When a battery is connected in series and parallel (commonly called a series-parallel bank), you achieve both higher voltage and higher capacity. Specifically, series wiring increases system voltage while keeping amp-hours (Ah) constant, and parallel wiring increases Ah while keeping voltage constant. For example, wiring four 12V 100Ah batteries in a 2S2P configuration yields a 24V 200Ah bank. This is the standard architecture for off-grid solar and marine systems that require 24V DC bus efficiency without stepping up to 48V.
System Architecture: Source to Load Block Description
Before cutting wire, you must understand the power flow. A complete 2S2P system follows a strict source-to-load block sequence:
- Source: Solar PV array (e.g., 4x 400W panels in 2S2P) or AC Grid/Generator input.
- Regulation/Conversion: MPPT Charge Controller (for DC) and Inverter-Charger (for AC grid/generator integration).
- Storage (The 2S2P Bank): Two series strings of two batteries each, wired in parallel. Protected by a main Class-T fuse and individual string breakers.
- Distribution: DC busbars feeding DC loads, and the inverter output feeding the AC subpanel.
| Configuration | Voltage Consequence | Capacity (Ah) Consequence | Best Use Case |
|---|---|---|---|
| Series Only (2S) | Doubles (12V → 24V) | Remains same (100Ah) | High-power, low-capacity needs |
| Parallel Only (2P) | Remains same (12V) | Doubles (100Ah → 200Ah) | RVs, 12V winch systems |
| Series-Parallel (2S2P) | Doubles (12V → 24V) | Doubles (100Ah → 200Ah) | Off-grid cabins, marine 24V buses |
Sizing Math: Peukert, Efficiency, and Real-World Capacity
Theoretical capacity rarely matches usable capacity. To size your bank accurately, we must apply Depth of Discharge (DoD), inverter efficiency, and Peukert's Law.
The Base Math: Four 12V 100Ah batteries in 2S2P = 24V nominal × 200Ah = 4,800Wh of total theoretical energy.
Peukert's Law Application: Peukert's Law ($t = H(C / (I \times H))^k$) dictates that as discharge current increases, usable capacity decreases. For Lead-Acid (AGM/Gel), the Peukert exponent ($k$) is typically 1.3. If you pull 100A from a 200Ah AGM bank, you will only get about 1.4 hours of runtime, not 2. However, for LiFePO4 (Lithium Iron Phosphate), $k$ is approximately 1.05. The Peukert effect is virtually negligible, meaning a 100A draw on a 200Ah LiFePO4 bank will deliver very close to the full 2 hours.
Depth of Discharge (DoD) & Efficiency:
- LiFePO4 DoD: 80% to 90% is standard for longevity. Let's use 80%. (4,800Wh × 0.80 = 3,840Wh usable).
- Inverter Efficiency: A high-frequency 24V inverter operates at roughly 93% efficiency under continuous load. (3,840Wh × 0.93 = 3,571Wh delivered to AC loads).
Therefore, your 4,800Wh theoretical bank safely delivers 3,571Wh to your AC appliances before the Battery Management System (BMS) triggers a low-voltage disconnect.
Inverter, Charger, and Wire Sizing for a 3000W Load
Let's size the inverter and charge controller for a continuous 3000W AC load on our 24V 2S2P bank.
Inverter Sizing:
Current ($I$) = Power ($P$) / Voltage ($V$).
3000W / 24V = 125A continuous DC draw. Factoring in 93% inverter efficiency, the actual draw from the batteries is $125A / 0.93 \approx 134A$. Adding a 25% safety margin for continuous duty (NEC-style guidance), we need wiring and fusing rated for 167A. A 3000VA 24V Inverter-Charger (like the Victron MultiPlus-II 24/3000) is the exact match.
Charge/Discharge Limits (C-Rate):
Most 100Ah LiFePO4 cells have a maximum charge C-rate of 0.5C (50A per battery). In a 2S2P setup, you have two parallel strings. Each string can accept 50A, meaning the total bank can accept 100A of charge current. Your MPPT charge controller must be configured with a hard charge limit of 100A to prevent BMS tripping or cell degradation. For a 24V system, a 100A charge limit equates to roughly 2,600W of solar input (26V charging × 100A).
Wire Sizing:
For the 167A battery-to-inverter run, 2/0 AWG THHN copper wire (rated 175A at 75°C) or 1/0 AWG pure copper welding cable (highly flexible, rated ~150A-190A depending on insulation) is required. Keep this run under 5 feet to maintain a voltage drop below 1%.
Critical Safety and Cell Matching Rules
Furthermore, torque matters. Loose terminal connections on a 2S2P bank create high-resistance points that generate localized heat. Use a calibrated torque wrench set to the manufacturer's spec (typically 5-7 Nm for M8 LiFePO4 terminals) and apply a dielectric grease or NO-OX-ID A-Special to prevent galvanic corrosion between copper lugs and brass battery posts.
Frequently Asked Questions
Can I connect different battery brands in series and parallel?
No. Connecting different brands in a battery connected in series and parallel configuration is a primary cause of premature bank failure. Different brands use different BMS low-voltage cutoff thresholds, internal cell chemistries, and internal resistances. In series, the weakest battery will hit the low-voltage cutoff first, shutting down the entire string while the other batteries still have capacity. In parallel, differing internal resistances will cause unequal current sharing, leading to over-stressing one brand's BMS while underutilizing the other. Always use identical make, model, and batch numbers.
What happens to the C-rate when a battery is connected in series and parallel?
The C-rate of the individual cells does not change, but the absolute amperage the bank can handle scales with the parallel connections. If a single 100Ah battery has a 1C max discharge rate (100A), wiring two in series (2S) keeps the max discharge at 100A (but at 24V). When you add a second identical string in parallel (2P), the total bank capacity becomes 200Ah, and the max discharge capability doubles to 200A. The C-rate remains 1C relative to the new 200Ah total capacity.
Do I need a separate BMS for each string in a series-parallel lithium setup?
Yes, if you are using drop-in 12V replacement batteries (like a Renogy or Dakota Lithium 12V 100Ah), each unit already contains an internal BMS. You do not add external BMS units to these. However, if you are building a bank from raw, individual 3.2V LiFePO4 prismatic cells, you wire 8 cells in series to make one 24V string, and then wire two of those strings in parallel. In this raw-cell scenario, you must use one high-amperage BMS (e.g., a 200A 8S BMS) per series string, and ideally, a BMS with Bluetooth balancing capabilities to monitor individual cell voltages across both parallel strings.
How do I balance a battery connected in series and parallel before the first use?
Top-balancing is mandatory before connecting raw cells in a series-parallel layout. Connect all cells in parallel at the 3.2V level and charge them to exactly 3.65V using a bench power supply until the current drops to near zero. This ensures every cell starts at the exact same state of charge (SoC). If you are using pre-assembled 12V drop-in batteries, charge each 12V battery individually to 100% on a bench using a dedicated 14.6V LiFePO4 charger before wiring them into your 2S2P configuration. This prevents massive equalization currents from flowing between a fully charged battery and a half-charged battery the moment you tighten the parallel busbar.
