Battery charger and method of charging batteries

Abstract

A battery charger and method of charging a rechargeable battery which comprises charging the battery with an initial charging current and measuring and comparing the charging temperature to an initial temperature to identify a temperature factor which either identifies a need to lower the current or terminate the charging current. The temperature factor may be a temperature gradient .DELTA.T. The open circuit voltage may be monitored from the beginning or may begin when .DELTA.T is identified. V.sub.OCV is plotted with respect to time t elapsed to identify a point or points on the V.sub.OCV (t) curve, such as an inflection point in the dV.sub.OCV /dt data or a transition point in the d.sup.2 V.sub.OCV /dt.sup.2 which indicate the onset of overcharge. If the open circuit voltage V.sub.OCV of the battery is sampled from the beginning of charging the second order differential information should be ignored until a predetermined temperature gradient is realized. If a point indicating onset of overcharge is not recognized then charging is continued and the steps above are repeated until a point or points indicating onset of overcharge are identified. As a safeguard against overcharge, the inflection point on dV.sub.OCV /dt curve is utilized to indicate onset of overcharge even if the inflection point on the d.sup.2 V.sub.OCV /dt.sup.2 curve is not identified and the negative slope--dV.sub.OCV /dt is utilized to indicate onset of overcharge if the inflection point on the dV.sub.OCV /dt data is not identified.