question

A couple of issues -

1. At 25A charger output current, the 20A fuse will blow! Maybe not immediately, but it will soon after charging is initiated. Fuses have an inverse time/current curve, so dependent on several factors (primary being load current), the time to fail may vary from instantaneous to sometime later.

2. Typically, the output fuse is not installed to protect the charger, It is installed to protect the charge circuit wiring (bus) from short circuit faults and load induced over-heating causing damage, possibly even a fire. I imagine that the charger has internal current limiting as protection.

3. Mounting and connection of any DC output fuse needs to be well considered to prevent false fails. The fuse is a 'fault limiter'. It is a deliberate weakness inserted in the circuit designed to heat up and 'fuse' on SC faults and sustained over-current faults. Alas, if you have a poor connection at the fuse, the IR losses at the connection will create heat that soaks into the fuse element, thereby de-rating the fuse from its nominal rating. The connections to the fuse need to be made with cable or solid bar of a size that will act as a heat sink to minimise this occurrence,

Hope this is helpful.

Just read your question again and note the fuse in question is 30A. My error. I am guessing that the 30A fuse and 20A charger combo is to allow for initial inrush or surge current, and possibly thermal issues to which I have alluded to. One thing is certain, you won't do any damage by running the 25A charger on the 30A fuse, except possibly to your wallet if it needs to be replaced.