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Here is an oldie but a goodie. Working a 87 Monte Carlo SS with the GM 12SI? alternator rated at 80 amps. I am making a long story short....
The vehicle will charge just fine with the blower motor on high, radio on, headlamps on low. If you flip the high beams on voltage drops to about 13.4 instead of 13.8 or more. Still OK, but iffy. Not sure on battery health but the car starts right up.
Its my buddies car and I'm working outside in the cold so I'm not playing with it a ton. If he uses the rear window defroster it REALLY draws down the alternator and will barely charge the battery like 12.7v. I used a Uscope with an amp clamp to see the load on the defroster circuit. I got about 19-16 amps or so. It does not get very warm and I found a voltage drop on the purple wire going to the grid from the swtich on the dash. About 1V. I really didn't feel like dropping the headliner and such crap as its a T-Top car and he was not too concerned about so I didn't pursue that part of it. Anyway I was curious to see what the alternator was putting out under this heavily loaded condition. I used the same settings and such as the previous 19-16 amps measurement. ON the alternator feed wire to the battery I was getting 20amps at idle????? No more with RPM's either. I also had 20 amps on the ground and positive battery cables. This does NOT make any sense. I tired re-zeroing my amp probe and messing with the settings but it either read something completely wrong or too odd to be real. 20 amps is NOT enough to run what I have on in the car. Did I miss something/screw up or is this just odd?
FYI, all feed/signal wires etc...to the alternator are OK and working.
The numbers add up at 13.4 v there is a small nett charge current making it into the battery, at 12.7v there is zero amps going into the battery perhaps even slight discharge.
The voltage of a fully charged lead acid battery is 12.5 to 12.8 v depending on temperature but the break even voltage is generally taken as 12.9 to 13.1v this also varies with temperature. Only when the charging voltage is over 13.1v will actual re-charging start.
Above this break-even voltage the current going into a battery depends on the state of charge more than alternator output voltage. Lead acid batteries don't work by simple ohms law. As a battery gets close to full charge it will fight against the incoming current. So with a fully charged battery as the RPM is increased and the alternator voltage rises don't expect to see a proportional rise in current.
The alternators on lot of older cars did struggle to re-charge the battery with all the electrical loads on so the current and voltage numbers you are seeing aren't surprising. As you have a scope you could look at the voltage ripple on the alternator B+ terminnal.
On older generation cars the heated rear windows generally operated at higher currents than those on modern cars but they were usually pretty good at defrosting so in this case there could be something wrong.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Last edit: 8 years 7 months ago by Andy.MacFadyen.
Sounds like the charging system maybe normal then. There is a definite issue with the defroster it does not work very well and consumes a ton of power.
I suspected the stock charging system was marginal and you have confirmed it.
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