Sodium-Ion Pack Math: Voltage Window, Series Count and BMS Settings

Sodium-ion cells swing over a wider voltage window than LFP or lithium-ion. They charge to 3.7 to 4.0 V, close to lithium-ion, and drain all the way down to 1.5 V. Same cell shape, same nickel strip welding, different voltage plan.

This guide works for both sodium chemistries we stock: NFPP and layered oxide. Swap in the values from the datasheet for your batch and the math still works.

Sodium vs LFP and lithium-ion, empty to full

• The top is familiar. Layered oxide charges to about 4.0 V and NFPP to about 3.7 V, between LFP (3.65 V) and lithium-ion NMC (4.2 V).
• The bottom is not. Both sodium cells are rated down to 1.5 V. LFP and NMC stop around 2.5 V. A sodium pack swings over about 2.5 V per cell, against about 1.2 V for LFP.
• The curve slopes. Sodium voltage falls steadily as the cell empties, so voltage tells you roughly how full it is. LFP sits flat for most of its charge.

Two problems the wide window creates

Fit the pack to the inverter

LFP and NMC packs were sized to fit inside a 48 V inverter. Sodium packs are not. Check your inverter's battery voltage range first, then work backwards:

1. Top sets the series count. Max series = inverter max voltage ÷ cell charge voltage. With a 60 V limit: layered oxide 60 ÷ 4.0 = 15S. NFPP 60 ÷ 3.7 = 16.2, so 16S.
2. Bottom sets how much you can use. Per-cell cut-off = inverter min voltage ÷ series count. With a 40 V floor: layered oxide 15S stops at 2.67 V per cell, NFPP 16S at 2.5 V per cell.
3. Know what that costs. In our bench test, about 15 to 20 % of a sodium cell's capacity sits below 2.5 V. Count on usable energy, not the label, when you size the pack.

Size for the current at your lowest voltage

A 3,000 W load on a 48 V class pack:

• Max current = load watts ÷ lowest pack voltage you will run to.
• Parallel count for current = max current ÷ per-cell continuous rating from the datasheet. Stay well under the rating for cooler cells and longer life.
• Parallel count for energy = usable watt-hours you need ÷ (series count × cell nominal V × cell Ah). Use whichever P is larger.

Four factors

Four factors cover every pack.

• Pack nominal voltage = series count (S) × cell nominal voltage
• Pack charge voltage = S × cell charge voltage
• Pack capacity (Ah) = parallel count (P) × cell capacity (Ah)
• Pack energy (Wh) = pack nominal voltage × pack capacity

Common packs with our NFPP and layered oxide cells

Both cells: 3.0 V nominal, 1.5 V absolute floor, 2.0 V working cut-off. Charge voltage differs: layered oxide 4.0 V, NFPP 3.7 V.

On a 48 V inverter, the working cut-off is usually the inverter's own low limit, not the cell's. Layered oxide at 16S charges to 64 V, above most 48 V inverters: use 15S, or charge lower and lose some top capacity.

Where to set the bottom

The datasheet floor is the lowest voltage the cell is rated for. It is a limit, not a working setpoint. In our bench test of sodium 18650s, about 15 to 20 % of the capacity sat below 2.5 V per cell, but only 3 to 5 % sat below 2.0 V.

• 2.0 to 2.2 V per cell keeps nearly all the capacity with margin above the floor. Best for DC loads that run that low.
• 2.5 V per cell or higher is what most 48 V inverters force on a 16S pack. It gives up about a fifth of the capacity.

Settings that change for sodium

Sodium-ion is still changing

Sodium-ion is cutting-edge, and the technology is still changing, voltage ranges included. Across three batches of our layered oxide 18650, the charge voltage moved between 3.95 V and 4.10 V and the floor between 1.5 V and 1.8 V.

• Use the datasheet for your batch, not this page and not last year's order.
• Don't mix batches in one series string. The smallest cell sets the capacity of the whole string.
• Buy every cell for a pack in one order, spares included.
• Re-check your settings when you reorder. 0.05 V per cell is 0.8 V across a 16S pack.