1990 Nissan 300ZX battery wont stay charged

Most people don't leave the jumper cables on long enough for their vehicle to back-charge your battery. OR, the cables are too small a gauge to handle the amperage to turn your starter. Anyway, it's one of two things. 1- your battery is bad and will accept or hold a charge, or 2- your alternator is defective and isn't charging the battery as the engine runs. From your description, I would guess it's the alternator. A simple load test can be performed by any shop to determine the problem. You can do this yourself with a volt meter. With engine running, connect the meter accross the pos and neg terminals of the battery. At an idle, with all accessories "off", the meter should read between 13 and 14.5 volts. If it reads below 13, the alternator is bad. Hope this helps.