Abstract

A battery power conversion circuit comprising a cell equalization circuit that ensures that each cell in a multiple battery cell stack is depleted of charge at an equal rate. The stack is coupled to provide current to a primary winding of a transformer. Each of the nodes between cells is coupled to the primary winding of the transformer through a transistor such that an equal number of turns of the primary winding are present between each node. An additional transistor coupled in series with the primary winding controls the current through the primary winding such that a capacitor coupled to a secondary winding of the transformer may be charged by an induced secondary current to a desired voltage level for powering a load. A controller circuit monitors the output voltage level and controls the transistor in series with the primary winding for maintaining the desired load voltage through a feedback loop. When the transistor in series with the primary winding is turned on to allow current to flow through the primary winding, each of the transistors coupled to each of the nodes between battery cells is also turned on by the controller. Only a small current travels through each of the transistors coupled to each of the nodes. Therefore, the voltage across each battery cell is equalized due to the equal number of transformer windings that are present between each of the nodes. This prevents any of the battery cells from being discharged to a voltage level that is lower than any other cell. Because each cell is maintained at a voltage level that is the same as each other cell, the entire battery stack may be discharged to a maximum degree without any one cell becoming depleted first or damaged. Other variations are also disclosed. For example, a single node may serve two battery cells. This results in looser regulation of the voltage to which each cell is discharged, but also results in a simpler and less expensive circuit. Also, the present invention may be used with battery cell stacks having any number of battery cells.