1963-1973 Mercruiser all Engines and Drives Service Manual Books 1 and 2, Page 206Get this manual
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STORAGE BATTERY MAINTENA NC tron ba lte ry mu be maintain dIf battery sh ows less th 9h vo lt wh und er startio loadit sh ou ld harge dCheck with DC vo ltme teread ing under 12 volts (meas ured at bat