Marine Battery Bank Calculator – Cable, C-Rate & Protection Sizing
Check & Size Your Marine Battery Bank
Do you know if every cable in your battery bank is correctly sized? This calculator helps you check your battery configuration, available power, main battery cables and the often-overlooked cables connecting the batteries themselves.
More Than a Battery Capacity Calculator
When building a marine battery bank, it is easy to focus only on voltage, amp-hours and the large cable going to the motor or main DC distribution.
But a battery bank can contain several different high-current cable sections, and they do not necessarily carry the same current.
A large main battery cable does not mean the rest of your battery bank is correctly wired.
The Baywatt battery bank calculator checks the complete architecture: the battery configuration, the current available from each battery string, the main battery cables and the cables connecting batteries and parallel strings.
What Can You Check?
Enter the voltage and capacity of one battery, its continuous discharge C-rate, and the number of batteries connected in series and parallel.
The calculator then estimates:
- Total battery bank voltage
- Total capacity in Ah
- Total stored energy in kWh
- Maximum continuous current per battery string
- Maximum continuous current available from the complete battery bank
- Main battery cable section
- Series cable section between batteries
- Parallel string cable section
- Preliminary breaker or fuse rating
- Battery disconnect switch rating
- Busbar rating
- Battery monitor shunt rating
Series and Parallel Batteries Are Not the Same
Batteries in Series
Connecting batteries in series increases the system voltage while the Ah capacity of the string remains the same.
For example, four 12 V 100 Ah batteries connected in series create:
- 48 V system voltage
- 100 Ah capacity
- 4.8 kWh nominal energy
The cables connecting those batteries together carry the current of that battery string.
Batteries in Parallel
Parallel battery strings increase the total Ah capacity and the total current potentially available from the battery bank.
Three 48 V 100 Ah strings connected in parallel create a 48 V 300 Ah battery bank.
Each string still carries only its own current, while the main battery cable can carry the combined current of all three strings.
The Cables Between Batteries Matter
This is one of the most commonly overlooked parts of a high-current battery installation.
A battery bank can have three different cable requirements:
- Main battery cables: carry the combined current between the battery bank, busbar, protection device and load.
- Series battery jumpers: connect batteries within one series string and carry the current of that string.
- Parallel string cables: connect each battery string to the common positive and negative busbars.
These cables should not automatically be assumed to require the same section.
For example, if three battery strings can each provide 100 A continuously, each string cable may carry approximately 100 A while the main battery cable may need to carry as much as 300 A.
Knowing Your Battery C-Rate Is Essential
p> That is exactly what the battery C-rate tells you.Without knowing the C-rate, you cannot properly determine:
- How much continuous current the battery can safely deliver
- How much current the battery bank can supply to a motor or inverter
- How fast the battery can be charged
- Whether the BMS can support the expected load
- How many parallel battery strings are required
For example, a 100 Ah battery rated at:
- 0.5C can deliver approximately 50 A
- 1C can deliver approximately 100 A
- 2C can deliver approximately 200 A
The same principle applies to charging. A battery that accepts 0.5C charge current can theoretically accept around 50 A of charge current if it has a 100 Ah capacity, while a 1C battery could accept around 100 A, provided the manufacturer and BMS allow it.
This is why entering the correct manufacturer C-rate is one of the most important parts of sizing a battery bank correctly.
Cable Size Depends on More Than Current
Cable section is influenced by both current and cable length.
A cable that is acceptable over 30 cm may not be appropriate over several metres because voltage drop increases with conductor length.
The calculator therefore asks for:
- Main cable one-way length
- Inter-battery or branch cable length
- Maximum acceptable voltage drop
For the main battery cable, enter the physical one-way distance. The calculator automatically includes both the positive and negative current paths.
Why Higher Voltage Reduces Current
High-power DC systems can require very large currents when operated at low voltage.
As a simple electrical comparison, a 6 kW load represents approximately:
- 500 A at 12 V
- 250 A at 24 V
- 125 A at 48 V
This is one reason why higher-voltage battery systems are commonly used for higher-power electric propulsion.
Protection and Distribution
Once the battery bank current is known, the calculator also provides preliminary sizing guidance for the main electrical hardware.
This includes the main breaker or fuse, battery disconnect switch, busbars, cable lugs and battery monitor shunt.
Protection sizing should always respect the relationship:
Normal Load Current ≤ Protection Rating ≤ Allowable Cable Current
The final protection device must also be compatible with the battery, BMS, motor controller, conductor installation and equipment manufacturer requirements.
Example: 48 V Battery Bank with Three Parallel Strings
Consider twelve 12 V 100 Ah batteries arranged as four batteries in series and three strings in parallel: 4S3P.
The resulting battery bank is:
- 48 V
- 300 Ah
- 14.4 kWh nominal energy
If each 100 Ah battery is rated for 1C continuous discharge, each series string can provide approximately 100 A.
With three strings in parallel, the complete battery bank can theoretically provide approximately 300 A continuously.
This means the main battery cables may need to carry 300 A, while the cables within each battery string carry approximately 100 A.
This difference is exactly why checking the complete battery bank wiring is important.
Check Your Battery Bank
Use the calculator above to check your battery configuration, current capability and the required cable sections throughout the battery bank.
It can be used for electric propulsion systems, marine house banks, inverters and other high-current DC installations.
Important
This calculator is intended as a preliminary marine electrical sizing tool. Final cable ampacity and protection ratings depend on conductor insulation, installation method, ambient temperature, cable bundling, battery and BMS limitations, equipment requirements and applicable marine electrical standards.
Always verify the final installation against the requirements of the equipment manufacturer and the rules applicable to the vessel.
