If the mutual inductance between two coils in parallel is M, the equivalent inductor is:  If formula_34  The sign of formula_36 depends on how the magnetic fields influence each other. For two equal tightly coupled coils the total inductance is close to that of every single coil. If the polarity of one coil is reversed so that M is negative, then the parallel inductance is nearly zero or the combination is almost non-inductive. It is assumed in the "tightly coupled" case M is very nearly equal to L. However, if the inductances are not equal and the coils are tightly coupled there can be near short circuit conditions and high circulating currents for both positive and negative values of M, which can cause problems. More than three inductors become more complex and the mutual inductance of each inductor on each other inductor and their influence on each other must be considered. For three coils, there are three mutual inductances formula_7, formula_8 and formula_9. This is best handled by matrix methods and summing the terms of the inverse of the formula_40 matrix (3 by 3 in this case). The pertinent equations are of the form: formula_41  The total capacitance of capacitors in parallel is equal to the sum of their individual capacitances:  The working voltage of a parallel combination of capacitors is always limited by the smallest working voltage of an individual capacitor. Two or more switches in parallel form a logical OR; the circuit carries current if at least one switch is closed. See OR gate. If the cells of a battery are connected in parallel, the battery voltage will be the same as the cell voltage, but the current supplied by each cell will be a fraction of the total current. For example, if a battery comprises four identical cells connected in parallel and delivers a current of 1 ampere, the current supplied by each cell will be 0.25 ampere. If the cells are not identical, cells with higher voltages will attempt to charge those with lower ones, potentially damaging them. Parallel-connected batteries were widely used to power the valve filaments in portable radios. Lithium-ion rechargeable batteries (particularly laptop batteries) are often connected in parallel to increase the ampere-hour rating. Some solar electric systems have batteries in parallel to increase the storage capacity; a close approximation of total amp-hours is the sum of all amp-hours of in-parallel batteries. From Kirchhoff's circuit laws we can deduce the rules for combining conductances.