doctors, confirm my diagnosis...
 

hello all! i think i have a bad battery and want a second opinion. i have no problems starting or running, but noticed the voltage kinda low when driving, around 13 volts. checked the battery voltage with the key off. reads 12.14. checked the water level, and one cell was low. topped off with distilled water. put on battery charger (12v, 2 amps) for a few hours. battery read 12.54 with the engine off and headlights on. i thought all may be well again. went to a car show today, and noticed the almost 13 volt condition again. starts and runs fine. checked the junction block and alternator post with the engine running, and confirmed the gauge in the car, 13.02 volts. that seems low, like a bad voltage regulator. turned off the engine, and the battery read 12.45. started the engine again with the multimeter hooked up and got the 13 volts again. disconnected the battery and it went up to 14.56 volts. so my conclusions are: good voltage regulator. battery has water and is of unknown age (at least 2 years old) and the date tabs were not marked. so, bad battery? do you concur?

I think the battery will show around 12V when the engine is off.Best way to check the battery is to have a load test done on it....JB.

the load isn't the issue; it can handle it. what makes me think it's bad is the fact that my electrical system will run at over 14 volts when the battery is disconnected and a 140 amp alternator can only put out 13 volts when it's connected. why would the battery be putting such a load on the alternator unless it was bad?

The alternator is running the car if the battery is disconnected and will show a higher output.With the battery connected,the car is drawing off the battery and the alt doesn't have to charge as much.......JB.

right, but why would a charged battery (already at 12.5 volts) bring a 140 amp alternator down to 13.0 volts?

Tell me if I'm wrong, but disconnecting a battery with the car running is a great way of destroying an alternator.

Do a little research on the electrical system of an automobile.....It's kind of hard to explain the concept if you are not familiar with it.Maybe Quick-Silver will chime in........JB.

If you disconnect the battery and the car stalls, the alt is not charging.Won't hurt it unless you short out the + cable........JB.

i just noticed that this was backwards. the alt is at 14.5, internally regulated. when the battery is at a lower voltage (and it always is), it draws current from the alternator (rate dependent on state of charge), except for momentary spikes of current flow. hence the question, why is my battery drawing so much current from my alternator?

Actually, disconnecting the battery cable can damage diodes in the alternator. The regulator can't respond instantly to reduce alternator output, so the voltage spikes when the load drops off (possibly well over 100 volts). The diodes aren't designed for that kind of voltage.

I can think of a few possibilities:

1) There's a significant difference between the voltage on the alternator sense circuit and the battery

2) The alternator's current output is low (possible caused by damaged diodes from disconnecting the battery)

3) A shorted battery is drawing a lot of current and pulling the voltage down.

It could be a combination of these -- if the alternator's good, it would take an awful lot of current going to a shorted battery to pull the voltage down. This would create a huge amount of heat in the battery.

I'd have the battery and alternator both tested.

When the disconnect test is done,all accessories should be off and only current drawn is the ignition......Never had any problems checking them that way as a mechanic for 40+ years........Not to contradict anyone,but that is the way it works for me......JB.

Sounds like the alternator is getting weak. A weak battery would make the alternator charge harder and it would show it testing voltage at the output post. The140 amp alternator should go past 14.5 volts charging normally.
Un-hooking a battery cable (engine running) tells the alternator the battery is plum dead and the alternator should start charging wide open. Cable un-hooked, voltage at the output post could go to 17 or 18 volts, with a stock alternator.
Even if the battery is bad, system voltage should stay up all the same, especially with a 140 amp alt.
You need to load test the battery and compare that to what it's rated at to see where it stands in it's life cycle.
Testing amp output at the alternator, with all accessories on, will tell if the alternator is up to par.

Make sure the belts not slipping before you do too much.

That alternator should be plenty strong enough to maintain 14.75 volts and be boiling a bad battery.

Thats what I understood, just couldn't remember why.

i just found the code stamped in the interstate marine cranking battery, and it was made in '02. might be time for a new one anyway. the alternator is going with me to kragen to see what it's up to. this is an alternator that's only 3 years old, but i recently replaced the case with new black powdercoated parts, so i may have dorked the up the internals in the process. if it's just a diode problem, that should be easy enough to fix. we'll she how she does...

Actually, a low battery has low resistance, so lots of current flow. Battery disconnected equals high resistance, so low current flow. That shouldn't make the alternator produce full output -- it's still sensing system voltage and trying to control to that.

I think the issue is that without the battery as a "shock absorber" the regulator can't accurately control system voltage, so the charging system overreacts to changes in load and the voltage swings wildly.

alternator checked bad! so, i'm off to test the one on the camaro, which is the same alt with the same changes (case replaced and coated). i hope i didn't mess that one up, too!

Are you going to replace your battery? I think it'd be smart to, since it's 7 years old.

yep. trying to decide which one to get. the 78 series is shorter and lighter, but the 24 series has more power. decisions, decisions...

so the camaro alt is good. likes to be at 14.0 volts, and with 2 18 amp fans on with headlights is at 13.6 volts. the battery is at 12.5 volts (after discharging capacitance with with the motor off, headlights on for 30 seconds or so) and is 4 years old.

so, i disassembled the suspect alternator. nothing looks bad or cooked. i did notice that the 2 prong plug connector had the voltage sensing connection backed out. what would the alt do if the voltage sensing circuit wasn't connected? maybe that was the problem? got a new interstate mega-tron. went with the 101 series. nice and small and matches the black engine compartment (how vain is that!). i'm tempted to reassemble that alt and put it back on and see what happens with a good connection to the sensing circuit. btw, both cars have the pigtail sensing circuit connected to the back post on the alt with a 8 gauge wire going to a junction/distribution block on the fenderwell. so the question now is: what does the alt do with a diconnected voltage sensing circuit?