If you get the same reading with the probe disconnected, then you probably don't have the probe wired correctly. I asked how you were connecting the probe to the DAQ card but you never responded. Do you have one of the accessory terminal block devices for interfacing?
Then at least provide the exact model of the 6259. There is a bnc and a mass termination type. Either one of these? If you are using the bnc, not sure this is correct for a scope probe.
yes i am using bnc. if it is not correct for voltage probe then which other DAQ can i use. if i use the screw teminal,since the voltage from the battery is giving more than 10 v.
I would recomend you start with the basics. First. connect the input to gnd and verify that is what you read. Then, directly connect to some smaller battery that is within the range of the DAQ. Then perhaps try a simple 10:1 voltage divider made from resistors with the 12V battery.
