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I lost all power a couple of days ago and the garage say that I need a new alternator fitted, approx cost £400.
I had a new alternator fitted 6/8 months ago because of the same problem.
Could there be another reason for this power loss, every time the garage charges the battery and drive the car around for few hours it looses all power, the battery is only 6 months old.
Thanks for any suggestions.

Assuming the battery and the alternator has been checked by the garage (both should be within manufacturers warranty BTW) then one other possibility is the rectifier... The alternator generates AC voltage and the rectifier (a series of diodes) convert this to DC voltage.

RP wrote:every time the garage charges the battery

If they put the charger leads on in REVERSE then they could cause damage to the alternator - easy to do, difficult to prove.

If it was done by them its under warrantee but you may have to pay there labour

What car, year and mileage ???