Anenji 100A MPPT solar charge controller: MPPT input voltage too low at full battery

Been having a strange one with my Anenji 100A MPPT controller and wondering if anyone else has seen this.

When my battery bank is nearly full — say 95-100% SOC — the controller seems to throttle back massively and I'm getting "MPPT input voltage too low" errors in the app, even on a bright sunny day with panels clearly producing. Voltage at the array is reading fine on my multimeter (around 75V open circuit on a 48V system), so it's not the panels.

My suspicion is the controller is deliberately reducing the operating voltage as it tapers the charge current near absorption, but something is going wrong in that process and it's tripping the low voltage fault rather than just quietly throttling. Does that make sense to anyone technically minded?

A few questions:

• Is this a known firmware bug with the Anenji units?
• Could it be related to my battery voltage set points? I've got a Fogstar Drift 48V LiFePO4 and I'm wondering if my absorption voltage is set slightly high, causing the controller to back off aggressively
• Would a Victron SmartSolar handle this more gracefully? Starting to lose confidence in the Anenji if I'm honest

It's frustrating because mid-day in decent weather I should be topping off nicely, but instead I'm getting false error states and the controller essentially going into a sulk.

Has anyone successfully resolved this with a firmware update or parameter tweak, or is this just a limitation of the cheaper Chinese controllers that I need to accept?

@Chris1966 this is completely normal behaviour and not a fault at all — it's just how MPPT works.

When your batteries approach full, the controller enters absorption phase and deliberately reduces current to maintain the target voltage. To do that, it effectively "derates" the panel input, which shows up as low MPPT voltage in the logs.

On my boat setup with a Victron SmartSolar 100/30, I see exactly the same thing. The controller is essentially throttling the panels back to avoid overcharging — it'll look like the MPPT input has dropped dramatically even on a bright day.

If you're using the Anenji's app or a display, check whether it's correctly transitioning through bulk → absorption → float. If it's jumping straight to float too early, that's worth investigating — could be your absorption voltage set too low for your battery chemistry.

What battery type are you running?

@ForestBoater is correct that throttling is normal, but there's a subtlety worth adding here.

When the controller enters absorption or float phase, it's deliberately reducing current to avoid overcharging. The panel voltage will appear to rise because the controller is no longer drawing maximum power — it's essentially "backing off" the MPPT tracking point.

What you're describing as "MPPT input voltage too low" is likely a display artefact — the controller is reporting a lower harvested voltage figure because it's pulling less current through the algorithm.

Worth checking:

• Your absorption voltage setpoint (too low = premature throttling)
• Whether you're on AGM/lithium/flooded profile — wrong chemistry setting causes early tapering

I saw identical behaviour on my shepherd's hut setup until I corrected the battery voltage profile. Anenji's default profiles tend to be conservative.

What battery chemistry are you running?

@Chris1966 one thing neither of the above have touched on — watch the actual Voc of your panels versus the controller's minimum MPPT input threshold. Some of these cheaper Chinese units (and I'm not saying Anenji are bad, just... budget) have a higher minimum tracking voltage than they'd have you believe in the specs.

On my boat setup I found the stated minimum was optimistic by a few volts.

So at absorption/float when the controller is barely pulling amps, if your panel voltage is also marginal for other reasons — high temperature coefficients, partial shading, dodgy connections — you can get the MPPT algorithm essentially giving up and dropping into a weird quasi-PWM-ish state.

Check your actual terminal voltages under those conditions with a decent multimeter rather than relying on whatever the controller's own display says. Those onboard readings can be... generous.

@CliffGazer raises a good point on Voc limits, but there's another angle worth checking too.

When the battery's nearly full and the controller's in absorption/float, it's deliberately pulling less current. The panels then "see" less load, so their voltage climbs toward Voc. If you're watching the MPPT input voltage at that moment, it'll look high — not low.

So when you say "input voltage too low" — are you reading that off the controller's display, or inferring it from low wattage? Because those are very different problems.

Low wattage at high SOC = totally normal
Low voltage reading = possible wiring issue or loose MC4

Check your cable run length and connector quality too. Cheap MC4s on budget installs cause all sorts of weirdness. What cable cross-section are you running from the array?