as illustrated in fig1 power to the load 2 is normally supplied by two banks of batteries , each of which is made up of a plurality of individual batteries cells mutually connected in series . typically , each bank includes a pair of batteries 10 , 20 and 30 , 40 , and each battery is made up of six two volt cells , with the two banks of batteries being connected to the load through respective switches 50 and 60 . when switches 50 and 60 are both closed , the two pairs of batteries are connected in parallel , and when either of the switches is open , the associated bank is disconnected from the load , leaving the other bank connected in series with the load . normally , both switches are closed to provide increased battery life , with the appropriate switch being opened to disconnect the corresponding pair of batteries in order to permit replacement or servicing of an individual battery . it will be appreciated by those skilled in the art that the batteries 10 - 40 may take a variety of forms , and that the invention is not limited to pairs of 12 volt batteries made up of individual two volt battery cells . each bank can be made up of an arbitrary number of batteries , each consisting of an arbitrary numbers of internal battery cells connected in parallel or series . however , irrespective of the battery structure , each of the batteries can be represented as an ideal voltage source and an internal resistance , so that the overall circuit can be represented by the equivalent circuit in which the total internal resistances r10 - r40 of the respective batteries are connected as illustrated in fig2 to a voltage source which represents the combined voltage output of the battery banks . in both fig1 and 2 , the series connections between the batteries in each bank are represented by letters a and c , and the common parallel connections by letters b and d . the use of parallel connected banks of batteries is of course conventional . the unique aspect of the circuit shown in fig1 and its equivalent shown in fig2 is that a measuring device 70 is connected between points a and c , as will be described below , for purposes of monitoring the condition of the batteries in the bank . while points a and c could in theory be located at points other than the midpoints of the two banks of batteries , at the physical connections between respective batteries in each pair , for purposes of convenience it is preferred that the total number of cells on each side of connection points a and c be equal , so that resistances r10 - r40 in the equivalent circuit of fig2 are equal when the batteries are in good condition . if not equal , then at least the ratio r10 / r20 should equal the ratio r30 / r40 . the objective of the circuit configuration illustrated in fig1 is to provide a convenient way of monitoring the conditions of the batteries in the respective banks and to provide a warning when one or more battery cells in either bank needs to be serviced or replaced . the reason the circuit is able to accomplish this objective is that when a battery cell fails , the internal resistance of the cell increases , at which time a potential difference occurs between points a and c , which can be measured by the measuring device 70 connected between the points . by connecting the measuring device across the midpoints of the banks of batteries , a baseline of zero can be used to indicated normal operation of the batteries , so that the presence of any non - zero potential difference indicates a failing cell . if the ratios of the resistances on either side of the ammeter have been chosen to be equal ( i . e ., r10 * r40 = r20 * r30 ) when the batteries are operating properly , so that no current will appear at device 70 , when the internal resistance of any cell rises as a result of aging or defect , then the ratios of the resistances will change and a current will be measured , the direction of the current depending on which bank includes the defective battery . the magnitude of changes that can be measured in this way depends solely on the sensitivity of the measuring device 70 , and thus it is possible to provide a warning for very small changes in battery condition , if desired . as a result , it can be seen that the measuring device 70 can be any current responsive device , including a micro - ammeter , lighting circuit , sound generator , computer , or combination of the above . in addition , a threshold activating current can be chosen so that the measuring device does not respond to minimal changes in battery function such as might be caused by temperature differences rather than battery cell deterioration , a computer can be used to monitor current fluctuation patterns in order to separate aging patterns from ordinary fluctuations . one of the advantages of the illustrated arrangement is the simplicity of the connection required to monitor battery condition , namely a single connection or bridge between points a and c . because of this simplicity it might , in hindsight , appear that this is the most obvious way to monitor the condition of a parallel connected bank of batteries . however , this is not the case . in general , the usual way to monitor battery condition is to periodically measure , using a voltage measuring device , the voltage output of individual batteries or the total voltage output of the banks of batteries . what the inventor has recognized is that , in a parallel configuration involving multiple batteries in each bank , the internal resistances of the batteries form what is referred to in the art as a wheatstone bridge . while wheatstone bridges are also known , the usual way of applying the wheatstone bridge concept is to connect a component whose resistance is to be measured as one of the resistors in the bridge , with the other three resistors being in the form of components of known resistance . to conceive of the parallel connected batteries as forming a bridge circuit actually allows simplification of the measuring circuit , and at the same time provides continuous monitoring and the ability to generate a warning without the relatively complex circuitry that would be required using conventional battery monitoring equipment . furthermore , it will be appreciated that the illustrated configuration not only provides continuous real time monitoring of cell condition , but also that the it provide an indication of which of the banks contains a defective cell . this can easily be accomplished by making measuring device 70 responsive to the polarity of the potential difference between points a and c . this is especially important where one of the banks must remain connected while the other is being serviced , as is the case for a ship at sea . thus , when the measuring device indicates that one of the banks needs to be service , the corresponding switch 50 or 60 can be opened , either manually or automatically under control of the measuring device , to disconnect the defective bank from the main power supply circuit . because of the parallel configuration , the remaining bank will still provide sufficient voltage to maintain operation of the load . as is apparent from the above , the invention requires a minimum of four discrete battery cells . however , the upper limit on the number of cells is unlimited . in addition , where the number of batteries is greater than four , additional monitors could be added by forming additional bridge connections , and the sophistication of the monitoring equipment represented by measuring device 70 can be increased as desired . consequently , it will be appreciated that the invention is not intended to be limited to the specific configuration shown , but rather should include all variations and modifications fairly included within the scope of the appended claims . while the invention has been described in connection with its preferred embodiment , it should be recognized and understood that changes and modifications may be made therein without departing from the scope of the appended claims .