Alternators
prelude1980 - 6/9/09 at 07:58 PM
I have a 40A small denso alternator fitted to a zetec with megasquirt, i seem to be having issues with the battery never getting a full charge i
think. Is the alternator man enough?
I do have a 55mm diameter pulley on it
the alternator is wired correctly and i've checked to see it the battery drains while not in use and it gets a reading of 0.05 A when you put
the multimeter between the positive battery and connection
I do have an electronic speedo, fuel injection, 3 digital gauges?
Should i get a bigger alternator?
big_wasa - 6/9/09 at 08:00 PM
40A Should be fine. More than plenty for a 7.
prelude1980 - 6/9/09 at 08:02 PM
quote:
Originally posted by big_wasa
40A Should be fine. More than plenty for a 7.
but the battery never seems to charge fully thou,
Is there any way to see how much current is being drawn whilst the engine is running?
prawnabie - 6/9/09 at 08:03 PM
When you say fully charged what is your reading. iirc 12.7v is fully charged and 12.1 is discharged.
02GF74 - 6/9/09 at 08:05 PM
assuming you haven't fitted under car lighting , spot lamps and a massssive boom boom stereo you should be ok.
ignition is gonna draw 4 A; fuel injection maybe 10 A so you still have well over 20 A in reserve.
alternator comes on line by 2,000 rpm so unmless you got massive gearing and never go about that rpm engine wise or have geared down the pullys, the
alternator should charge.
Headlamps would draw about 12 A so you should be ok even for driving at nights.
Instruments take next to nothing.
So 40 A should be adequate.
What capacity is your battery?
How new is the battery?
Have you checked all connections are clean and tight and wiring of adequate size?
What exactly do you mean by "i seem to be having issues with the battery never getting a full charge i think."?
Have you put multimeter to see what voltage is when revving at idle and 2,000 rpm?
[Edited on 6/9/09 by 02GF74]
big_wasa - 6/9/09 at 08:07 PM
Are you sure you havnt got a bad cell in the battery ?
If you think about what kit is running it wont be alot.
prelude1980 - 6/9/09 at 08:07 PM
i get 12.3 - 12.4, and when running i can sometime get 13.6 at idle and when you rev it a bit it drops to down to 12.2 then stablises at 12.6 ish
02GF74 - 6/9/09 at 08:13 PM
and what is voltage when not running?
sounds a bit on the low side to me.
Do you have a igntion lamp fitted, if so , does it turn on when engine is not running and go out when engine is running?
You have wired the alternator correctly?
The Denso needs a switched live input.
prelude1980 - 6/9/09 at 08:18 PM
quote:
Originally posted by 02GF74
and what is voltage when not running?
sounds a bit on the low side to me.
Do you have a igntion lamp fitted, if so , does it turn on when engine is not running and go out when engine is running?
You have wired the alternator correctly?
The Denso needs a switched live input.
i do have a light, it's on when not running and off when running,
i can't remember what the voltage is when not running and it's too dark to check with no garage lights LOL
02GF74 - 6/9/09 at 08:34 PM
quote:
Originally posted by prelude1980
i do have a light, it's on when not running and off when running,
that bodes well for the alternator; what about telling us more about the battery?
prelude1980 - 6/9/09 at 08:39 PM
i'll let you know tomo after i get home from work
craig1410 - 6/9/09 at 10:30 PM
quote:
Originally posted by prelude1980
i get 12.3 - 12.4, and when running i can sometime get 13.6 at idle and when you rev it a bit it drops to down to 12.2 then stablises at 12.6 ish
That doesn't sound right. I would expect 13.something at idle but it should stay at that level or even increase when revved. Most run at 13.8 or
14v and this should be regulated by the aptly named regulator. I've never known an alternator's output voltage to fall when revs are
increased.
My best guess is engine to chassis earth strap isn't clean and tight (or, less likely, alternator to engine connection). Also check the
alternator positive feed cables just to be sure. To test, just measure voltage between alternator body and the battery earth terminal under load. You
should not get more than about 0.25 volts.
Failing that I would expect you have an alternator fault. This is most easily diagnosed with an oscilloscope to check the output waveform or you can
do some diagnosis with a voltmeter on the AC volts range to look for excessive voltage oscillation which might indicate faulty rectifier diodes. The
other thing which can go wrong is the regulator but in my experience this tends to cause excessive charging.
Edit: One other thing - your battery voltage is quite low at 12.4. Try charging the battery from a mains charger for a few hours (or ideally
overnight) and then disconnect the charger. Check the voltage after 1 hour and see if it is more like 12.8 volts. If you don't get close to 12.8
volts then the battery is suspect. Note these readings are taken with the battery disconnected from the vehicle (earth lead only necessary). Note if
the battery is faulty then be careful it doesn't gas excessively and cause an explosion when you disconnect the charger. Switch the charger off
at the mains, wait 10 seconds and then disconnect the croc clips. Make sure you have good ventilation to avoid hydrogen buildup too during
charging.
I hope this helps.
Cheers,
Craig.
[Edited on 6/9/2009 by craig1410]
prelude1980 - 7/9/09 at 04:52 PM
well been testing it again and if i disconnect the battery i get 12.4v.
When the car is running i get 13.5v and rev it up 13.8v ish. (this now seems normal)
i'm gunna whip out the battery and stick it on charge see if it holds a charge
02GF74 - 7/9/09 at 06:47 PM
^^^ that sounds ok asfar as the alternator hcarging goes but you still haven;t asked my earlier question, whcih was:
What exactly do you mean by "i seem to be having issues with the battery never getting a full charge i think."?
craig1410 - 7/9/09 at 07:08 PM
quote:
Originally posted by prelude1980
well been testing it again and if i disconnect the battery i get 12.4v.
When the car is running i get 13.5v and rev it up 13.8v ish. (this now seems normal)
i'm gunna whip out the battery and stick it on charge see if it holds a charge
If your battery is more than 4 years old then it's due for replacement anyway.
If it is less than 4 years old then maybe a good overnight charge will do the trick. Bear in mind that if you are only using the car occasionally, the
battery will lose charge naturally even if disconnected and you may get to the point where you are not replacing the charge lost through cranking and
natural depletion.
If you are leaving the battery connected when not in use then even a low current like 0.05A will add up over time. In fact I would expect it to
discharge enough to prevent cranking within 3 weeks at this discharge rate.
Arithmetic: 0.05A * 24 hours = 1.2AHr per day * 21 days = 25.2 AHr. Assuming battery is 36-40 AHr then this will be more than half charge and may not
be enough to crank.
You mentioned your 55mm pulley - how big is the crank pulley in comparison? Measure from centre of crank to edge of belt and from centre of alternator
pulley to edge of belt. I think a 3:1 ratio (crank:alternator) is usual for a "normal" 6k RPM car engine. Is that about what you have?
prelude1980 - 7/9/09 at 07:30 PM
quote:
Originally posted by 02GF74
^^^ that sounds ok asfar as the alternator hcarging goes but you still haven;t asked my earlier question, whcih was:
What exactly do you mean by "i seem to be having issues with the battery never getting a full charge i think."?
sorry i forgot to answer, i have fully charged the battery off the car gone to use it a week later and the voltage is very low, alternator seems to be
charging but even after a run the next time i go to start it struggles
prelude1980 - 7/9/09 at 07:35 PM
quote:
Originally posted by craig1410
quote:
Originally posted by prelude1980
well been testing it again and if i disconnect the battery i get 12.4v.
When the car is running i get 13.5v and rev it up 13.8v ish. (this now seems normal)
i'm gunna whip out the battery and stick it on charge see if it holds a charge
If your battery is more than 4 years old then it's due for replacement anyway.
If it is less than 4 years old then maybe a good overnight charge will do the trick. Bear in mind that if you are only using the car occasionally, the
battery will lose charge naturally even if disconnected and you may get to the point where you are not replacing the charge lost through cranking and
natural depletion.
If you are leaving the battery connected when not in use then even a low current like 0.05A will add up over time. In fact I would expect it to
discharge enough to prevent cranking within 3 weeks at this discharge rate.
Arithmetic: 0.05A * 24 hours = 1.2AHr per day * 21 days = 25.2 AHr. Assuming battery is 36-40 AHr then this will be more than half charge and may not
be enough to crank.
You mentioned your 55mm pulley - how big is the crank pulley in comparison? Measure from centre of crank to edge of belt and from centre of alternator
pulley to edge of belt. I think a 3:1 ratio (crank:alternator) is usual for a "normal" 6k RPM car engine. Is that about what you have?
the battery is nearly 3 years old, the crank pulley is about 135mm dia
craig1410 - 7/9/09 at 09:24 PM
135/55 is 2.45:1 which is a bit on the low side I think. At an assumed idle speed of 800RPM that would only be 1960 RPM at the alternator which is a
tad lower than the RPM at which alternators start to produce a decent amount of charge. That might explain why you are only getting 13.5 volts at idle
instead of the full 13.8. In fact, many alternators produce 14v or slightly more.
Get your battery up to full charge and then try running your car at idle with headlights on full. If the alternator is charging sufficiently then the
voltage should stay steady at the same voltage as it is unloaded. If it drops then the alternator is either weak or more likely is not spinning fast
enough. To check this, repeat the test at 1000 engine RPM and see what the loaded and unloaded voltage is this time.
My bet is that your idle speed voltage of 13.5 volts will drop under load to closer to 13v. At 1000RPM I expect it will stay steady at 13.8v. That 0.8
volt difference will mean a big difference in terms of charging current. Bear in mind the charged battery should be about 12.8v so the excess voltage
(which drives the charging) is going from 0.2v to 1.0v - a factor of 5 increase. Even going from 13.5 to 13.8 is a decent increase with the extra
electrical load off.
You could always fit an ammeter? Just make sure your starter feed doesn't go through it... 
Cheers,
Craig.
prelude1980 - 8/9/09 at 07:41 PM
Right my battery is a 44Ah one, i have had it on charge for an hour and turned off the charger and the voltage settled at 12.8v (so i'm assuming
the battery is fine.
With regards to the pulley diameters, do i need a smaller one then? to increase the charge happening
craig1410 - 8/9/09 at 08:00 PM
Possibly but without knowing the maximum RPM of the alternator and the maximum RPM of the engine I can't advise you properly. Do you happen to
have the spec sheet for the alternator?
I'd try the test I suggested last time where you check charge rate at normal idle then try it again at fast idle. If electrical loads (ie.
headlights) cause the voltage to drop below 13.8v at idle but the same test shows no drop in voltage at fast idle then I would say you need a smaller
pulley. You really need the spec sheet of the alternator to set this correctly or look at the donor car and check the ratio (and max RPM) on that
engine.
Of course, you could argue - how much time is spent at idle speed in your car? Do you really need to have full charging capacity at idle speed?
Is it worth trying the car out with the freshly charged battery and see if that was all that was wrong?
02GF74 - 8/9/09 at 08:12 PM
^^^ wha the says.
charge the battery like you ahve done, disconnect from car for a week and see what happens when you try to start the engine.
car alternators typically run a 2 x engine speed, max rpm being 12,000.
for high revving engine the pulleys are resized so as not to exceed the max rpm.
prelude1980 - 8/9/09 at 08:19 PM
ok i'll give that ago, i had a look at the alternator it has a max rpm of 10,000. 40A
craig1410 - 8/9/09 at 09:50 PM
quote:
Originally posted by prelude1980
ok i'll give that ago, i had a look at the alternator it has a max rpm of 10,000. 40A
Are you sure of the max RPM? Obviously I don't know the exact model you are using but the following website suggests that 18000 RPM max is not
uncommon.
http://www.brise.co.uk/alternators.html
Also, if you look at the spec sheets for the 40A denso alternators on the above website it shows only 10.8A of output at 3086 RPM although strangely
it also shows 21.3A at 3079 RPM which suggests to me the regulator was in the process of switching on or something at the lower level. In any event,
if your pulley is 55mm and RPM is below 2000RPM at idle as discussed then I think it is likely that you are not getting enough alternator output to
drive the ignition and fuel injection never mind lights.
Interesting to note from the spec sheet that max output occurs at around 8500RPM but max RPM is 18000. I suppose this makes sense because you would
want your alternator to be at max efficiency at cruising speed but with the capacity to go to max engine RPM without exploding. I wonder if your 10k
RPM figure is max output speed rather than absolute maximum speed?
But yeah, as 02gf74 says, do a leakage test on the battery by leaving it for a week or so before deciding it is the alternator.
[Edited on 8/9/2009 by craig1410]
[Edited on 8/9/2009 by craig1410]