Swapped to lithium on my 28ft Moody – battery monitor keeps reading wrong

Seen this exact issue crop up repeatedly when people swap chemistry without reconfiguring the monitor. The BMV-712 (or whatever you're running) will have been calibrated for lead-acid charge/discharge curves — lithium has a completely flat voltage plateau through most of its SoC range, so voltage-based estimation is essentially useless. You must be running coulomb counting, and even then the Peukert exponent needs resetting.

Specific things to check:

• Peukert exponent — set to 1.05 for LiFePO4, not the 1.25-ish you'd use for AGM
• Charged voltage threshold — needs to match your BMS termination voltage exactly
• Tail current setting — lithium absorbs very little current at full charge, so this wants to be around 1-2% of capacity
• Synchronisation — have you manually synced it to 100% after a full charge since the swap?

My shepherd's hut setup had similar nonsense readings when I first fitted a Fogstar Drift 100Ah — the monitor was convinced it was perpetually at 73% regardless of what I did. Turned out the charged voltage threshold was set 0.3V below where the BMS was actually terminating, so it never registered a full charge and never reset the coulomb count.

Also worth checking whether your shunt is properly sized for lithium's higher potential discharge rates. Lead-acid users often run undersized shunts because the batteries self-limit. Lithium won't.

What monitor are you running and what BMS is in the new battery? That'll determine exactly which parameters need touching.

TonyRoss | Posts: 847

@SolarNotSure spot on about the reconfiguration. One thing worth adding – with LiFePO4 you'll want to set your Peukert exponent to 1.05 (or as close to 1.0 as your monitor allows), as lithium discharges far more linearly than lead-acid. The default Peukert figure on most monitors is set around 1.25 for AGM/flooded, which will absolutely throw your state-of-charge readings off, especially under heavier loads.

Also worth checking your charged voltage threshold and tail current settings – lithium has a much flatter discharge curve so the monitor needs different reference points to recognise "full." Give it a full charge cycle after reconfiguring and let it sync properly before trusting the readings. What battery monitor are you actually running? Some older units have limited adjustment range which can make accurate lithium monitoring a bit of a faff.

Boycie25 | Posts: 1,203

@TonyRoss @SolarNotSure — both right, but nobody's mentioned the Peukert exponent yet. On a BMV-712 that's been sitting on lead-acid settings, your Peukert will likely be around 1.25. Drop it to 1.05 for LiFePO4 — lithium barely exhibits the effect. Charged time correction factor needs adjusting too.

Also check your tail current setting. Lead-acid typically 4%, lithium wants to be closer to 1-2% or your monitor thinks it's never full and the SOC drifts into fantasy territory.

Done this twice on my narrowboat and once on a cabin backup system. Both times the monitor was reading 15-20% low after a week because nobody touched these parameters. Victron's own documentation buries this in a footnote somewhere, naturally. Classic.

Wez1961 | Posts: 2,341

@Boycie25 good shout on Peukert — lithium is basically 1.00 to 1.05, lead-acid sits around 1.25. If your BMV still has the old value in there it'll be wildly miscalculating capacity under load.

Also worth checking your charged voltage and tail current settings. LiFePO4 has a flat discharge curve so the monitor relies heavily on those thresholds to know when it's genuinely full and can resync to 100%.

On my setup I've got:

• Charged voltage: 14.2V
• Tail current: 4%
• Peukert: 1.05

Took me an embarrassing amount of time to sort this out when I swapped to Fogstar cells. Monitor was showing 40% when I was nearly empty. Not ideal.

Victron's own docs cover it but they're not exactly light reading.

GrumpyDrifter | Posts: 89

Does anyone know if the same reconfiguration applies to cheaper monitors — I've got a Renogy one on my static caravan setup and I'm wondering if it even has a Peukert setting to adjust? Looked through the manual and couldn't find it mentioned anywhere.

Also slightly off-topic but relevant — what's the correct charged voltage threshold to set for LiFePO4? On lead-acid I had mine set to 12.7V but I've read lithium sits at a different resting voltage and presumably that affects when the monitor decides the bank is "full" and resets the SoC counter?