absolute minimum voltage

[dark_knight]

i spent the better part if not ALL of yesterday wiring in some electronics. the radio was playing the entire time since morning and so at about 15h00, my stereo went off and my a'pexi t/timer was flashing digits like it was going into epileptic shock. i figured i must have killed my battery -- which was the case..

now here's the interesting part: i pushed and jump-started the car. it started but struggled to idle with my battery and EGT warning lamps illuminated on the dash. the car rough-idled for about 3 minutes then sputtered to death. i tried the jump-start again and same case story. so i got a battery and jumped the car.. & instantly all electrics came to life!

so my question is, does the alternator circuit have a minimum "jump" voltage that it needs to trigger charging..? because initially i thought i'd blown the alternator fuse (which was intact). my battery had drained to about 6.2 volts and of course that's way below the 9vdc minimum i've known about for these cars.
can anyone share a similar experience..?

 

[studoc72]

Nah bud but when you bump start a car your not giving it any extra power so therefore its using all the alternators power to charge while still draining power by running. Whereas when you connected the battery it was like having a new bat fitted so back to square 1. Im going to risk it and say that when you bump start it rather than idle you should hold the revvs up for a few minutes to help it with the charging but i cant garuntee that. Hope this helps.

 

[dark_knight]

odd scenario

but you see studoc72, the interesting thing with what happened in my case is that after bump-starting, i could only hold the revs at about 15 hundred even at WOT -- and this makes sense as the ignition system wasn't making/getting enough power to give a good strong spark for proper combustion any way.
i took readings off my batt terminals at this point and was still getting ~7 volts with the engine struggling to run and my batt warning lamp on.

now here is where it got really interesting, after jump-starting with a properly charged batt, seconds after the engine cranked, my terminal readings were up in their normal range of 13.8~14.4 volts.. even after disconnecting the good battery seconds after the car had cranked.. hence my suspicion for wondering if there is a trigger relay somewhere perhaps that only allows the charging system to engage after a minimum voltage is attained..

you follow..?

 

[studoc72]

Im with you that is strange i didnt realise it would only rise to 1500rpm with you holding the throttle open thought u meant you just left it to idle. That is strange i wonder if it could be related to the power going to the ecu and it not havin enough to give and measure correct readings. I havent seen anything like this before mate.

 

[dark_knight]

that's probably the reason studoc72 as my EGT dash warning lamp was also on during this entire time.. yet i'd just started the car.. so no way my EG temps were already dangerous.. lol

 

[Rev]

I have read there is a protection that wont let the car start if voltage is too low. Have experienced this when stero and radiator fan is spinning car fails to statrt.
Also if you had a good battery it will recover voltage after you turn off the power drain just takes a little time.

 

[weeJohn]

The battery was probably taking the lions share of the current to recharge the cells, leaving less for the rest of the electrics, the coilpack and fuel pump being the 2 that were probably affecting its running.

 

[steve-gers]

what is the minimum voltage required to start the car out of interest?

 

[dark_knight]

i hear it 9 volts steve-gers, but i've once cranked with 9.5 volts. starter was sluggish but it cranked to life..

 

[weeJohn]

Its not voltage its current that turns the starter, so it depends on what AH the battery is as well as the terminal voltage.

 

[dark_knight]

i concur weeJohn, it's current that cranks the starter, in as much as it's a co-relation of the available voltage. my battery is a 40Ah one (i think). will confirm..