Starter Voltage while cranking

If you have a trunk mount battery, power it runs up the body, thru the ground strap between the engine and body. In general, I would have 2 gauge wire for high amp connections (juicing the starter). I would have a 2 gauge wire to a shiny clean heavy thick part of the frame unibody bolted with a clean sanded bolt (or engine block if batt under hood), lead batt terminals and post clean, connection between neg cable and batt terminal done right and sanded shiny. Then move to positive wire, run a 2 gauge from batt to the starter relay and a 2 gauge from the relay to the starter, again all clean wire brushed connections in mint condition. Voltage drop is just resistance in the power wire or in the connections. The larger the conductor and the better the connections, the less voltage drop. Fine stranded copper works better with DC. With a large enough conductors (2 or 0 gauge) and pristine the connections, you should see very little voltage drop. Here is something to check: volts at battery during start (cranking), then volts at starter motor during cranking, theres the voltage drop. My last remote battery had 2 gauge for positive and negative battery leads. Neg went to frame in trunk. Then from frame in engine compartment to block (2 gauge as well). All connections points were sanded bare, connected tight, then a coat of anti-sieze over them for oxidation prevention. Frigging engine had 10.25 to 1 and spun like 300 rpms on the starter.. With this arrangement you should see about 1.15 - 1.22 volts drop from battery terminals to starter motor assuming 200 amp starter load during cranking. Your system has some loss in wiring and connectors currently.. Or the dang battery is shot (volts at battery during cranking are like 10 or drop to 10 or 9 quickly) Start testing by charging battery fully and having it tested at Autozone or with another load based method.. My two cents.