Series vs. parallel battery wiring: what's the difference?
Two batteries can be wired two fundamentally different ways, and they do different jobs. Series wiring adds voltage; parallel wiring adds capacity. Almost every real battery bank above the smallest sizes uses some combination of both.
Series wiring adds voltage while capacity stays the same as a single battery; parallel wiring adds capacity while voltage stays the same. Most real battery banks combine both — series strings to reach the target voltage, wired in parallel to reach the target capacity.
Series: voltage adds, capacity stays the same
Wiring batteries in series — positive terminal of one to the negative of the next — connects them end to end along a single current path. The voltages add together; the capacity (Ah) stays the same as a single battery. Two 12V 100Ah batteries in series make a 24V 100Ah bank, not a 24V 200Ah bank. This is how you reach a target system voltage (12V, 24V, 48V) from lower-voltage battery or cell building blocks.
Parallel: capacity adds, voltage stays the same
Wiring batteries in parallel — positive to positive, negative to negative — keeps every battery at the same voltage but combines their capacity. Two 12V 100Ah batteries in parallel make a 12V 200Ah bank. This is how you reach a target Ah capacity once the batteries you're using are already at your target voltage.
Series-parallel: doing both
Most banks that need both higher voltage and higher capacity than a single unit provides use a series-parallel configuration: build one series "string" to reach the target voltage, then wire multiple identical strings together in parallel to reach the target capacity. Four 12V 100Ah batteries as two series pairs, wired in parallel, gives 24V 200Ah. The series & parallel calculator works this out for any target voltage and capacity with a wiring diagram.
The rule that actually matters: never mix ages, brands or states of charge
The wiring topology is simple math. The real risk in series-parallel banks is imbalance: batteries of different age, brand, capacity, or state of charge wired together will fight each other — the weaker unit gets overworked in parallel strings, and a mismatched cell drags down (or gets overcharged relative to) the rest of a series string. Build every string from identical, same-age, same-state-of-charge units, and keep string lengths identical across parallel groups. This matters more for the bank's long-term health than the wiring diagram itself.
DIY cell-level series-parallel is the same idea, smaller scale
Building a pack from individual prismatic or cylindrical lithium cells uses identical logic at a smaller scale — commonly written as, for example, "4S2P" (4 cells in series for voltage, 2 groups in parallel for capacity). See the DIY cell-to-pack calculator for cell counts by target voltage, and why a BMS matched to the exact series count is not optional at this scale.
Related tools
Sources & standards
- IEEE 1013 — Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems — general sizing-practice reference.
- UL 1973 — Batteries for Use in Stationary and Motive Auxiliary Power Applications — safety standard most LiFePO4 packs are certified to.
- NFPA 70 (NEC) Articles 690 & 706 — PV source circuits and energy storage systems.
Educational content, not an electrical design. Verify with your equipment's datasheet and a licensed installer for anything permanently wired. See the methodology.