New lifepo4 batteries very slow top balance

Been through something similar with my cabin setup and it's worth sharing here.

When I first wired up my 4 × 280Ah cells in parallel (Fogstar Drift units, so slightly different beast), the top balance took ages — we're talking the better part of three days before the Victron SmartShunt was showing anything resembling a sensible state of charge.

The thing people don't always mention is that with multiple batteries each running their own BMS, they're essentially negotiating with each other at first. One pack reaches its high-voltage cutoff, backs off, the others catch up slowly. It looks broken but it usually isn't.

A few things that helped me crack it:

• Dropped charge current right down — counterintuitive, but slower at the top lets the cells breathe and balance properly
• Kept absorption voltage conservative — I was running 3.45V/cell rather than pushing to 3.65V
• Watched cell delta in the BMS app — once all packs were within about 20mV of each other, things settled dramatically

The Cerbo makes it easier to see what's happening across the system, which is a blessing. Before that I was squinting at individual BMS Bluetooth apps simultaneously like some kind of solar-powered circus act.

Curious whether others with Chinese BMS units (JK, Daly, that sort of thing) have found the inter-pack communication a factor, or whether it really does just come down to patience and letting the chemistry sort itself out?

Would love to know how others have handled this — especially anyone running more than four packs in parallel.

@WattHelen your post got cut off mid-sentence, looks like it didn't submit properly — worth editing it so we can see the full thing.

Had a similar slow top balance situation on my narrowboat build. What helped was just leaving the charger sitting at absorption voltage (3.45V/cell in my case) for a longer soak period than I expected — took a good few hours before the cells started pulling even. Patience is really the main tool there.

What BMS are you running? Sometimes the balancing current is just tiny (like 30–50mA on cheaper units) and it physically can't keep up if cell drift is significant. Worth checking the spec sheet.

@WattHelen yes please finish that thought — you've left us all hanging like a half-charged cell! 😄

On the actual topic though, I had something similar when I first set up my narrowboat emergency backup bank. Cells that had been sitting in a warehouse for months were all over the place voltage-wise before top balancing. Took ages to creep up to 3.65V on the stragglers.

One thing that helped massively was doing an initial charge at a much lower current than I thought necessary — basically letting them soak rather than rushing it. Are yours being charged at a sensible absorption current or are you hammering them?

Also worth checking whether your BMS is prematurely cutting off before the cells actually finish — mine did exactly that and I spent two days thinking something was wrong with the cells themselves. Proper embarrassing in hindsight.

@WattHelen yeah your post is cut off but I can guess where you're going with this — new cells often sit at slightly different resting voltages out of the box, even from the same batch. Wiring them in parallel before top balancing properly can mean the BMS just sees the weakest cell and throttles the charge rate right down.

What worked for me with my garden office setup was doing a proper bench top balance first — cells individually charged to 3.65V, held there until current dropped to near zero, then paralleled up. Took a couple of days but the bank has behaved itself ever since.

What BMS are you running? That can make a big difference to how aggressively it'll let the charger push current during balancing.

@WattHelen nothing like a cliffhanger to keep the forum engaged — your post has more suspense than my Victron Cerbo GX dashboard at 3am wondering why one cell keeps drifting.

While we wait for the full story, I'll add from my motorhome setup: patience is genuinely the key with top balancing fresh LiFePO4 cells — mine took a couple of full charge cycles before the BMS stopped throwing fits. New cells can have surprisingly uneven self-discharge rates straight from the factory, so don't panic if one sits stubbornly low for a while.

@WattHelen while we wait for the rest of your post to materialise from wherever the internet ate it — can I ask a practical question that might be relevant here anyway?

With my garden office build I've got 4 × 280Ah cells (different brand, not Fogstar) and I'm doing a bench top balance before even connecting the BMS. Using a bench power supply capped at 3.65V per cell, but it's taking ages — we're talking days, not hours.

Is that normal for large-format prismatic cells that have been sitting in a warehouse? I'm wondering whether:

• Initial self-discharge during storage causes cells to diverge more than expected
• The low finishing current (around 200mA) just inherently takes forever at this capacity
• I'm doing something fundamentally wrong 🤔

Anyone got ballpark timings for a proper top balance on fresh 280Ah cells?