I have an electrical problem with a 99 Honda Civic DX.
The battery is draining with the key off. I can jump it and run it until it charges, but sitting for 2 days (key off) drains it to the point that nothing electrical will work (start, radio, windows, locks, etc).
I had the battery, starter, and alternator all tested and all were determined to be good. No lights found left on, radio off, etc.
??

Battery Drain

Parasitic battery drain is the cumulative load produced by electrical devices (e.g. ECMs, clocks, security alarms, and radio presets), when the ignition is turned off. These items continue to operate, when the ignition has been switched off, and are becoming an ever increasing problem. Normal parasitic loads are below 75 milliamps (.075 amps). When parasitic load is greater than 75 milliamps, batteries will drain even faster. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed, or shorted alternator diodes, are common offenders. Cooling fans, power seat belt retractors, radios and dome lights left on, alarm systems, and electric car antennas can also cause batteries to drain overnight. Leaving headlights on will usually discharge a fully charged car battery, with 90 minutes of reserve capacity (36 amp hours), in a couple of hours.

Two methods are used to test for parasitic load (engine off, under hood light disconnected, accessories off, and vehicle doors closed):

1. The basic approach is to connect a 12-volt bulb across the positive and negative battery terminals to test the bulb and battery. If it glows brightly, remove the negative battery cable and connect the bulb in series between the negative battery cable connector and the negative battery terminal. If the bulb continues to glow brightly, start removing fuses or connections to the positive battery, one-at-a-time, until the offending electrical component is identified by the bulb dimming.

2. The best approach is to use a DC ammeter, connected in series between the negative battery cable connector and the negative battery terminal, or a clamp meter (set to DC amps), clamped around the negative battery cable. Starting with the highest scale (or set multimeter to auto range), determine current load. If load is above 75 milliamps, after the initial surge, start removing fuses or connections to the positive battery post, one-at-a-time, until the offending electrical component is identified by the parasitic load dropping to 75 milliamps.

I have this problem as well. I don't know if this is your problem, but by chance I noticed the dashlights on the factory radio were on 24/7 with the key off. They are so faint I never noticed in the daytime. And yes it takes a couple days to drain the battery. I am trying to find the fuse to temporarily disconnect the radio until I get this sorted- be aware this may affect the keyless entry.