Lithium batteries stop charging when going into absorbtion

Had something similar happen with my cabin setup last spring and it proper stumped me for a while.

I'm running a bank of Fogstar Drift 100Ah cells — four of them in parallel — hooked up to a Victron SmartSolar and a Phoenix charger. Everything was ticking along nicely until I noticed the batteries were barely pulling any current the moment the charger transitioned into absorption. Looked almost like they were full, but the resting voltage told a different story.

Took me a good week of head-scratching before I figured out what was going on. A couple of the batteries were slightly out of balance with each other — one BMS was tripping early and essentially dragging the others with it. Because they're non-communicating units, the Victron had no idea; it just saw voltage climb and backed off like it's supposed to.

A few things worth checking if you're in the same boat:

• Are all your parallel batteries genuinely matched in age and cycle count? Mixing older and newer cells is a recipe for this.
• What absorption voltage are you actually set to? Lithium is far less tolerant of sloppy settings than AGM.
• Have you tried isolating each battery individually and running a slow charge to see if one BMS is the culprit?

The Victron smart lithium preset is a decent starting point but it's not magic — it still needs the underlying batteries to play nicely together.

Anyone else found a more systematic way to diagnose this across a parallel bank without Bluetooth-capable BMS units? Curious whether a simple clamp meter on each feed cable is the way to go.

@MountainBarry the Victron absorption voltage is almost certainly the culprit here. Fogstar Drift cells have a fairly tight upper voltage tolerance, and if your DVCC settings aren't properly configured, the Cerbo can get confused about who's actually commanding the charge voltage.

Check your SVS (Shared Voltage Sense) is enabled in DVCC — if the BMS is reporting cell voltages slightly higher than what the SmartShunt sees at the battery terminals, the system can effectively throttle current to near-zero whilst technically still "in absorption."

Also worth verifying your BMS charge enable signal is wired to the VE.Bus BMS assistant on the MultiPlus, not just relying on CAN communication alone. I had a nearly identical situation with a 280Ah bank where the assist logic was cutting charge current prematurely because the CVL from the BMS was being interpreted as "battery full" rather than "target voltage."

What firmware version is your Cerbo running?

Great thread this. @MountainBarry I'd check what absorption voltage your Victron is actually set to — the Fogstar Drift cells are quite particular about not exceeding 3.45V per cell, so for a 12V bank that's roughly 13.8V absolute maximum. If your Victron's absorption is set anywhere near the default lead-acid figures (14.4V or higher), the BMS will kick in and disconnect before absorption even gets going properly.

Worth also checking whether it's the BMS cutting charge entirely or just the Victron backing off due to a communication issue, if you've got the VE.Can or VE.Direct link set up. Pull up VictronConnect and check the DVCC settings — sometimes they fight each other in unexpected ways.

What's your actual absorption voltage set to currently?

@MountainBarry what nobody's mentioned yet is the cell balancing state before absorption kicks in. If one cell in your parallel bank is sitting slightly higher than the others when the Victron ramps up voltage, the BMS will see that outlier hit its overvoltage threshold and pull the plug — looks exactly like what you're describing.

Worth grabbing a cheap cell-level voltage logger or just manually checking each Fogstar's terminals during a charge cycle. I've seen parallel banks behave beautifully for months then develop a rogue cell that throws everything off.

Also — what cable lengths are you running between each battery? Unequal resistance across a parallel bank causes uneven charging. Learned that the hard way on my motorhome build before I standardised every run to the same length and gauge.

@LiFePO4Nerd cell balancing point is the one that bit me — had a rogue cell in my static van bank doing exactly that, whole BMS just threw its hands up the moment Victron tried to push into absorption.

Ended up dropping my absorption voltage to 3.45V/cell (so 13.8V for 12V system) and the drama stopped immediately.

Also worth checking your absorption time — if Victron's set to hold it for ages, the BMS gets increasingly unhappy even if the voltage is borderline acceptable.

Fogstar Drift cells specifically seem sensitive to this — learned that the expensive way on my garden office setup too.