Battery Series Parallel Calculator | Voltage, Ah & Wh

Allocate battery stock and protect spare batteries using this page’s inputs.

Check a layout or allocate the batteries you already have

The original calculator checks a chosen series/parallel layout and draws its topology. The stock allocator instead finds how many complete matching strings can be formed at an exact target nominal voltage after protecting spare batteries. For the reverse question of how many batteries a load and runtime require, use the Inverter Battery Capacity Calculator.

Keep voltage, Ah and energy separate

For matching batteries, bank voltage = one battery’s nominal V × batteries per series string. Bank Ah = one battery’s Ah × parallel strings. Total battery count = series × parallel. Nominal Wh = bank V × bank Ah. Planned usable Wh applies your entered usable percentage to nominal energy; it is not a measured discharge capacity or an inverter-efficiency adjustment.

Four 12.8 V, 100 Ah batteries in each string and three matching strings give 4S3P: 12 batteries, 51.2 V, 300 Ah and 15,360 Wh. At 80% planned usable capacity, that is 12,288 Wh. Series changes voltage without multiplying Ah. Parallel changes Ah without multiplying voltage.

The optional continuous-current rating in the original form belongs to one matching battery. The ideal bank current sum multiplies that rating by parallel strings, not by the total number of batteries. Shared BMS, cable, connector and protection limits can be lower. Equal sharing is an assumption, not a measurement.

Allocate stock without incomplete strings

In the stock tool, available units = matching stock − protected spares. Required series count = exact target nominal voltage ÷ nominal voltage per battery. This must be a whole number. The tool does not round 48 ÷ 12.8 = 3.75 into an automatically compatible bank. A nominal 51.2 V target instead requires exactly four 12.8 V modules; equipment operating-voltage limits must still be checked separately.

Complete strings available from stock = floor(available units ÷ required series count). An optional known manufacturer maximum parallel count further caps the allocation. Used batteries = series count × allocated strings, and unconnected batteries = total stock − used. Protected spares stay within the unconnected count, alongside any remainder too small to complete another string.

For nine matching 12 V, 100 Ah batteries, one protected spare and an exact 24 V target, eight units are available. Two units per series string produce four complete strings: 2S4P, 400 Ah, 9,600 nominal Wh and one spare left. If the entered manufacturer limit is three parallel strings, only six units are allocated, leaving three unconnected. A 50% planned usable setting gives 3,600 usable Wh for that capped six-unit allocation.

Separate arithmetic from permitted hardware arrangements

A known series-count limit is checked independently. If the required series count exceeds it, the output identifies the failure even though it can display the arithmetic arrangement. Blank manufacturer limits remain unknown. If fewer available units exist than one full string needs, the worksheet reports the missing count and zero allocated energy. It never creates an incomplete string.

Only matching voltage, capacity and compatible battery types belong in this model. Manufacturer restrictions, battery condition, current balance, voltage windows and installation details can make a numerically neat arrangement unsuitable. The diagram is topology, not a terminal, cable or fuse drawing. The stock tool does not combine leftover units into another voltage class or assume that every battery can be placed in series.

Inputs, precision and related calculations

The stock allocator uses per-battery voltage, Ah, usable percentage and optional current from the original form. Its stock, spares, exact target and manufacturer count limits are separate; the original series/parallel fields are not overwritten or reapplied. Use whole stock/count values up to 1,000,000, with protected spares no greater than stock. Target voltage is positive up to 1,000,000 V; source voltage is positive up to 1,000 V and capacity up to 10,000 Ah. These software bounds do not describe equipment ratings.

Plain decimals up to 18 places are checked exactly for series divisibility before displayed values round to six places. Edits clear the old allocation. Calculations do not save an inventory or verify physical batteries. Use Battery C Rate for distinct whole-bank and per-string current limits, or Inverter Battery Backup Time for a configured bank and load.