Series wiring adds voltage (2× 12V = 24V), parallel wiring adds capacity (2× 100Ah = 200Ah). Build a bank by combining both, but always use matched batteries, connect in the right order, and protect each series string — mismatched or miswired banks fail early.

The Short Answer

Batteries store energy as voltage × capacity, and wiring changes the ratio. The underlying physics follows series and parallel circuit rules, applied to battery pack construction. In series, the positive of one battery connects to the negative of the next — voltages add, capacity stays the same. In parallel, all positives connect and all negatives connect — capacity adds, voltage stays the same. For a solar, RV, marine, or backup bank, you usually need both: series to reach the system voltage (12V, 24V, 48V), parallel to reach the required capacity. The golden rules: use identical batteries, charge them to the same state first, and never exceed the pack’s ratings. Always observe correct polarity when connecting.

Series Wiring: Voltage Up

Series wiring connects batteries end-to-end: battery 1 positive → battery 2 negative, and the load is taken across the two free terminals. Two 12V 100Ah batteries in series make a 24V 100Ah bank — voltage doubles, capacity (Ah) does not. Series is how you build a 24V or 48V system from 12V batteries, reducing cable size for the same power. The catch: the whole string is limited by its weakest battery, and if one battery fails open, the entire string is dead. Series strings also need a BMS per string (for lithium) because the pack voltage is supervised as a whole.

Parallel Wiring: Capacity Up

Parallel wiring connects all positives together and all negatives together. Two 12V 100Ah batteries in parallel make a 12V 200Ah bank — capacity doubles, voltage stays the same. Parallel is how you scale run time without changing voltage. The risks: current sharing between batteries depends on equal internal resistance and equal connection length — mismatched batteries or unequal cables cause one battery to work harder and age faster. Parallel banks also multiply fault current, so proper fusing per battery is essential. For lithium, each battery should have its own BMS, and parallel connection should follow the manufacturer’s guidance.

Series-Parallel Combinations

Most practical banks combine both: build series strings to reach the voltage, then parallel the strings to reach capacity. Example: four 12V 100Ah batteries to make a 24V 200Ah bank = two strings of two in series, wired in parallel. The rule: make all strings identical — same battery count, same capacity, same state of charge. Wiring order matters: connect the series strings first, verify each string’s voltage, then parallel them. This is the configuration used in LiFePO4 battery packs and most solar storage banks.

Voltage & Capacity Math

Note that energy (Wh = V × Ah) is conserved either way — series and parallel store the same total energy; they just present it at different voltage. Choose the configuration the system needs (inverter voltage, charger compatibility, cable sizing) rather than preference.

Why Cells Must Match

Battery banks work only as well as their weakest battery. Mismatched batteries — different age, capacity, internal resistance, or state of charge — cause uneven charging: in series, the weakest battery hits limits first and is forced past them; in parallel, the lower-resistance battery carries more current and ages faster. The industry rule: use batteries of the same brand, model, age, and charge state; ideally from the same production batch. When adding a battery to an existing bank, replace the whole set rather than mixing old and new. For lithium, this matters even more because the BMS protects the whole pack at once.

Common Mistakes

• Mixing different capacities: a 100Ah + 200Ah parallel pair is limited and ages unevenly.
• Mixing old and new: the old battery drags the new one down.
• Unequal cable lengths: the shorter cable carries more current; use equal-length cables for parallel banks.
• Wrong connection order: connect positive before negative, and check polarity before closing the circuit.
• No per-battery fusing: parallel banks need a fuse per battery for fault isolation.
• Series with different SOC: charging one battery while the other is full damages the full one.

Designing a 12V/24V/48V Bank

• 12V systems: single battery or parallel for capacity — RV, small solar, boat.
• 24V systems: two 12V in series (or 2S) — larger solar, vans, equipment.
• 48V systems: four 12V in series (4S) or eight (2P4S) — home storage, inverters, telecom.

Match the bank voltage to the inverter/charger’s input range — running a 24V inverter on a 12V bank needs a boost converter and is a common planning error. Jetray engineers custom bank configurations for solar, marine, and industrial applications, including proper BMS and fusing.

FAQ

What is the difference between series and parallel batteries?

Series adds voltage (12V + 12V = 24V) with the same capacity; parallel adds capacity (100Ah + 100Ah = 200Ah) at the same voltage. Energy is the same either way.

Can I mix different battery brands in a bank?

Not recommended. Mismatched internal resistance and capacity cause uneven charging and early failure. Use identical batteries of the same age and state of charge.

How do I wire batteries for 24V?

Connect two 12V batteries in series: positive of battery 1 to negative of battery 2, and take 24V across the two free terminals. Or use a purpose-built 24V battery pack.

Do parallel batteries need fuses?

Yes — each parallel branch should have its own fuse so a fault in one battery doesn’t draw the full bank’s current and cause a fire.

Is it better to run higher voltage or more capacity?

Higher voltage (e.g., 24V or 48V) reduces current for the same power, allowing thinner cables and lower losses — preferred for larger systems. More parallel capacity extends run time at the chosen voltage.

Bank-Ready Lithium Batteries from Jetray

Sources: Wikipedia – Series and Parallel Circuits.