My inverter is showing a low battery alarm

Been getting a low battery alarm on my Victron Phoenix 24/1200 this past week and trying to work out if it's the inverter settings, the batteries, or something else entirely.

My setup is in a shepherd's hut — been running it for about 4 years now on a 24V system with two 200Ah AGM batteries. The alarm is triggering around 23.2V which seems low but the batteries were showing a decent state of charge the night before.

A few things I'm trying to rule out:

• Battery age — the AGMs are coming up to 4 years old. Starting to wonder if one has a dead cell dragging the bank down
• High load spike — I've got a small 650W induction hob that I use occasionally. Could a momentary voltage sag be tripping the alarm even if the overall SOC is fine?
• Inverter threshold settings — can the low voltage cutoff be adjusted on the Phoenix, or is it fixed firmware?

I've noticed it mostly happens in the morning before the solar gets going, so there's definitely a overnight discharge element to it. Running 400W of panels through a Victron SmartSolar MPPT, which has been solid.

Has anyone else had this with an aging AGM bank? I'm half expecting the answer is "your batteries are knackered, replace them" but wanted to check if there's a config fix worth trying first before I start looking at Fogstar Drift lithium as a replacement.

What voltage do others set their low battery cutoff at on a 24V system?

@RussScott had exactly this on my narrowboat last winter. The Phoenix default low battery cutoff sits at 21.5V for a 24V system, which sounds reasonable until your bank is cold and under load — voltage sags faster than you'd expect.

Worth checking two things first:

1. Actual resting voltage after the bank's been sitting idle for a couple of hours
2. What's drawing current when the alarm triggers — kettle, inverter microwave, something spiky?

On my boat I nudged the low battery cutoff down slightly in VictronConnect and the false alarms stopped overnight. Not a permanent fix if the bank's genuinely struggling, but it'll tell you whether it's a settings issue or a capacity issue.

What batteries are you running? AGM, lithium, something else? Makes a big difference to how aggressively the voltage drops under load.

@RussScott what battery chemistry are you running — because if it's LiFePO4 and your Victron is still set to lead-acid defaults, that 21.5V cutoff @DerekDixon mentioned is way too low and your BMS will have already thrown a fit long before the Phoenix even blinks at you.

Connect VictronConnect via Bluetooth, check your Low battery shutdown and Restart voltage settings under the inverter config — for 24V LiFePO4 I'd set shutdown around 23.2V and restart at 25V.

Also worth ruling out a dodgy cell dragging the pack down under load; had exactly that grief with an older Fogstar pack in my motorhome before I realised one cell group was quietly giving up the ghost whilst everything looked fine on the surface voltage.

@RussScott to add to what @DerekDixon and @FormerCop have started — I went through exactly this diagnosis process on my motorhome last spring with a Phoenix 24/3000.

The bit people miss is checking the DC cable voltage drop under load. My alarm was triggering not because the batteries were genuinely low, but because I had marginal crimps on a long cable run — the inverter was seeing 21.3V whilst the batteries were actually sitting at 23.8V.

Grab a multimeter and measure directly at the battery terminals whilst the inverter is under load. Then measure at the inverter DC terminals. Any meaningful difference and you've found your culprit.

If the readings match and voltage is genuinely sagging, that points toward either a weak cell or an undersized battery bank for your draw. What's your battery capacity and typical load?

One thing I haven't seen mentioned yet — have you checked whether the alarm is happening at a specific time of day?

In my garden office setup I was getting false low battery alarms every morning before the solar kicked in. Turned out my overnight loads were higher than I'd estimated and the batteries were genuinely sagging, just not as dramatically as I'd assumed from the alarm.

Worth logging actual voltage over 24 hours via the VictronConnect app (assuming you've got Bluetooth on that Phoenix) before changing any settings. Gives you a proper picture rather than chasing ghosts.

Also — what's your battery capacity vs the 1200W inverter? That's a reasonable load and if your bank is undersized you could be hitting legitimate low voltage under surge conditions rather than a settings issue.