Use a digital multimeter across battery posts, everything off, what is the reading? The generic spec for a full charge is 12.6 volts. If that checks out, leave the meter leads touching the battery posts, have helper turn key to crank, how far does the reading drop, or does it drop much at all? Generally it shouldn't drop below 9 volts, different makes and models may be slightly different readings, if it drops way down there, that won't work. Did you try jumper cables? Note the wiring diagram for starter motor. Proper voltage and ground is important.
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