71 roadrunner not enough power to charge the battery

[RJRENTON]

The alternator is not intended or designed to charge the battery. It only maintains the charge and powers the rest of the cars electrical demand. The battery is to start the car and excite the diodes in the alternator to induce a magnetic field. Dead battery, dead alternator. Slow charge the battery to full charge. It could take several hours. Test to at least 12.6 volts at rest. Load test pulling a 50% load. Should maintain 9.5-6 volts under load. If it doesn't, needs new battery. You can't check an electrical system without having a fully charged battery first. Otherwise all other test are not going to be accurate.

Totally incorrect.....the alternator's function is to charge the battery AND to supply ancillary power for electrical accessories.
The battery does not "excite" the diodes. The magnetic field is produced by the ROTOR, inducing an AC current (and voltage) in the STATOR WINDINGS, which are rectified, or changed, to DC voltage and current by the diodes, configured in a three phase full wave bridge rectifier circuit. The rectified power is presented at the output terminal and ultimately to the battery and the car's electrical system.
Regarding proper test procedures, your best bet is to follow the FSM for your particular vehicle. Using a variable load device, measure the battery's terminal voltage, following FSM values, typically not less than 10.5 volts at the perscribed load (450 amps) for a perscribed amount of time (15 seconds). REMEMBER...a lead acid storage battery does not store electrical energy but stores chemicial energy. The battery's ability to absorb and release energy, on a cyclical basis over time deterriorates. The longer the battery's ideal state of charge (14.0-14.5 volts) can be maintained, by a functioning alternator, the longer the battery will last. Another test of a battery's state of charge is to measure the specific gravity of the electrolyte, when possible. An ideal specific gravity would be 1.260-1.270. As the battery's charge level goes down, the specific gravity will drop.
Just something else to consider.
BOB RENTON

 

[Photon440]

Working in an auto parts store, I can confirm that while it is POSSIBLE to charge a dead battery with the alternator, when buying a replacement alternator and the vehicle has a dead battery, the user is warned to make sure that the battery has been charged beforehand and not use the alternator to do the charging. Maxing out the current rating of the alternator for charging is not what it was designed for.

 

[Don Frelier]

I've probably charged a dead battery at least 20 times (maybe even 30) by jumping and running the car.
First I'll let the jump vehicle idle for a while, then start the car.
After it's running I'll let it idle for a while so it can't put out full output from the alternator watch the ammeter when
the RPM is increased to see if the battery is getting charged then finally drive it. You can tell alot by watching the ammeter.
Whenever possible I'll try and put it on a trickle charger, otherwise the high charge rate is tough on everything and should be avoided.

 

[Dibbons]

I was informed years ago to turn on the headlights before removing jumper cables to prevent damage to the alternator. Might not be a bad idea. Don't know the science behind it though. I don't remember if this advice pertains to the vehicle doing the jumping or the vehicle being jumped.