Decision table
| Factor | 48/51.2 V LFP | Second-life EV pack |
|---|---|---|
| Condition | New products/cells can have known specifications and warranty | History and cell spread require evidence and testing |
| Usable fraction | Defined by product/BMS limits or DIY design | SOH × selected window; weakest group may constrain |
| Voltage | Common nominal 51.2 V (16S) | Usually hundreds of volts; pack-specific range |
| Current at 5 kW | About 98 A at 51.2 V before losses | About 14 A at 360 V before losses |
| Controls | BMS plus a supported low-voltage inverter protocol | OEM BMS plus CAN translation/controller and compatible HV inverter |
| Thermal design | LFP-specific sensing and enclosure plan | Retained pack cooling approach and stationary duty must be assessed |
| Warranty/support | Possible with integrated/rack products; DIY terms vary | Normally no stationary-system product warranty |
| Best fit | Predictability, modularity and common equipment ecosystems | Experienced projects valuing large capacity and reuse |
Use one denominator
Calculate purchase £/nominal kWh, purchase £/estimated usable kWh, then complete £/usable kWh. Include every component needed to create the same functional boundary: battery through inverter, protection, containment and installation.
Illustration A cheap EV pack can lead on the first metric and lose on the third. Conversely, a technically sound large pack paired with an already-compatible inverter may retain an advantage. Without dated quotes and measured SOH, declaring either route cheapest is speculation.
Avoid chemistry slogans
LFP generally trades lower energy density for strong cycle-life and thermal-stability characteristics, while NMC/NCA offer high energy density. Pack-level risk also depends on cell condition, mechanical protection, sensing, fault interruption, thermal propagation design, installation and controls. “LFP safe / NMC unsafe” is not an engineering assessment.
A practical choice process
- Define required delivered kWh and kW.
- Choose low-voltage or high-voltage inverter architecture.
- Obtain documented battery limits, condition and compatibility.
- Cost the complete protective and control chain.
- Compare expected delivered lifetime energy, warranty and serviceability.
Compare household scenarios first.
Architecture follows the load, tariff and economic case.