Update on my home stormwater pump backup using a 48V MP2 with 5kWH LFP battery, no PV

So I've been running a Victron Multiplus-II 48V setup paired with a 5kWh Fogstar Drift LFP battery — no solar involved, purely mains-fed backup — to protect a critical drainage pump in my yard. The pump sits at the lowest point of my property and, without it, I get flooding into the garage within about 20 minutes of a serious downpour. Painful experience taught me that lesson.

The pump draws roughly 1.2kW but only cycles for about 60–90 seconds at a time, maybe four or five times during a heavy storm event. So actual energy consumption per event is genuinely tiny — the battery capacity is almost comically oversized, but I wanted the headroom and the peace of mind.

My question is around the MP2's ESS configuration specifically. I've got it set up in "Keep Batteries Charged" mode because there's no PV in the picture, but I'm second-guessing whether that's truly the optimal mode for a pure backup scenario like this. The manual isn't exactly light bedtime reading.

A few things I'm uncertain about:

• Is "Keep Batteries Charged" actually correct here, or should I be looking at a different ESS assistant configuration?
• Does the MP2 handle the inrush current from a pump motor (capacitor-start type) gracefully, or should I expect any issues there?
• Anyone know whether Cerbo GX logging will capture individual pump cycle events at a useful resolution, or is the data too coarse?

I realise this is quite a niche use case — it's not the typical motorhome or solar-storage scenario — but the underlying principles must surely apply. Has anyone done something similar here in the UK?

@PanelLouise interesting setup — a few questions if you don't mind:

1. How are you handling the charge cycle from mains? Are you using a dedicated AC charger or relying on the MP2's internal charger?
2. What's your typical draw when the pump kicks in — have you measured peak inrush current? Drainage pumps can be spiky.
3. Is the MP2 set to UPS mode so switchover is seamless, or is there a gap?

Reason I'm asking — I've been considering something similar for my garden office, not for a pump but for keeping critical kit alive during outages. The mains-only approach (no PV) seems underrated on this forum. Curious whether Victron's ESS assistant is involved or if this is purely a straightforward backup config with no grid interaction beyond charging.

@SmartSolarMaster great questions for @PanelLouise — I'd be curious about the charge settings too. One thing worth flagging: with a purely mains-fed setup like this, you'll want to make sure the MultiPlus-II is configured with a sensible absorption voltage and tail current for LFP — the Fogstar Drift cells don't appreciate being held at float the way lead-acid would be. In VEConfigure you can set a proper LFP charge profile and disable float altogether if needed. Also worth checking whether you've got the input current limit set appropriately so you're not surprising your DNO. Are you using a CCGX or Cerbo to monitor state of charge, @PanelLouise? Accurate SOC tracking without PV is straightforward enough but relies heavily on having a decent shunt calibrated correctly from the off.

Good to see a proper use-case thread with actual real-world stakes — drainage backup is no joke, especially if you're in a flood-prone area.

@PanelLouise welcome to the forum, great first post. This is exactly the kind of specific, well-documented setup we need more of here.

One thing nobody's flagged yet: with a pure mains-fed UPS-style config on the MP2, make sure your Dynamic Current Limiter is enabled if you're on a standard domestic supply. The MP2 can get greedy on inrush during bulk charge and trip your DNO fuse — seen it happen on similar setups on my narrowboat when shore power is limited.

Also worth checking your Low Battery Restart threshold in VEConfigure. Default settings aren't always sensible for a standby system that rarely discharges.

What firmware version are you running on the MP2?

@PanelLouise worth checking your ESS assistant configuration carefully here. On a pure mains-backup setup with no PV, the MP2 can sometimes behave oddly around the "keep batteries charged" vs "optimised with BatteryLife" modes — I'd strongly recommend "keep charged" with a scheduled discharge window disabled entirely, since you're not cycling for economics, you're cycling for resilience.

On my narrowboat I run a similar non-solar Multiplus setup and the minimum SOC threshold is critical. Set it no lower than 20% but I'd argue 30% given pump inrush current — LFP sags harder than people expect under motor loads, and the Fogstar Drift BMS will disconnect if cell voltage drops sharply during startup.

Also worth fitting a contactor or transfer switch rated for the pump's locked-rotor current, not just running amps. That's often the overlooked failure point.