The ELK-BLT Battery Life Tester or .Mhos Meter. is a compact, easy to use service tool for testing 12 Volt rechargeable batteries. The BLT measures internal conductivity (Mhos), which is the best indicator of a battery’s health and life expectancy. Mhos happens to be the inverse of Ohms (conductance as opposed to resistance). The BLT operates as follows: For each battery brand and size installed you should start by measuring and recording a .baseline. Mhos reading. Then, during any service call or inspection simply measure and record the current (most recent) Mhos reading. Always compare the current Mhos reading with the original reading.