Why can't lead-acid be discharged to 100 percent?

Deep discharge accelerates sulfation and sheds active material, so a lead-acid bank rated for perhaps 1200 cycles at 50 percent DoD may last only 300 or fewer at 80 percent. The datasheet cycle curves are quoted at a fixed DoD for that reason. Lithium iron phosphate tolerates 80 to 90 percent daily DoD, which is why an LFP bank of the same usable energy is roughly half the nameplate size.

Why does cold weather shrink the capacity?

Electrochemical kinetics slow down as temperature falls. A lithium battery at minus 10 degrees Celsius delivers roughly 80 percent of its rated capacity, lead-acid about 80 percent too but with less usable energy at high current. Below 0 degrees Celsius LFP must not be charged at all without a heater, because charging plates lithium metal onto the anode permanently.

What is C-rate and why does it matter?

C-rate is discharge current divided by capacity. Drawing 50 A from a 100 Ah bank is 0.5 C. Lead-acid in storage duty is comfortable at 0.2 C or less — the Peukert effect means a heavily loaded lead-acid bank delivers far less than its label. LFP handles 1 C continuously, but sustained high rates add heat and shorten cycle life.

Battery Bank Sizer

Q: What does end-of-life headroom mean?

Batteries fade with age. A bank specified at 80 percent state of health will still meet your autonomy on the day you retire it. Sizing without that headroom means the system silently loses an autonomy day every couple of years until the loads start dropping out.

Turn a daily energy figure into an actual battery bank: nameplate kWh, module counts, and a discharge-rate check. The derating chain — depth of discharge, temperature, end-of-life fade — is shown line by line so you can defend the number. Runs entirely in your browser.

What this calculator does

It converts "my site uses 10 kWh a day and I want to ride through one dark day" into a bank you can actually order: nameplate kWh, Ah at your DC voltage, and how many modules in series and parallel. It also refuses to flatter you — temperature, cycle life and ageing are subtracted before the answer is given.

The calculation

E_dc = E_load ÷ η_inv AC load referred to the DC bus
E_cell = E_dc ÷ √η_rte discharge-side loss
C_use = E_cell × N ÷ DoD autonomy at the allowed depth
C_temp = C_use ÷ f_T cold-weather derating
C_eol = C_temp ÷ SOH_EOL nameplate so it still works when old
C-rate = P_max ÷ (V × C_eol) checked against chemistry limit

The round-trip efficiency is split into a discharge-side half (√η_rte) because the charge-side loss is already paid upstream by the PV array. Presets: LFP 0.95, NMC 0.94, AGM 0.85, OPzV 0.86.

The three deratings people skip

Depth of discharge. Lead-acid cycle life roughly halves for every extra 10–15 % of daily DoD; an AGM bank good for 1200 cycles at 50 % DoD may give 300–400 at 80 %. LFP tolerates 80–90 % daily, which is why the same usable energy needs half the nameplate.
Temperature. Capacity is quoted at 25 °C. At −10 °C expect ~80 % from lithium and lead-acid alike; below 0 °C an LFP bank must not be charged without a heater — the calculator flags this case explicitly.
End of life. Sizing at 100 % SOH means the system loses its last autonomy day silently, a couple of years in, on the coldest night of the year. 80 % is the usual floor.

Worked example

An off-grid cabin: 10 kWh/day AC load, 1 day autonomy, 48 V LFP bank, 0 °C cellar, 5 kW peak inverter, 51.2 V / 100 Ah rack modules.

E_dc = 10 ÷ 0.95 = 10.53 kWh
E_cell = 10.53 ÷ √0.95 = 10.80 kWh
C_use = 10.80 × 1 ÷ 0.90 = 12.0 kWh
C_temp = 12.0 ÷ 0.90 (0 °C) = 13.3 kWh
C_eol = 13.3 ÷ 0.80 = 16.7 kWh nameplate
Layout: 1s × 4p of 5.12 kWh = 20.5 kWh, 400 Ah at 51.2 V
C-rate = 5000 W ÷ (51.2 V × 400 Ah) = 0.24 C ✓

Notice the installed 20.5 kWh delivers only 10 kWh on the design night — the other half is DoD cap, cold and end-of-life margin. That is normal, and it is the number a datasheet will not volunteer.

What this tool does not do. It sizes energy, not power electronics: wire, fuse, busbar and breaker sizing, short-circuit ratings, string fusing and BMS communication compatibility are separate design steps. Grid-code and interconnection requirements are yours to confirm.

Frequently asked

Why can't lead-acid go to 100 % DoD?

Deep discharge accelerates sulfation and active-material shedding. Cycle curves are quoted at a fixed DoD for a reason: 1200 cycles at 50 % can become 300 at 80 %. The calculator caps the preset at 50 % but lets you override it — at the cost of cycle life.

What does end-of-life headroom mean?

Sizing at 80 % SOH means the bank still meets autonomy on the day you retire it. Without it, capacity fades silently until loads start dropping out.

Why does cold shrink capacity?

Electrochemical kinetics slow down. At −10 °C expect roughly 80 % of rated capacity. Charging LFP below 0 °C plates lithium onto the anode — permanent damage — so charge lockout or a heater is required.

What is C-rate?

Discharge current divided by capacity: 50 A from 100 Ah is 0.5 C. Lead-acid in storage duty is happy at ≤0.2 C (the Peukert effect eats real capacity above that); LFP handles 1 C continuous but runs hotter and ages faster near the limit.

Should I round modules up or down?

Up, always. The calculator returns whole modules and shows the installed kWh, which will exceed the required figure. A bank is never "close enough" one module short.