1987 intruder 700 battery not holding charge

Load test the battery. (most bike shops will do it for free) Check charging system. (should be 13.5vdc running at 3000rpm)
Also, sounds like you may have a bad ignition switch?

Check it is charging. The voltage of the battery should be about 14 volts with the engine running at just above idle. If it is lower than this (which I suspect) you will need a new regulator or rectifier. to identify which of these you need will require a visit to an auto electrician.

Sounds like you have a bad stator or rectifier. If you like to work on your own bike get a repair manual it will give you several test you can do. Charge the battery to full charge, take a volt meter and put in on the battery leads pos to plus and neg to ground. start the bike and watch the meter it should be putting out over 13 volts if it is not have your rectifier and stator checked by a pro. also check the compression valve cable it may be out of adjustment . I had one of those bike for 13 years a great bike

Alternator, rectifier or regulator. Chances are greater that it's the electronics rather than the coils. Check the AC voltage going into the electronics, and then check the DC voltage exiting it.

The easiest way to check if the battery is charging is to put a volt meter on the battery.
See if the voltage increases when you rev the engine beyond the battery's engine-off voltage.

the stator will have three phases (three same color wires) any pair of these should put out maybee 50Vac, and none of them short to earth.
If you have AC, then stick the multi meter across the battery,it should rise past 14 vdc with the bike running

The oil breaks down the insulation of the stator windings over time

Sounds like you battery isn't holding charge overnight. If it was the alternator, it wouldn't start during the day so the alternator sounds fine.

Make sure the battery that is on it is the right one for the bike.

Try taking the battery off the bike and using a battery charger to charge it overnight. If that doesn't work, I'm afraid it's going to need a new battery.

Good Luck

It's possible the voltage regulator output is too high and boiling all the fluid out of the battery. Do you have to add fluid to the battery at all? Have you done a volt output test of your alternator?

The battery probably needs replacing. They are small and it is over 12 years old.

It's not charging, or the battery won't hold a charge? Get a multimeter and test the generator with the bike running, should be 13.5-14.5 volts. If you get this, you need a battery, if not need to find the bad component in the charging system.