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I have a 1999 civic and a 2006 hyundai coupe.
civic battery is discharged and just for testing purposes Im thinking why dont I put the hyundai battery in the civic and see if all is well then I can go buy the replacement battery.
the civic battery spec is 45ah I think whereas the hyundai is 70. And the cca etc will be different.
Any harm in trying this?
There is no harm as long as the batteries are the same voltage, which they should be. It is recommended not to install a car battery that is LESS in CCA rating than the manufacturer recommends, though installing a higher CCA battery will do no harm, and may actually assist with starting in cold weather conditions.
Both batteries put out 12.x volts. You'll be fine.
Voltage and CCA would be fine, however you may not have room physically to fit the the Hyundai battery in the Honda civic; you may also find that the terminals are on the opposite sides.
A better solution would be to take the Civic battery, put it in the trunk of the Hyundai, and take it to a car parts supplier. They will be able to test it for you. If it's OK, but just discharged, they'll usually charge it for you overnight for free. Once you get the Honda running, take it back to the same store, and they'll do a comprehensive charging system check for you.
You could also (as an alternative) try to jump-start the Honda from the Hyundai. Once it's running, drive to the car parts supplier for a diagnostic test.
The only downside of the above is, what will you do if they tell you that the battery is OK, but the alternator needs replacing? They'll sell you an alternator, but they won't fit it for you.
