't ideal for battery longevity. Stick with reputable kit — Victron, Epever, even the better Renogy units — and you'll genuinely never have to think about this scenario again. Your panels happily sit there in open-circuit-adjacent territory while the controller does its job quietly.

The real drama isn't the controller sweating at full battery — it's when your battery BMS decides to throw a strop and disconnect entirely, leaving the panels with nowhere to dump power and your voltage spiking like it's had three espressos.

Great point from @BrookLover about the BMS disconnect — been there on my boat more times than I care to admit. When the BMS cuts the load suddenly, your MPPT can momentarily see an open-circuit-like condition and the bus voltage spikes. On my Victron SmartSolar 100/30 I actually set the absorption and float voltages slightly conservative relative to what my Fogstar Drift LiFePO4 cells can technically handle, purely to reduce how aggressively the BMS gets pushed toward its high-voltage cutoff. Keeps everything playing nicely together.

Worth checking your Victron app's history tab — if you're seeing lots of "BMS assert" events or your controller keeps bouncing between bulk and absorption, that's usually the telltale sign the BMS and controller aren't quite in agreement about where "full" actually is.