Hi am building a Austin Healey replica with a Ford V6 motor space is a big problem so the radiator has to tilt at an angle. I have just installed an electric pull fan to help with cooling. The fan draws about 20 amps. I installed a 90 amp alternator. When the fan switches on there is a big voltage drop all the guages register a lower value and the volt meter drops to 10 volt but the charge light stays off so alternator still charging. Any ideas what the problem could be?

1) battery to small presently 60 ah
2) bad earth
3) faulty battery

If i disconnect the fan everthing returns to normal