I suspect you're chasing the wrong problem. Intermittent starting problems while lights and radio continue to work are symptomatic of a bad connection to the starter, not a drained battery. If the lights go out when you hit the starter, the bad connection is likely right at the battery. Pull the cables off and clean the insides of the cable clamps and the battery terminal until bright metal is showing. Dull gray isn't good. Reassemble and tighten securely.
If that doesn't fix it, follow this procedure the next time the vehicle fails to start.
- Put your multimeter directly across the battery terminals, not on the cables or connectors. You should measure > 12 volts.
- Now have someone hit the starter while you watch the meter. If the battery voltage drops below 10V, you have a discharged or bad battery. If it stays above 12V and the starter doesn't turn you have bad connection to the starter.
- To find the bad connection, keep moving the positive probe of the multimeter toward the starter until you find a spot where the voltage drops significantly when your assistant engages the starter. That will indicate you've moved across the bad connection. All the while keep the negative probe of the multimeter on the negative terminal of the battery.
- If you don't find the voltage drop by moving the positive probe, repeat the process by placing the positive probe on the positive battery terminal, and move the negative probe progressively along the grounding cable toward the engine block until you find the point of voltage drop. That will be your bad ground connection.
