Dell Latitude D600 Start-up problem?

I read with interest the thread in this forum about start-up problems with these laptops, but my problem is different, so I am starting a new thread.
The machine was working fine. Then it was kept indoors but not used for around 5 months. When I tried to turn it on, having connected external power and made sure the battery was charged, nothing happened except for the power button light coming on and the power on indication at top right corner. The hard drive activity lamp did not come on. There was no screen illumination, but I could hear the hard drive spinning.
So I fitted a new CMOS battery. No change. Next I fitted a brand new main battery. No change. Finally I removed the main battery and just ran the machine on external power. Behold, it started up and worked fine.

So the fault is that when any main battery is fitted, it will not start up. Also, if a battery is fitted while it is running on external power the machine freezes - but display remains illuminated and showing whatever was there when the battery was inserted.