Battery & Power
Build a series-parallel battery bank
Enter one battery’s rating and the planned layout to calculate the complete bank.
Calculation breakdown
Layout preview
Planning calculation only. Verify battery matching, BMS limits, fusing, cable size, charger voltage and manufacturer connection limits before installation.
How to Use the Battery Series Parallel Calculator
Enter the voltage and amp-hour rating of one battery, followed by the number of batteries in each series string and the number of identical strings connected in parallel.
The calculator returns the configuration label, total battery count, bank voltage, Ah capacity, nominal energy, planned usable energy, and an optional maximum-current estimate.
Use identical battery specifications throughout one calculation. The mathematical result assumes each battery has the same voltage, capacity, and continuous-current rating.
Information You Will Need
Voltage per Battery
Enter the nominal voltage printed on one battery, such as 6V, 12V, 12.8V, 24V, or 25.6V.
Use nominal voltage rather than a temporary meter reading. A fully charged 12V-class battery may measure above 12V, but its bank configuration is still normally described using its nominal rating.
Capacity per Battery
Enter the rated amp-hour capacity of one battery.
Do not add the capacities yourself before entering the value. The calculator applies the number of parallel strings to the single-battery Ah rating.
Batteries in Each Series String
Enter the number of batteries connected end-to-end in one string.
Increasing the series count increases the complete bank voltage.
Parallel Strings
Enter the number of identical completed series strings connected in parallel.
Increasing the number of parallel strings increases the bank’s Ah capacity.
Planned Usable Capacity
Enter the percentage of nominal battery energy you plan to make available before recharging or reaching the system cut-off.
This produces a battery-side energy allowance. It does not by itself calculate runtime or account for inverter losses.
Maximum Continuous Current per Battery
This optional value should come from the specification for the exact battery model.
The calculator uses it to estimate a theoretical bank-current limit based on the number of parallel strings.
Reading a Battery Configuration Label
Battery-bank arrangements are commonly written in the form:
xSyP
where:
- S = batteries in each series string;
- P = number of identical parallel strings.
| Label | Arrangement | Total batteries |
|---|---|---|
| 1S1P | One battery | 1 |
| 2S1P | Two batteries in one series string | 2 |
| 1S2P | Two single-battery strings in parallel | 2 |
| 2S2P | Two batteries per string and two parallel strings | 4 |
| 4S3P | Four batteries per string and three parallel strings | 12 |
Total battery count is calculated by multiplying the two counts:
Total batteries = Series count × Parallel strings
A 4S3P bank therefore requires:
4 × 3 = 12 batteries
What Series and Parallel Connections Change
The complete bank calculation follows four basic relationships:
| Result | Calculation |
|---|---|
| Total batteries | Series count × Parallel strings |
| Bank voltage | Voltage per battery × Series count |
| Bank capacity | Ah per battery × Parallel strings |
| Nominal energy | Bank voltage × Bank capacity |
Series connections increase voltage.
Parallel strings increase amp-hour capacity.
Both affect the total battery quantity and therefore the bank’s total stored energy.
A useful cross-check is:
Bank voltage × Bank Ah = One-battery voltage × One-battery Ah × Total batteries
Both sides should produce the same nominal watt-hours.
Worked Example: A 4S3P LiFePO4 Bank
Suppose twelve identical 12.8V 100Ah batteries are arranged as 4S3P.
Total Battery Count
4 series batteries × 3 parallel strings = 12 batteries
Bank Voltage
12.8V × 4 = 51.2V
Bank Capacity
100Ah × 3 = 300Ah
Nominal Energy
51.2V × 300Ah = 15,360Wh
The resulting bank has a nominal rating of:
51.2V, 300Ah, or 15.36kWh
The energy cross-check gives the same answer:
12.8V × 100Ah × 12 = 15,360Wh
If planned usable capacity is set to 80%:
15.36kWh × 0.80 = 12.288kWh
The calculator will therefore display approximately:
12.29kWh planned usable energy
This confirms the arithmetic of the entered configuration. It does not confirm that the exact battery model permits four batteries in series and three strings in parallel.
Why Series and Parallel Can Produce the Same Total Energy
Consider two identical 12V 100Ah batteries.
They can be arranged as:
- 2S1P: 24V and 100Ah
- 1S2P: 12V and 200Ah
The first configuration contains:
24V × 100Ah = 2,400Wh
The second contains:
12V × 200Ah = 2,400Wh
Both arrangements use the same two batteries, so the total nominal stored energy remains the same.
What changes is how that energy is expressed through voltage and Ah capacity.
The correct layout should therefore be chosen according to the required DC system voltage and the supported battery and equipment configuration—not according to which arrangement displays the larger Ah number.
A 24V-class inverter requires a compatible battery-bank voltage. Connecting the same batteries in a 12V parallel layout simply to obtain a larger Ah figure does not make that bank suitable for a 24V inverter.
Understanding Planned Usable Energy
Nominal watt-hours describe the bank’s rated energy before applying the selected usable percentage.
The calculator uses:
Planned usable energy = Nominal energy × Usable percentage
For example, a 10kWh nominal bank entered at 80% gives:
10kWh × 0.80 = 8kWh
This value remains a battery-side energy estimate.
It does not automatically include:
- inverter efficiency;
- cable losses;
- discharge-rate effects;
- temperature effects;
- battery ageing;
- standby consumption;
- equipment losses.
If you already know the bank configuration and want to estimate how long it may support a UPS load, use the UPS Battery Backup Time Calculator.
For an inverter-based system, the Inverter Battery Backup Time Calculator handles the runtime question separately.
If the goal is to estimate how long the configured bank may take to recharge, use the Battery Charging Time Calculator.
Keeping these as separate calculations prevents battery-bank configuration, runtime, and charging time from being mixed into one result.
Understanding the Maximum-Current Result
Series and parallel connections also affect the optional current estimate.
One series string retains the entered continuous-current rating of one battery.
Parallel strings can increase the theoretical total:
Theoretical bank current = Continuous current per battery × Parallel strings
Suppose each battery is rated for 100A continuous and the bank contains three parallel strings:
100A × 3 = 300A
The series count is not included in this multiplication.
Four 100A-rated batteries connected only in series still form a 100A-rated string under this simplified calculation, not a 400A string.
The theoretical bank-current result assumes that parallel strings share current evenly.
Actual allowable system current may be lower because of:
- BMS limits;
- inverter limits;
- charger limits;
- fuse and disconnect ratings;
- cable capacity;
- busbar capacity;
- terminals and connectors;
- unequal current sharing.
The practical system limit is determined by the lowest applicable component or manufacturer restriction.
If you want to compare a current value with the bank’s Ah capacity, the Battery C-Rate Calculator handles that relationship directly.
Why Parallel Strings Need Balanced Current Paths
Identical parallel strings do not automatically guarantee identical current sharing.
Small differences in electrical resistance can make one path carry more current than another.
A balanced installation may therefore require attention to:
- cable type and cross-sectional area;
- positive and negative path lengths;
- lugs and connection hardware;
- terminal condition and torque;
- busbar arrangement;
- clean and secure connections;
- string-level protection where required;
- placement of the main system connections.
The Victron Energy battery-bank wiring guidance illustrates why equal current paths matter in parallel banks and shows recommended connection arrangements.
The calculator does not model individual cable or connection resistance, so it cannot predict how current will divide between real parallel strings.
Why Batteries in One Bank Should Match
The calculator assumes identical batteries throughout the bank.
In a real installation, batteries connected together should normally be compatible in characteristics such as:
- chemistry;
- nominal voltage;
- Ah capacity;
- model;
- charging requirements;
- current limits;
- age and service history;
- state of charge before connection.
In a series string, the same current flows through every battery. A weaker battery can therefore reach an operating limit before the rest of the string.
Parallel-connected batteries can also exchange substantial equalization current if they are connected at materially different voltages or states of charge.
Follow the preparation, balancing, and connection procedure specified for the actual battery system.
Matching nominal voltage alone is not enough to establish complete compatibility.
What the Layout Preview Shows
For smaller configurations, the calculator displays a simplified battery arrangement.
Each row represents one parallel string.
The battery blocks in that row represent batteries connected in series.
For a 3S2P arrangement:
- the preview contains two rows;
- each row contains three batteries;
- each row represents one complete series string;
- the two completed strings are treated as parallel paths.
The preview is a configuration aid, not an electrical wiring diagram.
It does not show:
- positive and negative terminal orientation;
- cable routing;
- fuses or circuit breakers;
- busbars;
- disconnects;
- BMS communication wiring;
- midpoint monitoring;
- grounding or bonding;
- physical spacing;
- ventilation.
Large configurations may omit the complete visual preview to keep the result readable on smaller screens.
How to Read the Results
Configuration
The abbreviated arrangement, such as:
2S2P or 4S3P
Total Batteries
The number of identical batteries needed to build every complete string.
Bank Voltage
The nominal voltage of one complete series string and therefore the voltage of the parallel bank.
Bank Capacity
The combined Ah capacity created by the parallel strings.
Nominal Energy
The rated watt-hours or kilowatt-hours before applying the planned usable percentage.
Planned Usable Energy
The selected fraction of nominal battery energy.
It should not be interpreted as guaranteed usable AC output or runtime.
Energy per String
The nominal energy stored in one complete series string.
Multiplying this value by the number of parallel strings should reproduce the total nominal bank energy.
Maximum Continuous Bank Current
An optional theoretical current estimate based on the entered single-battery continuous-current rating and the number of parallel strings.
Calculation Checks That Catch Common Input Errors
Before treating the result as a proposed configuration, verify the following.
1. Battery Count
Series count × Parallel strings
must equal the number of batteries required.
2. Voltage
Battery voltage × Series count
should match the intended nominal DC system voltage.
3. Energy
Bank voltage × Bank Ah
should equal:
One-battery voltage × One-battery Ah × Total battery count
4. String Consistency
Every parallel string should contain the same intended number and compatible type of batteries.
5. Product Limits
Confirm that the exact battery and BMS support the proposed series and parallel counts.
6. Equipment Compatibility
The charger, inverter, battery monitor, protection equipment, and DC loads must support the resulting bank voltage and operating limits.
These checks separate the arithmetic of a configuration from approval of the physical installation.
Common Configuration Errors
Adding Series and Parallel Counts
A 4S3P bank contains:
4 × 3 = 12 batteries
not seven.
The values represent rows and positions within the bank rather than two quantities to add together.
Multiplying Ah by the Series Count
Series connections increase voltage.
A series string made from four 100Ah batteries remains a 100Ah string.
Multiplying Voltage by the Parallel Count
Parallel strings increase Ah capacity while retaining the voltage of one complete series string.
Treating Watt-Hours as Watts
Watt-hours (Wh) measure stored energy.
Watts (W) measure power.
A 10kWh battery-bank result does not state how much instantaneous power the battery, BMS, cables, or inverter can safely deliver.
Multiplying Current by Every Battery
Only the number of parallel strings is used in this calculator’s theoretical current total.
Batteries connected in series do not add current capability in the same way.
Assuming Correct Arithmetic Approves the Hardware
A configuration can be mathematically valid while exceeding:
- permitted series count;
- permitted parallel count;
- BMS limits;
- current limits;
- equipment voltage limits;
- protection requirements.
The calculator confirms electrical totals from the entered values. It does not certify the proposed hardware arrangement.
Practical Configuration Questions
Why Does a Four-Battery 12.8V Series Bank Show 51.2V Instead of 48V?
“48V” is commonly used as a nominal system class.
Four batteries with individual nominal ratings of 12.8V produce:
12.8 × 4 = 51.2V
Actual charging and operating voltage ranges depend on the battery chemistry and product specification.
Connected equipment must support the battery manufacturer’s real operating range, not just the informal system-class label.
Can Different Ah Batteries Be Connected in the Same Series String?
The arithmetic can be calculated, but the simple bank result would not represent a well-matched series string.
The same current passes through every battery in series, so a lower-capacity or weaker unit may reach an upper or lower operating limit first.
Use combinations specifically permitted by the relevant battery manufacturers.
Can the Calculator Work Backward From a Target Voltage and Energy?
This calculator is designed primarily to evaluate a series/parallel layout entered by the user.
For a basic reverse estimate, the required series count can be approached from:
Target bank voltage ÷ One-battery nominal voltage
and the parallel requirement can then be considered from the energy available in one complete series string.
However, a practical bank sized around a required load and backup duration must also account for usable capacity and conversion losses.
For that task, the Inverter Battery Capacity Calculator provides the more appropriate starting point.
Limits of the Calculation
This calculator determines the electrical totals associated with the entered series/parallel arrangement.
It does not select or verify:
- battery chemistry or model;
- manufacturer connection limits;
- BMS compatibility;
- charger settings;
- inverter power rating;
- cable or busbar size;
- fuse or circuit-breaker ratings;
- fault-current protection;
- disconnect requirements;
- grounding or bonding;
- enclosure design;
- ventilation;
- electrical-code compliance.
Battery banks can deliver high fault current even when the nominal system voltage appears relatively low.
For high-energy, safety-critical, or permanent installations, verify the proposed configuration against the specifications for the exact battery, BMS, charger, inverter, wiring, and protection equipment before construction.