When a battery wired in series is added to a string, the system voltage increases while the Amp-hour (Ah) capacity remains exactly the same as a single unit. If you wire four 12V 100Ah lithium iron phosphate (LiFePO4) batteries in series, you get a 48V (nominal 51.2V) bank with 100Ah of capacity, yielding 5,120 Watt-hours (Wh) of total energy. You do not get 400Ah. This is the most common point of failure in DIY solar builds: builders wire in series expecting more runtime, only to trip their BMS at high loads because they misunderstood the Ah math.
The System Block: Source to Load in a Series String
A properly engineered series battery bank is not just batteries linked together; it is a managed power pipeline. Here is the exact physical topology for a 48V series string, from the source to the load:
- Source (The Battery String): Four 12V batteries. The positive terminal of Battery 1 connects to the negative of Battery 2, and so on. The free negative on Battery 1 and free positive on Battery 4 become your main 48V bank terminals.
- Overcurrent Protection: A 250A Class T fuse mounted on the main positive output, within 7 inches of the terminal, per NEC-style guidance for ungrounded DC conductors.
- DC Disconnect: A 48V-rated, 250A rotary disconnect switch to isolate the bank from the inverter.
- Inverter/Charger: A 48V DC to 120/240V AC split-phase inverter/charger (e.g., Victron MultiPlus-II 48/5000).
- Load: The main AC subpanel or dedicated critical loads panel.
Series vs. Parallel: Consequences for Voltage, Ah, and Inverter Sizing
Choosing between series and parallel dictates your wire gauge, your inverter selection, and your system efficiency. Parallel wiring increases Ah (capacity) at the same voltage. Series wiring increases voltage at the same Ah. Here is the direct consequence for a 5,120Wh bank:
| Configuration | Nominal Voltage | Total Ah | Total Wh | Current at 4000W Load | Required Wire Size |
|---|---|---|---|---|---|
| 4P (Parallel) | 12V (12.8V LFP) | 400Ah | 5,120Wh | ~347A DC | 4/0 AWG (Minimum) |
| 2S2P (Series-Parallel) | 24V (25.6V LFP) | 200Ah | 5,120Wh | ~173A DC | 1/0 AWG |
| 4S (Series) | 48V (51.2V LFP) | 100Ah | 5,120Wh | ~86A DC | 2 AWG or 1/0 AWG |
Inverter Sizing for the Stated Load
Assume a continuous AC load of 4,000W. Inverters are not 100% efficient; a high-frequency 48V inverter operates at roughly 90% efficiency under heavy load.
The Math: 4,000W AC / 0.90 (efficiency) = 4,444W DC input required.
In a 4S series configuration (51.2V nominal LiFePO4), the DC current draw is 4,444W / 51.2V = 86.8 Amps.
This 86.8A draw is well within the 100A continuous output rating of most premium 12V 100Ah LiFePO4 BMS units, and it keeps voltage drop under 1% over a 10-foot run of 1/0 AWG copper. If you attempted this same 4,000W load on a 12V parallel bank, you would pull 347A, requiring massive, expensive 4/0 AWG welding cable and generating significant heat at the busbars.
Sizing Math: Peukert’s Law, Efficiency, and C-Rate Limits
When sizing your series string, you must account for how battery chemistry reacts to high current draws. This is where Peukert's Law and C-rates come into play.
Peukert’s Law (Lead-Acid vs. LiFePO4)
Peukert’s Law states that as your discharge current increases, the usable capacity of the battery decreases. The formula is t = H × (C / (I × H))^k, where k is the Peukert exponent.
- Flooded Lead-Acid (FLA): Has a k value of ~1.3. If you pull 100A from a 100Ah FLA battery, you will only get about 60Ah of actual runtime before voltage collapse.
- LiFePO4: Has a k value of ~1.05. The voltage curve is incredibly flat. If you pull 100A from a 100Ah LiFePO4 battery, you get roughly 95Ah of usable capacity.
Note: If you are wiring AGM or Gel batteries in series, you must oversize your Ah by at least 40% to compensate for Peukert losses at high inverter loads.
Charge and Discharge Limits (C-Rates and DoD)
Every battery has a maximum C-rate (charge/discharge rate relative to capacity). For a 100Ah battery, 1C = 100A. Most off-grid LiFePO4 batteries are rated for 0.5C continuous discharge (50A) and 0.5C charge.
If your inverter pulls 86.8A continuously, a single 100Ah battery in a 4S series string will be pushed to 0.86C. This will trigger the BMS over-current protection and shut down your power. The fix: You must either use batteries rated for 1C continuous (like the SOK 12V 100Ah or Epoch 12V 100Ah) or wire two parallel strings of 4S batteries (a 4S2P configuration) to halve the current per battery.
Regarding Depth of Discharge (DoD): Never size a lead-acid series bank for more than 50% DoD. LiFePO4 can safely be sized to 80%–90% DoD, meaning a 100Ah LiFePO4 series string yields 80Ah–90Ah of usable daily energy, whereas a 100Ah Lead-Acid series string yields only 50Ah.
Lithium Fire-Safety and BMS Topologies in Series
When a LiFePO4 battery is wired in series, the internal Battery Management System (BMS) must be rated to handle the stacked voltage, or you must use an external BMS topology.
Most cheap, drop-in 12V LiFePO4 batteries contain an internal BMS designed strictly for 12V operation. If you wire four of them in series, the negative terminal of Battery 4 is sitting at -36V relative to Battery 1. If a ground fault occurs, the internal BMS MOSFETs will experience voltage breakdown, arc, and fail shorted.
Safe Series BMS Options:
- Internal BMS Rated for Series: Brands like SOK, Renogy, and Epoch explicitly state in their manuals that their internal BMS supports up to 4 in series (48V). They use higher-voltage-rated MOSFETs and isolated communication lines.
- External BMS (Victron Smart Lithium): You buy raw 12V lithium batteries without internal BMS, wire them in series, and use a centralized external BMS (like the Victron VE.Bus BMS) that monitors individual cell tap wires. This is the safest, most robust method for marine and high-end off-grid builds.
For further reading on safe DC topologies, refer to the Victron Energy Wiring Unlimited guide, which details exact fault-current paths in series-parallel battery banks.
Decision Tree: Picking Your Series Battery and Components
Do not guess your configuration. Use this decision path to select the exact parts for your 48V series build based on your continuous load requirements.
| If Your Max Continuous AC Load Is... | And Your Inverter Efficiency Is... | Then Your DC Current at 51.2V Is... | Required Battery Spec (per 4S string) | Concrete Part Pick (4S Configuration) |
|---|---|---|---|---|
| < 2,500W | 90% | ~54A | 12V 100Ah (0.5C BMS limit) | 4x Renogy 12V 100Ah LiFePO4 |
| 2,500W – 4,500W | 90% | 54A – 97A | 12V 100Ah (1.0C BMS limit required) | 4x SOK 12V 100Ah LiFePO4 |
| > 4,500W | 90% | > 97A | Requires 4S2P (Two parallel strings of 4S) | 8x SOK 12V 100Ah (Configured 4S2P) |
The Default Recommendation for a 4,000W System
If you are building a standard off-grid cabin or backup system with a 4,000W continuous load target (running a well pump, fridge, and microwave simultaneously), terminate your decision here:
- Batteries: Buy four SOK 12V 100Ah LiFePO4 batteries (SKU: SOK-12V-100Ah). They feature a 100A continuous BMS (1C rate), Bluetooth monitoring, and explicit manufacturer approval for 4S series wiring up to 51.2V.
- Interconnects: Use 1/0 AWG copper busbars or custom-cut 1/0 AWG copper cable with 3/8' lugs crimped using a hex-crimper. Do not use aluminum interconnects.
- Inverter/Charger: Victron MultiPlus-II 48/5000/70. This 48V inverter handles the 5,000VA (approx 4,000W continuous) load effortlessly and includes a 70A AC charger to replenish the 100Ah series bank in roughly 1.5 hours from a generator.
- Fusing: Install a Blue Sea Systems 250A Class T Fuse on the main positive 48V output line.
For deeper analysis on how Peukert's exponent affects your specific load profiles over time, consult the Peukert's Law breakdown on Battery University. By wiring your battery bank in series to achieve 48V, you drastically reduce DC current, minimize I²R heat losses in your cabling, and allow standard, cost-effective wire gauges to safely deliver whole-home power.
