Ned
Life time member
Number of posts : 644
I agree. It sounds like the battery is flat and that you are not getting any charge from the charge circuit.
When an other battery is connected, you are using it to run everything. once disconnected, there is nothing to support the electronics and the bike dies.
Which is it? Well as you suggested new battery should work, so I suspect that the charge circuit has a problem or/and the battery is draining quickly (bad connection or lights are on or ?).
You will have to get one of the el cheapo multimeters and start checking.
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Ned
05/1986 (K55) K100RS Motorsport (Europe), Production Code: 0503, 110k km, VIN:0140519 (SOLD)
1976 Honda Goldwing GL1000 (naked)
1997 BMW K1200RS red, VIN: WB10544A1VZA22667