Mark Goodall wrote:

> The fans run fine straight off the battery

You might was well connect the fans directly to the battery (or to the
switched 12V bus that becomes active when-ever the ignition is on).

> although one fan has a bit of resistance when I turn it by hand

You need to oil or lubricate that fan - or replace it. That sticking
fan is drawing more load from your electronic relay, which is causing
your relay failure.

> so I'm thinking about wiring them to the battery and fuse box so
> they only run with the key on.

But you need to lubricate that fan so it turns freely, otherwise it will
eventually fail.