Quote:
Originally Posted by COLVERT
You won't charge a flat battery on a 450 mile run.
Lets say you averaged 50 mph on that run. This would mean that the best you could hope for is 9 hours on charge.
( 450 divided by 50. )
The regulator in the alternator would limit the charge going into your battery at about 5 amps. ( about the same as your average charger. )
You need at LEAST 24 hours on a charger to get your battery anywhere near fully charged. So your 9 hours running would just not cut it.
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9h x 5A = 45Ah, A 'flat' car (typical, not stop start or deep discharge type) battery is typically at 70-80% of full charge when the voltage drops below 10.5v - too low to start the car. 45Ah is over half the capacity of the battery so should be sufficient. kicker here is that the alternator needs enough voltage from the battery for the field coils in the first place, which it won't get from a flat battery - which is the real reason it won't charge on a 450mile run (you won't get 5A spare charging out of it + running the car and it's accessories).
24hrs on a 2A charge, or 12 on a 4/5 Amp charge will be enough. Failing that charge well enough off of the car (i.e. 3-4 hours at 4/5A) to give it enough to work properly and the long run should do the rest.