When you start building a solar bank, UPS, or off-grid power system, the sheer volume of battery names can be paralyzing. You will see alphanumeric codes like 18650 and 21700, alongside terms like Group 31, 4D, and 8D. These are not random marketing labels; they are strict dimensional and standardized codes that dictate your physical layout, busbar sizing, and ultimately, your system's C-rate limitations.
If you misinterpret a battery name, you end up with a bank that physically won't fit your enclosure, or worse, a high-current draw that melts your terminals. This guide decodes the naming conventions, runs the hard sizing math, and terminates in concrete part picks for your next build.
The Anatomy of Battery Names: Cylindrical vs. BCI Standards
Battery names generally fall into two categories in the DIY and prosumer space: metric cylindrical dimensions for raw cells, and BCI (Battery Council International) group sizes for enclosed 12V/24V drop-in blocks.
Cylindrical Cell Names (The Metric Standard)
Raw lithium-ion and LiFePO4 cells use a 4- or 5-digit metric name representing their diameter and length.
- 18650: 18mm diameter, 65.0mm length. The legacy workhorse. Typical capacity: 2500mAh–3500mAh (NMC) or 1500mAh (LiFePO4).
- 21700: 21mm diameter, 70.0mm length. The modern high-density standard. Typical capacity: 4000mAh–5000mAh (NMC).
- 32700 (or 32650): 32mm diameter, 70.0mm length. Common in raw LiFePO4 prismatic/cylindrical hybrids. Typical capacity: 6000mAh.
BCI Group Sizes (The Enclosed Standard)
When you buy a sealed 12V AGM or a 12V LiFePO4 drop-in battery, it will carry a BCI Group name. This defines the physical footprint, terminal placement, and approximate capacity range. According to the International Electrotechnical Commission (IEC) and BCI standards, physical dimensions must remain consistent even as internal chemistry upgrades from lead-acid to lithium.
| BCI Group Name | Dimensions (L x W x H) | Typical Lead-Acid Ah | Typical LiFePO4 Ah |
|---|---|---|---|
| Group 24 | 10.25' x 6.8' x 9.4' | 70-85 Ah | 100 Ah |
| Group 27 | 12.5' x 6.8' x 9.4' | 85-100 Ah | 100-120 Ah |
| Group 31 | 13.0' x 6.8' x 9.4' | 100-130 Ah | 100-200 Ah |
| 4D | 20.75' x 6.8' x 9.4' | 180-220 Ah | 200-300 Ah |
System Block Architecture: Source to Load
Before sizing, we must define the system block. A standard off-grid or backup power system flows in a strict sequence:
- Source: Solar array (e.g., 4x 400W panels in 2S2P) or Grid Generator.
- Charge Controller: MPPT steps high DC voltage down to battery charging voltage.
- Battery Bank: The storage buffer. Configured in series/parallel to hit target voltage and capacity.
- Inverter/Charger: Converts DC to AC for loads; includes an AC-to-DC charger for grid/generator top-ups.
- Load: AC Subpanel or DC fuse block.
Series vs. Parallel Consequences
How you wire your batteries fundamentally changes your wire gauge requirements and inverter selection.
- Series Wiring: Voltages add, Amp-hours (Ah) remain the same. Four 12V 100Ah batteries in series yield 48V at 100Ah. This is ideal for high-power systems (3000W+) because higher voltage means lower current, allowing you to use smaller, cheaper AWG wire (e.g., 2 AWG instead of 4/0 AWG).
- Parallel Wiring: Voltage remains the same, Ah adds. Four 12V 100Ah batteries in parallel yield 12V at 400Ah. This is strictly for low-power 12V DC systems (like vans or small marine setups). Pulling 3000W from a 12V bank requires 250+ Amps, which generates massive heat and requires massive busbars.
Sizing Math: Peukert, Efficiency, and C-Rates
Let's size a battery bank for a specific load: running a 1,500W microwave and fridge combo for 2 hours during a grid outage.
Step 1: Calculate DC Draw with Inverter Efficiency
Inverters are not 100% efficient. A good high-frequency pure sine wave inverter operates at about 85% efficiency under heavy load.
- AC Load: 1,500W
- Required DC Power: 1,500W / 0.85 = 1,764W
- Current at 48V nominal: 1,764W / 48V = 36.75 Amps
Step 2: Calculate Required Ah and Apply Depth of Discharge (DoD)
Over 2 hours, the raw Ah consumed is 36.75A × 2h = 73.5 Ah.
However, you cannot drain a battery to absolute zero. LiFePO4 batteries safely allow an 80% to 90% DoD. Lead-acid/AGM batteries should be limited to 50% DoD to prevent sulfation and rapid degradation.
- LiFePO4 (80% DoD): 73.5 Ah / 0.80 = 91.8 Ah minimum bank capacity.
- AGM (50% DoD): 73.5 Ah / 0.50 = 147 Ah minimum bank capacity.
Step 3: The Peukert Penalty (Why Battery Chemistry Matters)
If you chose AGM, you must factor in Peukert's Law. Peukert's law states that as the rate of discharge increases, the available capacity of a lead-acid battery decreases exponentially. A 150Ah AGM battery rated at the 20-hour rate (7.5A draw) will only deliver about 90Ah of usable capacity when hit with a 36A continuous draw. To get 147Ah under a high C-rate load, you would actually need to buy nearly 250Ah of nameplate AGM capacity.
LiFePO4 chemistry is virtually immune to the Peukert effect. A 100Ah LiFePO4 battery will deliver ~95Ah whether you pull 10A or 100A. Battery University extensively documents how lithium's flat discharge curve and low internal resistance eliminate the high-draw penalties seen in lead-acid.
If you are building a DIY pack using raw 18650 or 21700 cylindrical cells, never parallel mismatched cells (different brands, ages, or capacities). Mismatched cells cause internal cross-currents that bypass the BMS, leading to thermal runaway. Always use a high-quality BMS (like a Batrium or JK BMS) rated for your peak discharge C-rate, and compress prismatic cells to manufacturer specs (usually 12-15 PSI) to prevent internal delamination and subsequent short circuits. Follow NFPA 855 guidelines for spacing and fire separation when installing large lithium banks indoors.
Inverter and Charger Sizing for the Bank
Your battery name and configuration dictate the supporting hardware.
Inverter Sizing
For our 1,500W continuous load, motors (like in the fridge compressor) require a surge multiplier of 1.5x to 2x for startup.
Pick: A 3,000W continuous / 6,000W surge 48V Pure Sine Inverter (e.g., Victron MultiPlus II 48/3000 or Growatt SPF 3000TL). This provides headroom for the surge without tripping the BMS low-voltage cutoff.
Charger Sizing (C-Rate Limits)
Batteries have strict charge C-rate limits.
- LiFePO4 Limit: Typically 0.5C (50A charge for a 100Ah battery). Charging faster degrades the anode.
- Lead-Acid Limit: Typically 0.2C (20A charge for a 100Ah battery). Charging faster causes outgassing and thermal damage.
Decision Tree: Picking the Right Battery Format
Stop guessing. Use this decision matrix to select the exact battery name and format for your specific application.
| If Your Scenario Is... | Required System Voltage | Choose This Battery Name/Format | Concrete Part Pick (2026 Market) |
|---|---|---|---|
| DIY Portable Power Station / High-Density E-Bike (Needs max energy density, lightweight, custom shape) |
12V to 52V (Custom Series/Parallel) | 21700 Cylindrical NMC (Avoid 18650; 21700 offers 20% better volumetric density and fewer spot welds) |
Molicel P42A 21700 (4200mAh, 45A continuous discharge. Build with Vruzend DIY kit or nickel strip spot-welding) |
| RV / Marine 12V Drop-In Replacement (Needs to fit existing AGM battery boxes, 12V alternator charging) |
12V Nominal | BCI Group 31 LiFePO4 (Fits standard marine/RV trays, internal BMS handles alternator voltage spikes) |
Epoch 12V 100Ah Group 31 or Victron Smart Lithium 12.8V/100Ah (Includes low-temp charge cutoff and Bluetooth monitoring) |
| Home Off-Grid / Whole-House Backup (Needs 3000W+ inverters, high cycle life, easy server-rack scaling) |
48V Nominal (51.2V actual) | 19-inch Server Rack LiFePO4 (Standardized 3U/4U rackmount, uses standard CAT5 for BMS comms) |
EG4 48V 100Ah LL (Server Rack) or SOK 48V 100Ah (Buy 2 in parallel for 10kWh. Uses Grade-A EVE or Lishen prismatic cells) |
| Heavy Duty Solar / Industrial Forklift (Extreme abuse tolerance, budget constrained, doesn't care about weight) |
48V | 8D or 4D Flooded Lead Acid (FLA) (Requires monthly watering, but survives massive surge loads and high ambient heat) |
Trojan L16P 6V 400Ah (Wire 8 in series for 48V. Requires ventilated battery box and hydrogen sensors) |
By understanding what battery names actually mean, you move past marketing fluff and start engineering your power system around physical realities, C-rate limits, and verified dimensional standards. Pick the format that matches your voltage architecture, size the wire for the worst-case continuous draw, and lock in your BMS parameters before you ever tighten a busbar lug.
