Just an FYI......an automotive electrical system is designed to be run having the alternator provide all the needed voltage and amperage that it will need to operate. The battery is in the equation to provide a source for starting the engine and for being a temporary cushion for temporary voltage and amperage spikes when electrical components turn on and off. The vehicle actually runs on the alternator. If the battery is put into play to make up for deficiencies in the charging system it soon will be under charged, and for anything except for a deep cycle battery, that soon means an untimely death. As an aside, deep cycle batteries don't last very long if they are used in place of an automotive style battery and AREN'T drawn down quite a ways and then fully recharged. Hope this helps.
Rob Hawes-Anchorage Rod Works-Anchorage, Alaska