How much current does an alternator supply in this situation?
How much current does an alternator supply in this situation?
(OP)
The van's alternator is rated at 125A.
The battery is 130Ah (RC 190 mins 1000CCA).
I think in theory the battery should be able to deliver 115A for 40 mins (less in reality due to heat build-up). When I ask it to do that the voltage drops to 11.7V & it holds there for at least a few mins (I haven't let it go to for longer).
With the same load with the engine idling, the alternator manages to keep the voltage at the battery terminals above 12V (can't remember what exactly) but how do I work out how much current it's contributing? I guess it can't make 115A at those revs so I'd like to estimate the draw on the battery so I know how long I've got (roughly!)
All cables & contacts are sturdy (don't get too hot).
Cheers





RE: How much current does an alternator supply in this situation?
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RE: How much current does an alternator supply in this situation?
'RC 190' on the battery means it's rated to deliver 25A for 190 minutes, so let's say it can do 50A for 60 minutes & let's say the alternator can make 50A at idle revs, that means I've got a stable situation for an hour.
That's a lot of guessing & I haven't got an ammeter big enough to measure the alternator current so I suppose I'll just have to test it for longer watch the voltage to work out what's going on...
RE: How much current does an alternator supply in this situation?
Alternator characteristics are:
at 1000 rpm (alternator, not engine) 0A
at 2000 rpm 60%rated output
at 6000 rpm rated output
at 15000 rpm 110%
for a couple of different designs I looked at those curves don't change much.
It is not unusual, although it is annoying, to run with the battery discharging slightly at idle if you have any of the usual loads running.
Cheers
Greg Locock
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RE: How much current does an alternator supply in this situation?
My second potentially useless comment is to Greg: that "C1.2" doesn't look right. Is that supposed to mean 1.2 * C? C is 130A, so 115A is actually 0.88C and (Mark) the perfect case capacity is 53 minutes, not 40.
RE: How much current does an alternator supply in this situation?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: How much current does an alternator supply in this situation?
As for the "50A for 60 minutes & let's say the alternator can make 50A at idle revs" At idle, a typical car electrical load is only the order of 20A, so the alternator would not supply more than that. Additionally, it 's not exactly a "stable" condition. Assuming a battery starts at full charge, and 50A is drawn, the battery continues to discharge until the threshold of the alternator kicks in, at which point, the alternator supplies 50A. But, since the load is 50A, the alternator will not be charging the battery until the load goes away. If the system is stopped completely, the battery is still down the energy drawn before the alternator kicked in.
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RE: How much current does an alternator supply in this situation?
RE: How much current does an alternator supply in this situation?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: How much current does an alternator supply in this situation?
RE: How much current does an alternator supply in this situation?
I've found a quick and easy way to measure max alternator output is to simply place a standard 100amp battery tester load on the battery and, with the current probe on the alternator output lead. The load tester will pull the battery voltage down low enough that the alternator should go to max current-limited output..
A 100 amp current load is within range or in many cases much less that the typical engine starter current load, depending on whether the starter motor is a reduction gear permanent magnet starter motor (far more efficient) or the older generation wound field and armature starter motor.
I've measured PM motors starting Ford 460cu size industrial generator engines only pulling about 70amps to start and the old wound field and armature motors sometimes having an initial current surge approaching 10x this value for same size engines.
Regarding final charge voltage levels, remember that alternators have (now a days many internal to the alternator using a hybrid chip regulator) regulate battery charge voltage levels to follow a battery temperature compensation curve, or done by external regulator. You can find systems for typically marine applications that do this far more precisely with temperature probes that mount on the battery positive terminal for the regulator to more precisely follow the temperature compensation recommended by the battery manufacturer. Many automotive applications consistently undercharge batteries due to not using more precise temperature compensation control and I guess to cheaply hedge against very high battery temperatures in an under hood environment.
RE: How much current does an alternator supply in this situation?
Wired a 200A 75mV shunt resistor in between the van's electrics & the battery positive terminal. 1mV drop across the shunt with engine idling & no extra electrical load (14.3V at battery).
With a 1150W 240VAC load on the 85% efficient inverter wired to the battery, the drop across the shunt was 36mV (13.3V at battery).
Looks like the alternator is supplying most of the current I need?
(After 5 minutes the shunt was 34mV & battery was 13.2V)
Interesting about under-charged OE batteries & a current probe would have been great but not cheap! A friend has a couple but they're AC only
RE: How much current does an alternator supply in this situation?
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RE: How much current does an alternator supply in this situation?
After 5 mins the alternator's contribution is down to 90A - I guess that is due to heat build-up?
Will do more testing but it's looking good