Oscillation in Battery SOC. Charge/discharge cycle under ESS Optimized with Batterylife

Been noticing something a bit odd with my setup over the past couple of weeks and wondering if anyone else has seen the same.

Running a Victron Multiplus II with a Fogstar Drift 24V battery bank through Venus OS, ESS set to Optimised with BatteryLife. We've had a run of proper grey November weather here — the kind where the panels barely break a sweat — so the active SOC limit has dropped down to around 78-80%.

Here's what's catching my attention. During the day, even with weak solar coming in, the batteries will sit right at that BatteryLife limit. Then as loads kick in — kettle, laptop, the usual cabin stuff — they'll dip a few percent below it. Solar catches up, pushes them back to the limit, and the whole thing repeats in this slow oscillation. Discharge a bit, charge back up, discharge, charge. Almost like a heartbeat on the CCGX screen.

It's not causing any problems as far as I can tell, and the system seems to be doing what it's supposed to. But I'm not sure whether this is completely normal behaviour or whether there's some threshold or hysteresis setting I should be tweaking to smooth it out.

A few specific questions:

• Is this oscillation just BatteryLife doing its job correctly?
• Is there a minimum charge/discharge cycle depth I should be aware of that affects battery longevity with LiFePO4?
• Any value in adjusting the minimum SOC manually rather than leaving it dynamic?

Would appreciate hearing from anyone who's dialled this in on a similar Victron ESS setup.

@OffGridTerry my Fogstar does the same little SOC tango — I've started calling it "BatteryLife Boogie" and charging it to spectators in the shepherd's hut.

@OffGridTerry this is pretty well documented Victron behaviour actually. The Multiplus II under ESS Optimised with BatteryLife uses a dynamic minimum SOC that shifts based on recent history — it's essentially trying to predict your consumption pattern. What you're seeing is it hunting around that threshold.

Worth checking your Grid Setpoint in the ESS assistant config — if it's set too close to zero the system gets twitchy trying to balance import/export and that feeds into the oscillation.

Also had this on my narrowboat setup until I bumped the Minimum SOC (unless grid fails) up by about 5% and set a proper charge schedule via VRM. Settled right down after that.

Check the DVCC settings too — if you're running without a proper CANBUS link to the Fogstar, the Victron is essentially guessing on SOC accuracy which doesn't help.

Seen this exact thing on my static van setup. The oscillation is usually the BatteryLife algorithm doing its job — it's deliberately nudging the minimum SOC floor upward when it thinks conditions warrant it, then relaxing again. Looks alarming on the graphs but it's largely benign.

One thing worth checking though: if your Fogstar BMS is reporting SOC slightly differently to what the Cerbo/CCGX is calculating internally, you can get the two fighting each other a bit, which exaggerates the wobble. Make sure you've got DVCC properly configured and that the BMS is set as the trusted source for SOC.

Also worth logging a week of data on VRM before you start tweaking anything — half the people I see "fixing" this end up creating a bigger mess than they started with.

@MrBodge65 is right but worth adding — if the oscillation is driving you mad, check your minimum SOC setting in the ESS menu. If it's set too close to your actual resting voltage, the algorithm hunts constantly trying to hold that line.

I had this on my own Multiplus II / Fogstar setup last winter. Dropping the minimum SOC by about 5% gave the BatteryLife algorithm enough headroom to stop bouncing. Also worth checking your DVCC settings — if charge current limit is too tight it can make the oscillation worse.

One thing I'd rule out first though: are your battery voltage readings stable? A dodgy cable connection can mimic this behaviour entirely and send the ESS logic a bit haywire.

Good shout from @MarineGaz on the minimum SOC setting — that's often the first dial to tweak.

One thing nobody's mentioned yet: if your Fogstar Drift cells are relatively new, the BMS's internal resistance figures will still be settling, which can cause the Multiplus II to misread true SOC and exaggerate the oscillation. Give it a few full cycles to stabilise.

Also worth checking your tail current setting in VEConfigure. If it's set too high, the charger declares the battery "full" prematurely, then BatteryLife kicks in and pulls it back — classic see-saw behaviour. I had exactly this on my narrowboat setup before I dropped tail current to around 1–2% of battery capacity. Sorted it completely.

Node-RED on a Cerbo can also help smooth the transitions if you want finer control than the stock ESS logic allows.

Good points all round from @FormerMechanic and @MarineGaz.

One thing worth checking that nobody's touched on yet — your ESS charge current limits in relation to your BMS's own charge acceptance curve. If the Fogstar Drift's BMS is throttling charge current at certain SOC thresholds (fairly common behaviour around 80-90%), the Multiplus can momentarily see the battery as "full," flip to discharge briefly, then restart the charge cycle. Looks exactly like oscillation on the VRM graphs but it's actually two separate systems disagreeing about state of charge.

On my own 24V Fogstar setup I cured something similar by enabling DVCC and setting a controlled charge current limit slightly below what the BMS would naturally throttle at — smoothed the whole thing out almost immediately.

Worth pulling your VRM data and checking whether the oscillations correlate with those upper SOC thresholds specifically.