State-of-charge determination of lead-acid batteries using wire-wound coils

Non-intrusive monitoring of the state-of-charge of sealed lead-acid batteries using wire-wound coils is described. Coils were attached to the plastic case of the batteries, adjacent to the negative end plate, and excited using ac current at 10 kHz. As the batteries were cycled, the inductance of the coils tracked the change in metallic content of the end plates. Data is presented for different discharge rates and temperatures. A variation of the technique is also described that uses less expensive equipment to measure the voltage drop across the coil, rather than inductance.