Battery wiring guide
Battery series vs parallel
Series adds voltage. Parallel adds amp-hours. A series-parallel bank combines both, while total stored watt-hours still comes from the number and rating of the matching batteries.
Series connections
Connect the positive terminal of one battery to the negative terminal of the next and the voltages add. Amp-hour capacity stays at the rating of one matching battery. Two 12V 100Ah batteries in series form a nominal 24V 100Ah bank.
total voltage = unit voltage × batteries in series
Parallel connections
Connect matching positive terminals together and matching negative terminals together and the amp-hours add. Voltage stays at the rating of one battery. Two 12V 100Ah batteries in parallel form a nominal 12V 200Ah bank.
total Ah = unit Ah × parallel strings
Series-parallel banks
A series-parallel bank builds equal series strings first, then places those matching strings in parallel. Four 12V 100Ah batteries in a 2S × 2P layout create two 24V 100Ah strings. Connecting those strings in parallel produces 24V 200Ah, or 4.8kWh nominal.
| Layout | Total voltage | Total Ah | Total energy |
|---|---|---|---|
| 4S × 1P | 48V | 100Ah | 4.8kWh |
| 2S × 2P | 24V | 200Ah | 4.8kWh |
| 1S × 4P | 12V | 400Ah | 4.8kWh |
Check a proposed layout with the battery series-parallel calculator. It assumes every battery in the bank has the same rating.
Can you connect batteries with different Ah ratings in parallel?
Two batteries at the same nominal voltage may add their rated Ah on paper, but the current may not split in proportion to capacity. Internal resistance, state of charge, BMS limits, age, and temperature can make one battery charge or discharge harder than the other.
Do not connect batteries at different voltages. Use a mixed-capacity bank only when the manufacturer documents that arrangement and its protection requirements. Otherwise, use batteries that match in voltage, capacity, chemistry, age, and health.
Worked example: solve a target
You have 12V 100Ah modules and need a nominal 24V bank with at least 250Ah. The voltage requires two batteries in series. Capacity requires ceil(250 ÷ 100) = 3 parallel strings. The smallest layout is 2S × 3P, six batteries total, producing 24V 300Ah or 7.2kWh nominal.
The extra 50Ah is not a calculation error. It is the result of rounding to whole matching batteries.
Topology is not installation detail
A series-parallel sketch does not specify conductor size, equal-length paths, busbars, disconnects, fuses, grounding, terminal torque, ventilation, enclosure, or BMS communication. Those decisions depend on equipment limits and the installation.
Read the 12V vs 24V vs 48V guide before choosing a nominal voltage. Once the bank layout is fixed, use the battery runtime calculator to estimate how long it can support a load.