Can't keep my lithium blue batteries synced in my 5th wheeel

Had almost exactly this with my van build last spring — one of my Fogstar Drift 100Ah cells would hit its high-voltage cutoff and drop out of the circuit whilst the other was still happily sitting at 60% state of charge. Maddening.

The culprit in my case was the inter-battery cabling. I'd run equal-length positive leads but completely neglected the negatives, so resistance was slightly uneven across the two batteries. Doesn't sound like much, but at higher charge currents it was enough to push one pack ahead of the other.

A few things worth checking in your setup:

• Cable lengths and cross-section — both positives and negatives should be matched as closely as possible
• Connection points — are you charging and discharging from the same terminals on each battery, or have you got a proper busbar arrangement?
• BMS settings — some lithium batteries let you tweak the cell overvoltage threshold; even a small mismatch in factory settings between two units can cause this
• Victron MPPT or charger profile — if you're running absorption at 14.6V, that's fine for a single pack but can stress one battery if the two aren't properly balanced to begin with

The "safe mode" cutoff you're describing sounds like the BMS doing exactly what it should — protecting the battery — but it's masking an underlying imbalance issue rather than solving it.

Anyone else running parallel lithium packs in a larger rig? Curious whether a Victron SmartShunt on each individual battery helped anyone diagnose this kind of thing more cleanly.

@ValleyOffGrid makes a good point about the high-voltage cutoff issue, but I'd add one thing specific to Lithium Blue batteries that catches people out: their internal resistance can vary quite significantly between units, especially if they've had different usage histories.

What you're likely seeing is unequal state of charge at rest, which compounds itself every cycle. A few things worth checking:

1. Are your batteries connected with equal length, equal gauge cables? Even a small resistance difference causes preferential charging.
2. What's your charge current? Lithium Blues can be fussy above roughly 0.5C if cells are already mismatched.
3. Have you tried a manual top-balance — connecting them in parallel at full charge for 24-48 hours with no load?

I did exactly this on my narrowboat setup with two Fogstar units and it genuinely resolved the drift within two cycles.

Good shout from both @ValleyOffGrid and @DevonDweller. One thing worth checking specifically with Lithium Blue units is whether your charge source has a proper absorption/bulk profile matched to lithium — if you're running a generic lead-acid profile on a Victron SmartSolar or similar, you'll often see one battery reach threshold voltage fractionally ahead of the other due to minor internal resistance differences.

In my static caravan setup I resolved persistent sync issues by enabling battery balancing within the Victron MPPT settings and setting charge voltage to exactly 14.2V rather than 14.6V — gives the cells more headroom to equalise before any BMS trips.

Also worth physically checking your busbar connections — unequal cable lengths between batteries create resistance imbalances that compound the problem significantly over time.

Sounds like a classic balance issue but also — are your cables matched length and gauge between the batteries? Uneven resistance means one battery always does more work, charges faster, hits cutoff first. Seen this loads with parallel setups.

Also worth checking your charge voltage is set conservatively — don't just leave it at factory default. I run my Fogstar pair at 27.4V absorption rather than the full 28V and they stay much tighter together.

What's doing your charging — Victron MPPT or a DC-DC? Some cheaper chargers have rubbish absorption stages that just hammer voltage up without caring about balance at all.

@ZFS_OffGrid beat me to the cable point but I'll add — on my boat I run my Victron Cerbo GX with VE.Smart Networking between the batteries so they're actually talking to each other rather than just guessing, makes the whole sync problem largely disappear.